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NI-DSA Software User Manual

1. National Instruments Corporation Exact Preferred k Frequency Frequency k 1 3 octave Hz Hz 1 1 octave 19 12 4 12 5 18 15 62 16 6 17 19 69 20 16 24 8 25 15 31 25 31 5 5 14 39 37 40 13 49 61 50 12 62 5 63 4 11 78 75 80 10 99 2 100 9 125 125 3 8 157 49 160 7 198 43 200 6 250 250 2 5 314 98 315 4 396 85 400 3 500 500 1 2 629 96 630 1 793 7 800 0 1 000 1 000 0 1 1 259 92 1 250 2 1 587 40 1 600 3 2 000 2 000 1 4 2 519 84 2 500 6 27 NI DSA Software User Manual Chapter 6 Advanced Concepts Table 6 2 Exact and Preferred Octave Analysis Frequencies Continued Exact Preferred k Frequency Frequency k 1 3 octave Hz Hz 1 1 octave 5 3 174 80 3 150 6 4 000 4 000 2 7 5 039 68 5 000 8 6 349 60 6 300 9 8 000 8 000 3 10 10 079 37 10 000 11 12 699 21 12 500 12 16 000 16 000 4 13 20 158 74 20 000 Figure 6 9 shows the preferred midband frequencies for some of the BPFs of the 1 3 octave analyzer These BPFs are constant Q filters the bandwidth of the filter increases with the increase in the midband frequency such that the ratio f bandwidth a constant value All the bandpass filters have the same value of Q Attenuation dB Frequency
2. see 6 17 PSV CLA DUE s i eet stet de etre rto i eI uen 6 17 Sweep Frequency and Auto Resolution see 6 17 Input Auto Ranging E recte tbe ie 6 18 Source Auto Level and 6 19 Swept Sine Measurements 6 20 SDectr m ee te a 6 20 C TOSS S rye te tees seas e eee 6 20 Frequency Response 6 20 THD Total Harmonic Distortion esses 6 20 SIND atn eet 6 21 Swept Harmonics on teer ROI RO qe 6 22 RMS Squared aciei ene ee 6 22 National Instruments Corporation ix NI DSA Software User Manual Contents Octave Analyzer ene ee edet pete certe a en Rs 6 23 Octave and Level Analyzers Software Architecture 6 24 Architecture of the Fractional Octave Analyzer 6 24 Exact and Preferred Frequencies eese 6 26 Summary of NI DSA Octave Mode Functions 6 29 Level Measurements eere ig ete 6 30 Averaging 6 32 Linear A Vera gin oe ucc tapete eene 6 32 Exponential Averaging esee 6 34 Impulse Linear Averaging eee 6 35 Peak Averaging eie tette ettet 6 35 Equal Confidence Averaging see 6 35 A B and C Weighting
3. 3 12 Step 4 Control eite ete ee e ER Ue e terea e ee pease red RD 3 13 Using Capture Mode to Acquire Time Domain Data e 3 14 Using Engineering Units seien epe Pr E RU 3 15 Configure Engineering Units essere enne 3 15 LabVIEW Example ttt tertie terere 3 16 C Example esce ede deo 3 16 Chapter 4 Swept Sine Mode Programming Step 2 Configure the Swept Sine Analyzer essere 4 3 Configuring Inputs ett ri e t e te e e 4 3 Configuring the SOUrCe stie tette 4 3 LabVIEW Example deste e Ee Re CEU ES 4 4 assente e e ERE 4 4 Notes on Example8 oec te re eet tees 4 4 Configuring Harmonics Measurements Optional 4 4 Configuring Custom Frequencies 4 5 Step 3 Read Swept Sine Measurement essere 4 5 Reading a New Measurement sess eee ene 4 5 LabVIEW Exatmple retener hee e Petre eee ees 4 7 C Example ied e utet O Re e e eene telo 4 7 Step 4 Control dte ete e Rob San ERE PEERS 4 0 LabVIEW Ex imple etre eee p E ere te 4 10 G Example vie Nt vnc MG aN tate a eee e 4 10 source NI 4551 Only rrr a ret oie ire Te tes 4 11 Step 2 Configure SOUICe 3 dpa e ee a e te Pee ces tod eee eet 4 11 LabVIEW Examples e i
4. esee 6 36 Considerations for Octave and Level Measurements 6 36 Configuration Limits eese 6 38 Frequency eterne 6 41 Setting Time e ib dee m t t ce neci dua 6 41 Overload Detection ers esses css ecce teet ede t tip Pert tpe 6 42 Appendix A Common Questions FET Mode zi speed ERE EDU DR T D HU fee tels 1 FET Analysis Questions irte eere A 1 Zoom FFT Analysis Questions essere A 3 SourceMOode ese ence eerte a a PRSE COH NGA pee ERR TN 5 Swept Sme Mode t ertt ree E ER USE EE eee Y EE etg 7 Octave and Level Analyzer Mode Add On esee A 9 Appendix B Technical Support Resources Glossary NI DSA Software User Manual X ni com Introduction to NI DSA Thank you for buying a National Instruments Dynamic Signal Analyzer DSA which includes NI DSA National Instruments driver for its DSA devices With NI DSA you can program your NI 4551 or NI 4552 to analyze time and frequency domain data onboard in real time This manual shows you how to use your application development environment ADE with NI DSA to program your DSA instrument Getting Started with NI DSA 1 Install your ADE the NI DSA driver and your NI 45XX For installation instructions refer to Where to Start with Your NI 45XX for PCI Dynamic Signal Analyzer which came with your hardware 3 Note Read
5. Lo NIDSA Configure EU Label vi NIDSA Configure EU Spa Ll NIDSA Configure EU dB Reference vi input channel pump eu Yeu error in no error Figure 3 6 Configuring Engineering Units C Example Configuring engineering units A microphone is connected on Channel 0 Units Pa NIDSA configure eu label DSASession 0 EU LABEL PA EU Microphone sensitivity 100 mV Pa NIDSA configure eu scale DSASession 0 0 1 EU SCALE V EU Reference level 20 NIDSA configure eu dbref DSASession 0 20e 6 NI DSA Software User Manual 3 16 ni com Swept Sine Mode Programming You can use swept sine mode to perform frequency response measurements with a higher dynamic range than is usually possible with FFT mode For information about how swept sine measurements are performed refer to the Swept Sine section of Chapter 6 Advanced Concepts Note Only the NI 4551 be programmed swept sine mode since this mode requires the instrument to generate stimuli You can find a swept sine programming example 455x Swept Sine on your NI DSA CD Figure shows the recommended program flow for swept sine mode programming National Instruments Corporation 4 1 NI DSA Software User Manual Chapter 4 Swept Sine Mode Programming NI DSA Software User Manual Initialize Configure Mode Swept Sine Config
6. ea eee eet 2 2 Notes on EX amples 4 ouo eed re RR er e OR ee SE 2 2 Step 2 Configlre Ret ro p HERO 2 3 LabVIEW Example ee re pete 2 3 GC Example 4 ug a eade eod esie A E egeo deg 2 4 Notes OM Example Sanam trente erede rte in 2 4 Step 3 Re d iet eh ew t eR er UP Red 2 4 Step 4 dControl iet o tet epe t i it teo a it dee piod 2 5 Step 9 Closes tists detecte a ee ee aa ila a mie ere 2 5 LabVIEW Examples teens 2 5 C Example ee 2 5 Chapter 3 FFT Mode Programming Step 2 Configure the FFT Analyzer essent eere 3 4 LabVIEW Examples tede been 3 5 C Examples geli ede cte ien te eu deni 3 5 Baseband FFT 3 5 Zoom FFT Example oe ete qo a 3 6 Notes on Examples reete ae Rete dies 3 6 RoUutiDg tuteionere eee e te te Rr ret 3 6 Configuring the FFT Engine eene 3 7 National Instruments Corporation vii NI DSA Software User Manual Contents Setting the Span dae 3 7 Configuring the FFT 3 8 AVGPASIN 8 55 seus Share Sitges enh aime nid aie 3 8 Step 3 Read FFT Measurements essent eene eene rennen 3 9 Reading a New Measurement sese nennen eene 3 9 LabVIEW Example ire eeoaeeoncemno EROR 3 12 C Example RIO
7. Configure Swept Sine Source sets the amplitude characteristics of your sweep Important parameters include e Amplitude sets the sine source peak level in volts unless Auto Level is enabled Autolevel enable when TRUE the source amplitude is automatically maintained at the level specified by Ideal Ref Level The output of the system under test is acquired on an input channel When Auto Level is enabled the amplitude of the source is adjusted to maintain a constant input channel level e Ramping enable when ramping is enabled ramping enable TRUE the source amplitude changes at the rate specified by Ramping Rate if FALSE the source amplitude is allowed to change instantaneously National Instruments Corporation 4 8 NI DSA Software User Manual Chapter 4 LabVIEW Example C Example Swept Sine Mode Programming NIDSA Configure Swept Sine i NIDSA bom NIDSA Configure 800 00 Swept Configure DSA 1200 00 Sine Swept Sine Mode vi Average vi Source Figure 4 2 Configuring the Swept Sine Analyzer Configuring Swept sine analyzer NIDSA configure dsa mode gDSASession SWPSINE MODE NIDSA configure swept sine gDSASession startFreq stopFreq numberOfSteps 0 0 0 512 1 0 6 0 NIDSA configure swept sine average gDSASession 0 01 1 0 01 1 NIDSA configure swept sine source gDSASession amplitude 0 1 1 0 3 0 3 0 10 0 0 1 0 Notes on Exa
8. NI 4551 Figure 6 6 Typical Swept Sine Measurement Application Why Swept Sine Measurement The transfer function of a system can be measured by both FFT and swept sine techniques Since the swept sine measurement generates and measures a single frequency at a time it has several advantages over traditional FFT measurements Instead of using the same settings on all frequency points as in the FFT case a swept sine analyzer can optimize the measurement at each individual frequency with auto ranging and auto level features National Instruments Corporation 6 15 NI DSA Software User Manual Chapter 6 NI DSA Software User Manual Advanced Concepts Auto ranging extends the dynamic range and auto level increases the signal to noise ratio If the system s transfer function has large variations within the measurement span a swept sine analysis can often give you greater dynamic range than an FFT analysis Because the FFT measures all the frequency components at the same time the source must contain energy at all of the measured frequencies Generally each individual component s amplitude is about 30 dB less than the overall source amplitude in the time domain So each frequency is actually measured at 30 dB relative to full scale This effectively reduces the dynamic range of the measurement A swept sine measurement on the other hand can eliminate the 30 dB loss because it measures one frequency at a
9. Slow the time constant for slow exponential averaging is 1 0 second This type of averaging is useful for tracking the energy levels of signals whose energy levels vary slowly e Fast the time constant for fast exponential averaging is 0 125 seconds This type of averaging is useful for tracking the energy of signals whose energy levels are quickly varying e Custom you can specify the time constant suitable for your particular application e Impulse the time constant for signals with increasing energy levels is 0 035 seconds whereas for signals with decreasing energy levels is 1 5 seconds This type of averaging is useful for tracking sudden changes in the signal and holding on to these changes so that they do not disappear too fast While exponential averaging is in progress the time for which the averaging process has elapsed is returned in the averaging seconds output of Read Octave Measurements and Read Level Measurements There is a one to one correspondence between the time returned and the measured values read from these functions 6 34 ni com Chapter 6 Advanced Concepts Impulse Linear Averaging Impulse linear averaging is available only for level measurements It is a combination of linear averaging and exponential impulse averaging Impulse averaging is performed over the time duration specified in Configure OLM Averaging and the corresponding measured values are weighted equally to form the average
10. arg X k tan REZO Im X amp The power spectrum reflects the energy content Power Spectrum X k National Instruments Corporation 6 3 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual The power spectrum normalized to 1 Hz bandwidth gives the power spectral density PSD PSD X k 2 Df The use of the FFT for frequency analysis implies two important relationships e highest frequency that can be analyzed is related to the sampling rate e The frequency resolution is linked to the total acquisition time which is related to the sampling rate and the block size of the FFT e Acquisition time 1 f where f sampling rate and N number of samples To illustrate the above relationships suppose that a digital time record obtained from discrete samples taken at a selected sampling rate is used to calculate a corresponding frequency spectrum composed of discrete frequency samples or bins For real signals the frequency spectrum has half as many unique frequency bins as the time domain record has points If the FFT size used is 1 024 that is 1 024 samples are acquired and the sampling rate is 204 8 kS s the complete time record acquisition will take 5 ms This means that the lowest frequency in the spectrum corresponds to the period of the time record itself x 200 Hz ms The FFT of a 1024 point record has 475 alias free lines o
11. Add On How do I configure the duration for linear averaging To set the time interval for linear averaging for both octave and level measurements use the Duration parameter of Configure OLM Linear Averaging The same value is used for the duration of linear averaging for both octave and level measurements and for all channels You cannot set the duration for linear averaging of octave measurements independently of that for level measurements nor can you set the duration of linear averaging independently for different channels How do I make level measurements To configure level measurements use Configure Level Measurements To read the level measurements first call Get Level Length to determine the number of measurements to read then call Read Level Measurements to read the measurements National Instruments Corporation A 9 NI DSA Software User Manual Appendix A Common Questions NI DSA Software User Manual How do I configure weighting e Use Configure Weighting Filter to select the type of weighting filter A B or C weighting Configure Weighting Filter selects the weighting filter for the entire device that is for all channels for both octave and level measurements e For octave weighting use Configure Octave Weighting to turn weighting on or off for a particular channel e For level weighting use Configure Level Measurements to turn weighting on or off for a particular channel and a particular le
12. center frequency by sacrificing the number of level measurements but you can keep both a high center frequency and up to eight level measurements by using exponential fast averaging instead of impulse averaging For 1 12 octave analysis using impulse averaging the highest center frequency possible is 5 339 Hz ANSI or 5 496 Hz IEC without weighting and without level measurements You can add A weighting but to keep the highest center frequency the same you must change to a less processor intensive averaging mode If you add eight level measurements per channel the highest possible center frequency drops to 4 490 Hz ANSD or 4 621 Hz IEC Note run the 1 12 octave analyzer to even higher center frequencies use Set OLM Sampling Rate to reduce the sampling frequency to 25 6 KHz NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual NI 4552 The NI 4552 has four input channels so the number of channels configured for octave measurements can be a more important factor in processor performance than it is with a NI 4551 If you configure one or two channels for octave analysis configurations that allow real time operation of the NI 4551 generally will for the NI 4552 as well However if you configure three or four channels for octave analysis the additional processing demands reduce the frequency range over which the processor can operate in real time To get a higher frequency range make the
13. coming from the test unit S C The diagram below illustrates frequency response H f of a system at one particular frequency f A and A represent the amplitudes of the stimulus and response signals respectively P and P are the phase of the stimulus and response signals Li Py NI DSA Software User Manual Figure 6 2 Frequency Response Measurement Most frequency response measurements involve calculating the magnitude and phase of the complex quantity H f The magnitude and phase can be related the physical diagram above by the following formulas d HG DAD ZH f tan mer P f PAf This frequency response function assumes that the unit under test is a perfectly linear noise free system In practice there is a finite nonlinearity present in all systems Any noise in the output is generally uncorrelated to the input signal Thus the quality of the frequency response measurement can be improved by averaging several spectra to help cancel random noise 6 6 ni com Chapter 6 Advanced Concepts The coherence function provides a measure of the correlation between the system response and stimulus is an array quantity with a value at each FFT frequency value f In a perfectly linear system has a value of 1 Another way of expressing this is that all the response signal was completely caused by the stimulus A value of 0 indicates no relation between th
14. 12 seconds The share 8046 of their points those between T 5 02 and T 5 10 Thus the FFT overlap is 8096 in this case Increasing the overlap actually generates more FFT curves in any given time period Time increment is simply defined as 100 minus the overlap so in the above example the time increment is 20 In software you control overlap time increment using Configure Base FFT Settings and Configure Zoom FFT Settings Time increment is one of the input parameters for these functions you should enter a number between 0 100 Enter a number greater than 100 to skip data points Zoom FFT Analysis Questions What is the major benefit of using a zoom FFT The zoom FFT technique is very useful for improving the frequency resolution of an FFT measurement if you are interested in a limited range of frequencies For instance assume you want to analyze the frequencies from 5 000 Hz to 6 000 Hz If you use an extended range baseband span with 950 lines you can look at all frequencies from 0 6 000 Hz In this case your frequency resolution is 6 000 Hz 950 lines 6 32 Hz A zoom FFT allows you to examine a frequency starting at a value greater than zero Using the zoom FFT you can obtain up to 1 600 frequency resolution from 5 000 Hz to 6 000 Hz The frequency resolution improves dramatically 1000 Hz 1600 lines 0 625 Hz National Instruments Corporation A 3 NI DSA Software User Manual Appendix A Common Questions NI DSA So
15. HARDWARE MALFUNCTIONS COMPUTER OPERATING SYSTEM SOFTWARE FITNESS FITNESS OF COMPILERS AND DEVELOPMENT SOFTWARE USED TO DEVELOP AN APPLICATION INSTALLATION ERRORS SOFTWARE AND HARDWARE COMPATIBILITY PROBLEMS MALFUNCTIONS OR FAILURES OF ELECTRONIC MONITORING OR CONTROL DEVICES TRANSIENT FAILURES OF ELECTRONIC SYSTEMS HARDWARE AND OR SOFTWARE UNANTICIPATED USES OR MISUSES OR ERRORS ON THE PART OF THE USER OR APPLICATIONS DESIGNER ADVERSE FACTORS SUCH AS THESE ARE HEREAFTER COLLECTIVELY TERMED SYSTEM FAILURES ANY APPLICATION WHERE A SYSTEM FAILURE WOULD CREATE A RISK OF HARM TO PROPERTY OR PERSONS INCLUDING THE RISK OF BODILY INJURY AND DEATH SHOULD NOT BE RELIANT SOLELY UPON ONE FORM OF ELECTRONIC SYSTEM DUE TO THE RISK OF SYSTEM FAILURE TO AVOID DAMAGE INJURY OR DEATH THE USER OR APPLICATION DESIGNER MUST TAKE REASONABLY PRUDENT STEPS TO PROTECT AGAINST SYSTEM FAILURES INCLUDING BUT NOT LIMITED TO BACK UP OR SHUT DOWN MECHANISMS BECAUSE EACH END USER SYSTEM IS CUSTOMIZED AND DIFFERS FROM NATIONAL INSTRUMENTS TESTING PLATFORMS AND BECAUSE A USER OR APPLICATION DESIGNER MAY USE NATIONAL INSTRUMENTS PRODUCTS IN COMBINATION WITH OTHER PRODUCTS IN A MANNER NOT EVALUATED OR CONTEMPLATED BY NATIONAL INSTRUMENTS THE USER OR APPLICATION DESIGNER IS ULTIMATELY RESPONSIBLE FOR VERIFYING AND VALIDATING THE SUITABILITY OF NATIONAL INSTRUMENTS PRODUCTS WHENEVER NATIONAL INSTRUMENTS PRODUCTS ARE INCORPORATED IN A SYSTEM OR APPLICATION INCLUDING W
16. Hz Figure 6 9 Preferred Midband Frequencies for 1 3 Octave BPFs NI DSA Software User Manual ni com Chapter 6 Advanced Concepts Summary of NI DSA Octave Mode Functions The functions are divided into several categories depending on their operation These categories are Configuration functions that configure the instrument for parameters such as the frequency range or the averaging mode Example Configure Octave Measurements Control functions that control the operation of the instrument such as restarting averaging for a particular channel Example Restart OLM Averaging Read functions that read measurements such as octave band powers octave preferred or exact frequencies or level measurements Example Read Octave Measurements Status functions that return the status of the instrument such as whether or not an overload has occurred at the input is the instrument operating in real time or the time that has elapsed since averaging was begun Example Get OLM Status Figure 6 10 is a flow diagram for level measurements Configure Configure Configure Configure OLM Engine I Weighting OLM Linear gt Level gt Filter Averaging Measurements Configuration a M Restart OLM Check New Get OLM Read Level Averaging Measurement Status Measurements Control Status Read Figure 6 10 Level Measur
17. Instruments Corporation All rights reserved Important Information Warranty The media on which you receive National Instruments software are warranted against defects in materials and workmanship for a period of 90 days from the date of shipment as evidenced by receipts or other documentation National Instruments will at its option repair or replace equipment that proves to be defective during the warranty period This warranty includes parts and labor The media on which you receive National Instruments software are warranted not to fail to execute programming instructions due to defects in materials and workmanship for a period of 90 days from date of shipment as evidenced by receipts or other documentation National Instruments will at its option repair or replace software media that do not execute programming instructions if National Instruments receives notice of such defects during the warranty period National Instruments does not warrant that the operation of the software shall be uninterrupted or error free A Return Material Authorization RMA number must be obtained from the factory and clearly marked on the outside of the package before any equipment will be accepted for warranty work National Instruments will pay the shipping costs of returning to the owner parts which are covered by warranty National Instruments believes that the information in this document is accurate The document has been carefully reviewed for tech
18. LSB of the worst case deviation from the ideal A D or D A transfer characteristic of the analog I O circuitry the current that flows into the inputs of a circuit the measured resistance and reactance between the input terminals of a circuit the difference in the input bias currents of the two inputs of an instrumentation amplifier a set of high level software functions that controls a specific GPIB VXI or RS 232 programmable instrument or a specific plug in DAQ board Instrument drivers are available in several forms ranging from a function callable language to a virtual instrument VI in LabVIEW a circuit whose output voltage with respect to ground is proportional to the difference between the voltages at its two inputs an ADC whose output code represents the average value of the input voltage over a given time interval a computer signal indicating that the CPU should suspend its current task to service a designated activity the relative priority at which a device can interrupt input output the transfer of data to from a computer system involving communications channels operator interface devices and or data acquisition and control interfaces current output high current output low interrupt request a type of signal conditioning in which you isolate the transducer signals from the computer for safety purposes This protects you and your computer from large voltage spikes and makes sure the measurements from the DAQ
19. RMS magnitude of the response e Swept noise reads the total noise excluding the fundamental LabVIEW Example NIDSA Read NIDSA Get NIDSA Get Measurement vi NIDSA Get NIDSA Check New Lohr NIDSA Read Swept Sine Measurement vi Measurement vi i Status vi Figure 4 3 Reading Swept Sine Measurement C Example Read Swept sine measurements Reading Swept Sine measurements is a 2 step process Read Frequency axis informations then read Frequency response informations These 2 arrays will be displayed in a XY graph Frequency response vs Freq Axis int CVICALLBACK TimerCB int panel int control int event void callbackData int eventDatal int eventData2 ViStatus sweepStatus ViStatus NewMeasurement National Instruments Corporation 4 7 NI DSA Software User Manual Chapter 4 Swept Sine Mode Programming Vilnt32 overload Vilnt32 measError Vilnt32 sweepState NI DSA Software User Manual Nariables used for Frequency axis Vilnt32 RealOrComplex Vilnt32 Length ViReal32 ViReal32 x0 Nariables used for Frequency response Vilnt32 respRealOrComplex Vilnt32 respLength ViReal32 respdx ViReal32 respx0 switch event case EVENT TIMER TICK NIDSA get swept sine status gDSASession amp sweepState amp measError amp overload Sweeping in progress if sweepState 0 Check if a new measurement is ready NIDSA check new measurem
20. Zone you can easily access the latest example programs system configurators tutorials technical news as well as a community of developers ready to share their own techniques Customer Education National Instruments provides a number of alternatives to satisfy your training needs from self paced tutorials videos and interactive CDs to instructor led hands on courses at locations around the world Visit the Customer Education section of ni com for online course schedules syllabi training centers and class registration System Integration If you have time constraints limited in house technical resources or other dilemmas you may prefer to employ consulting or system integration services You can rely on the expertise available through our worldwide network of Alliance Program members To find out more about our Alliance system integration solutions visit the System Integration section of ni com National Instruments Corporation B 1 NI DSA Software User Manual Appendix B Technical Support Resources Worldwide Support National Instruments has offices located around the world to help address your support needs You can access our branch office Web sites from the Worldwide Offices section of ni com Branch office Web sites provide up to date contact information support phone numbers e mail addresses and current events If you have searched the technical support resources on our Web site and still cannot fin
21. a harmonic level of 200 dB if the harmonic lies beyond the acquisition bandwidth How do the averaging settings work with swept sine analysis You can use Configure Swept Sine Average to control how much time goes into a swept sine measurement Up to a point longer test times can produce more accurate and repeatable frequency response measurements Configure Swept Sine Average has four input arguments e Settle time this value gives how many seconds should pass before each new frequency from the swept sine list is generated Since the units for this input are in seconds absolute time the length of this delay does not depend on the frequency being generated e Settle cycles like settle time this value also defines the length of the settling period However it is given in units of signal cycles rather than absolute time This means that the corresponding time changes as the test frequency increases smaller frequencies produce longer settling times Settle time and settle cycles both control the same physical parameter the length of the settling period between measuring individual frequencies At any given frequency the software pauses for either settle time or Settle Cycles Frequency which ever is greater For example assume settle time is 0 01 s and settle cycles is 10 cycles In this case the physical settling time is 10 Frequency for all frequencies below 1 kHz and 0 01 s for all greater frequencies A 8
22. analyzer can skip points in the range where the frequency response is flat and proceed step by step where a sharp transition occurs This results in fewer measurement points without a loss of frequency resolution 6 16 ni com Chapter 6 Advanced Concepts Swept Sine Controls and Settings The swept sine measurements are controlled by the following settings e Averaging e Sweep frequency and auto resolution e Input auto ranging e Source auto level and ramping Averaging Averaging is controlled by the settling time and integration time Settling time is specified by time and cycles The longer period specified by these two determines how long the instrument will wait before the integration starts after the source changes the frequency This waiting period allows the device under test to respond to the frequency change so the measurement can be made based on a settled signal Like settling time integration time is specified by time and cycles Time is converted to the next largest whole number of cycles The larger of these two will be used as the integration time The integration time must be an integral number of cycles This guarantees that the measurement results do not include DC or harmonics of the source frequency Swept sine integration is a process of multiplying the time domain data with a complex signal cos 2mft 4 j sin 2mft and averaging the results over the integration time The frequency f of the complex signal is th
23. analyzer to simply acquire a waveform The FFT parameter automatically sets the time parameter since an FFT cannot be performed without a waveform Setting auto sets the time and FFT parameters and those functions are performed as well as measuring the auto power spectrum Setting cross sets all of these parameters When you set the cross parameter the two baseband channels are linked to ensure that the settings for each channel are the same Setting changes made to channel 0 causes the channel settings to be changed to match channel 0 e Time Only Time domain information is available e FFT FFT Mag phase info is available e Auto Auto power spectrum Phase information is lost e Cross Cross power spectrum measurements Note Cross power spectrum measurements require much more processing than time domain acquisitions For the best performance be sure you choose the mode that provides only the information you need in real time and reserve other measurements for your computer a Tip Setting the Channel parameter to 1 selects all channels on your DSA device Setting the Span Set Classical Baseband Span can only be used to specify a classical span one that corresponds to 400 lines for a 1 024 point FFT You can set the span parameter to any value from 1 953 125 to 80 000 000 Hz but it is coerced to the nearest classical span Use Set Baseband Span to perform an extended FFT Sy Note Changing the bas
24. and Preferred Frequencies You can choose for the bandpass filters for fractional octave analysis to conform to either of the following standards e ANSI S1 11 1986 Specification for octave band and fractional octave band analog and digital filters e IEC 1260 1995 07 Electroacoustics Octave band and fractional octave band filters Since the NI DSA fractional octave analyzer has been designed to operate over the audio frequency range the reference frequency f chosen is 1 kHz The exact midband frequencies fmn for each are calculated according to this formula fm fx 2 where is an integer and b 1 for 1 1 octave analysis b 3 for 1 3 octave analysis and b 12 for 1 12 octave analysis For example for a 1 3 octave analysis for k 0 you have f x 203 1000 Hz For k gt 0 the calculated values of f are above 1000 Hz and for k lt 0 the calculated values of are below 1000 Hz The standards also specify what are known as preferred frequencies for 1 1 octave and 1 3 octave analysis Preferred frequencies are not specified for 1 12 octave analysis The exact and preferred frequencies for 1 3 octave analysis for different values of are shown in the Table 6 2 The values in bold type are exact and preferred frequencies for 1 1 octave analysis and the corresponding values 6 26 ni com Chapter 6 Advanced Concepts Table 6 2 Exact and Preferred Octave Analysis Frequencies
25. applying the Fast Fourier Transform FFT to acquired time domain data The FFT analyzer mode can also be used to provide non transformed time domain data to the host computer In FFT analyzer mode NI DSA and your NI 45XX can perform up to two baseband FFTs and two zoom FFTs in real time or four baseband FFTs Features of FFT analyzer mode include e Baseband FFTs of 50 to 800 lines e Zoom FFTs of 100 to 1 600 lines Classical range up to 80 kHz e Extended range up to 95 kHz e Overlap processing e Phase suppression e Averaging THD THD Noise SINAD The DSA FFT analyzer engine runs on the onboard processor and can analyze data from up to four input channels in real time using two dual channel analyzers one contains two baseband FFT engines the other contains two zoom FFT engines The dual channel baseband analyzer allows you to select an independent frequency span from DC to 95 kHz and up to 800 lines of frequency resolution for each of two channels The dual channel zoom analyzer lets you use any starting and ending frequencies for the zoom span The two channels can have different zoom spans with up to 1 600 lines of frequency resolution depending on the National Instruments Corporation 6 1 NI DSA Software User Manual Chapter 6 Advanced Concepts baseband span The relationship between the zoom span and baseband span is explained in the Baseband and Zoom Frequency Spans section Any of the four input channels
26. channel 0 channel 1 or both The spectrum is complex meaning it contains real and imaginary components with an amplitude of the measured signal and phase relative to source The phase measurement of a single channel measuring the system output but not the input for example is not generally meaningful Cross Spectrum The cross spectrum of channel 0 and channel 1 is given by this formula Cross spectrum S x S The cross spectrum is complex Frequency Response Frequency response is a comparison of the gain and phase of the signal input to a device under test to the gain and phase of the device output Specifically channel 0 source input over channel response input It is calculated as follows Y f output HQ X f input The result is complex THD Total Harmonic Distortion The calculation of THD is based on the measurement of the 2nd to the 64th harmonics as shown in this formula THD 100 x Aj is the RMS value of the nth order harmonic A is the RMS value of the fundamental frequency The number of harmonics involved in the THD calculation is bounded by the Nyquist frequency If the frequency of the mth 6 20 ni com Chapter 6 Advanced Concepts order harmonic exceeds the Nyquist frequency half of the sampling frequency is set to zero NI DSA computes the THD on channel 1 which is connected to the output of the system under test With Read Measurement parameter dB units set t
27. computer for processing 2 collecting and measuring the same kinds of electrical signals with A D and or DIO boards plugged into a computer and possibly generating control signals with D A and or DIO boards in the same computer decibel the unit for expressing a logarithmic measure of the ratio of two signal levels dB 20log 1 9 V V gt for signals in volts direct current NI DSA Software User Manual Glossary DC coupled DDS default setting delta sigma modulating ADC device DGND DIFF differential input differential measurement system digital port digital trigger DIO DMA NI DSA Software User Manual allowing the transmission of both AC and DC signals direct digital synthesis a default parameter value recorded in the driver In many cases the default input of a control is a certain value often 0 that means use the current default setting For example the default input for a parameter may be do not change current setting and the default setting may be no AMUX 64T boards If you do change the value of such a parameter the new value becomes the new setting You can set default settings for some parameters in the configuration utility or manually using switches located on the device a high accuracy circuit that samples at a higher rate and lower resolution than is needed and by means of feedback loops pushes the quantization noise above the frequency range of interest This out of band noise is ty
28. eerie 4 13 C Examples dete UE OU De radit 4 14 step 3 Generate Signal oe e e Ee er e y o to DIRE 4 14 LabVIEW Example tele eme gd eR 4 14 NI DSA Software User Manual Viii ni com Contents Chapter 5 Octave Analysis Add On Mode Programming Step 2 Configure the Octave Analyzer esee 5 3 Configuring Your Octave and Level Measurement sees 5 3 LabVIEW Examples tee ete ere det een 5 4 C Example naad eia hee eth tt er e m ORE Te ett 5 5 Step 3 Read Octave Measurement sssesseeeseeeeeeee eene enne 5 5 LabVIEW Examples etr t o ED I e eer 5 7 eene edema eee aequis 5 7 ERES 5 9 Chapter 6 Advanced Concepts FET Analyz tiz ute eo e REI EF TUE 6 1 FET Measurements Ete eH EO LEO 6 2 Dual Channel FFT Analysis esee 6 5 Baseband and Zoom Frequency Spans eee 6 7 Alias Free Bandwidth esee 6 7 Classical and Extended FFT Size sse 6 8 Baseband 5parns tee ete e RR 6 8 Zim SPANSE ceto er ete dete ned aee e ER es 6 9 The Real Time Zoom FFT Process esee 6 9 Zoom Span Characteristics esee 6 11 Nndowing eee dob etd ade 6 12 BET Averagitg erem tet t meet 6 14 SWeptzS1Dne E DOOR ert enda aet 6 15 Why Swept Sine Measurement eene 6 15 Swept Sine Controls and Settings
29. energy contained in the signal and octave measurements return the amount of energy contained in different frequency bands National Instruments Corporation 6 23 NI DSA Software User Manual Chapter 6 Advanced Concepts Octave and Level Analyzers Software Architecture Figure 6 7 shows the overall architecture for making octave and level measurements Input Time Domain Signal x n 1 1 e ha C S Level Analysis N gt Averaging r0 ia Level Analysis1 Averaging Ww Weighting Ww Ly e Octave Analysis J3 9 Averaging poe U NI DSA Software User Manual Figure 6 7 Octave and Level Measurement Functional Diagram You can perform octave and level measurements on one two or four channels simultaneously You can configure and perform up to eight simultaneous level measurements but only one set of octave measurements The time domain input signal is passed through a weighting process if desired then it is passed to the level analyzer and fractional octave analyzer and finally goes through an averaging process The input to either the octave or level analyzers can be either the weighted W or unweighted U time domain signal For any single channel it is possible to simultaneously obtain either a weighted or unweighted octave measurement but not both plus weighted unweighted or both weighted
30. following changes to your configuration e Use Set OLM Sampling Rate to set the sampling rate to 25 6 kHz e Use Configure OLM Engine to turn off level measurements Using the fast averaging mode these changes enable you to set the highest center frequency at 8 kHz for 1 1 octave analysis 10 kHz for 1 3 octave analysis and 2 997 Hz ANSI or 3 084 Hz IEC for 1 12 octave analysis Table 6 6 shows different configurations for which real time processing on the NI 4552 is possible and illustrates trade offs that you can make The table is not exhaustive but a guide other configurations are possible For this table the following assumptions are made the sampling rate is set at 25 6 kHz all four input channels are used and no level measurements are made Table 6 6 Real Time Octave Analysis Configuration Guide for NI 4552 Fractional Octave Highest Fractional Averaging Fractional Octave Octave Mode Weighting Center Frequency 1A Impulse A 8 kHz 1 3 Impulse A 10 kHz 1 12 Impulse A 2 520 Hz ANSI or 2 594 Hz IEC 1 12 Impulse None 2 828 Hz ANSI or 2 911 Hz IEC 1 12 Fast None 2 997 Hz ANSI or 3 084 Hz IEC 6 40 ni com Chapter 6 Advanced Concepts With computationally intensive impulse averaging and A weighting both 1 1 octave and 1 3 octave analysis are possible on all four channels simultaneously to a maximum center frequency of 8 kHz and 10 kHz respectively For 1 12 octave analy
31. hold time domain data as the DSA device acquires it If you are acquiring data from a known length of time that does not exceed a few seconds you can use the one shot capture option If you need to acquire more than a few hundred thousand data points or do not know the acquisition time in advance perform a continuous capture The continuous mode allocates a circular buffer After you have configured all the analysis parameters for the DSA device call Control Capture setting Capture Mode Start At this point the RAM buffer you set aside begins filling with data To pull data out of this buffer into you application call Read Capture Single Chan or Read Capture Multi Chan If you are performing a continuous capture operation it is important to read this data from the buffer periodically to prevent the buffer from overflowing If you fail to read the data often enough and the number of points in buffer exceeds its capacity NI DSA returns an error NI DSA ships with a LabVIEW example program that illustrates how to use capture mode to stream gap free time domain data to a file for offline processing 3 14 ni com Chapter 3 FFT Mode Programming Using Engineering Units Measurements made with NI DSA are expressed in volts V by default In many industries or applications it is more appropriate to express measurements in other units Sound pressure for example is usually measured in pascals Pa The NI DSA engineering units functi
32. improves accuracy in the resulting digitized signal and reduces noise a measure of how close to constant the gain of a circuit remains over a range of frequencies a lowpass filter preceding an ADC usually a brickwall filter that rejects signal energy above the Nyquist frequency 1 2 the sample rate of the ADC so that the ADC does not mistake out of band signals for in band signals a lowpass filter after a DAC usually a brickwall filter that rejects signal energy above the Nyquist frequency 1 2 the sample rate of the DAC in order to suppress out of band images of the in band signal created by the D A conversion process analog output ground signal Application Specific Integrated Circuit a proprietary semiconductor component designed and manufactured to perform a set of specific functions for a specific customer 1 hardware a property of an event that occurs at an arbitrary time without synchronization to a reference clock 2 software a property of a function that begins an operation and returns prior to the completion or termination of the operation G 2 ni com attenuate attenuation ratio b B bandwidth base address binary bipolar BNC brickwall filter buffer burst mode bus bus master National Instruments Corporation G 3 Glossary to decrease the amplitude of a signal the factor by which a signal s amplitude is decreased bit one binary digit either 0 or 1 byte
33. in the LabVIEW context help Tolearn about the electrical and mechanical aspects and features of your National Instruments DSA hardware refer to the NI 4551 4552 User Manual e Forthe latest versions of drivers manuals and example programs visit ni com instruments for free downloads NI DSA Measurement Modes NI DSA has three measurement modes including e FFT analyzer mode e Swept sine analyzer mode e Octave analyzer mode Real Time Octave Analyzer add on In addition to the measurement modes NI DSA has a source mode for use with only the NI 4551 The mode you use depends on the measurements you want to take The following sections describe the modes and their specific features Note The Real Time Octave Analyzer RTOA software is available as an add on for NI DSA If you have not obtained and installed the RTOA you cannot perform the octave and level measurements described in this manual FFT Analyzer Mode NI DSA Software User Manual Use FFT analyzer mode for the following operations e Real time FFT analyze two baseband spans and two zoom spans or four baseband spans e Auto power spectrum e Frequency response e Cross power spectrum 1 2 ni com Chapter 1 Introduction to NI DSA e Coherence e Power spectral density e Bandpower e Harmonic analysis THD THD Noise and SINAD e Time domain data acquisition Swept Sine Analyzer Mode NI 4551 Only Use swept sine analyzer mod
34. its extended representation in order to select the correct extended baseband span It is important to notice that when you select any zoom span NI DSA automatically coerces it to the closest possible zoom span available with the current baseband span The zoom span limits have to be within the baseband span and the maximum extended baseband span is 95 KHz You can select a zoom FFT of 100 200 400 800 or 1 600 lines National Instruments Corporation 6 11 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual When the selected zoom span is larger than the classical baseband span selected including the maximum classical baseband span of 80 kHz the driver will make the zoom analyzer behave as a baseband analyzer and eliminate the modulation and decimation stages By using the zoom analyzers as baseband analyzers the two dual channel analyzers architecture lets you perform an FFT analysis in the same frequency span on four channels at the same time Windowing In practical applications you can obtain only a finite number of samples of a signal The FFT assumes that this time record repeats If you have an integral whole number number of cycles in your time record the repetition is smooth at the boundaries However in practical applications you usually acquire a nonintegral number of cycles In such cases the presumed periodicity of the sampled signal results in discontinuities at the b
35. line as the current source Italic text denotes variables emphasis a cross reference or an introduction to a key concept This font also denotes text that is a placeholder for a word or value that you must supply Text in this font denotes text or characters that you should enter from the keyboard This font is also used for the proper names of disk drives paths directories programs device names functions variables filenames and extensions This manual also uses this font as a naming convention to jointly refer to LabVIEW VIs and C language function calls For example Read Measurement when shown in this font refers both to the LabVIEW NI DSA Read Measurements VI and to the C function NIDSA read measurement Contents Chapter 1 Introduction to NI DSA Getting Started with NI DSA nete tnter tentent 1 1 Related Documentation sssri e ana enne remet 1 2 NI DSA Measurement Modes prcnis nennen eterne 1 2 EFT Analyzer eret t e ene terre e RR ERE ees 1 2 Swept Sine Analyzer Mode NI 4551 Only eee 1 3 Octave Analyzer Mode Optional eese 1 3 Source Mode NI 4551 Only essent enne 1 4 Chapter 2 Programming with NI DSA Basic Concepts doce eegecetence eie eio eie entes 2 1 Notes C Programmers 2 nete dee entree 2 2 Step 1 Initialize e eR tA ER eruere 2 2 LabVIEW Example 2 tte HE EROS 2 2
36. measurements you specified with Configure Base FFT Engine Configure Zoom FFT Engine and Configure Base FFT Settings Configure Zoom FFT settings Each FFT analyzer measurement type and the valid type parameters for reading that measurement type is shown in the following list National Instruments Corporation Time measurement Time 0 reads time waveform on analyzer 0 Time 1 reads time waveform on analyzer 1 Windowed time 0 reads windowed time waveform on analyzer 0 Windowed time 1 reads windowed time waveform on analyzer 1 FFT measurement Base FFT 0 reads baseband FFT on analyzer 0 Base FFT 1 reads baseband FFT on analyzer 1 Auto power measurement Base auto power 0 reads baseband auto power from analyzer 0 Base auto power 1 reads baseband auto power from analyzer 1 3 9 NI DSA Software User Manual Chapter 3 FFT Mode Programming Cross power measurement Base cross power Base freq response Base coherence Base coherent power ZoomFFTO Zoom FFT 1 Zoom auto power 0 Zoom auto power 1 Zoom cross power Zoom freq response Zoom coherence Zoom coherent power Additional read measurement parameters that you must set are NI DSA Software User Manual startIndex starts returning data from this data point usually 0 to return the entire measurement Length number of data points to return View which part of the selected me
37. of the NI 4552 or the two input channels of the NI 4551 can be routed to any analyzer channel or to multiple analyzer channels For example you can have one input channel routed to different analyzer channels with different settings in order to view one part of the frequency spectrum at a higher resolution than the other parts The functional block diagram for a dual channel analyzer either baseband or zoom is shown below Any input channel can be routed to either channel A channel B or to both channels Average Average ch A gt Time Window Auto FFT Spectrum ch B bh Time Frequency Response Average Function Cross Spectrum Coherence Average Average Window Auto Spectrum FFT FFT Measurements NI DSA Software User Manual Figure 6 1 Dual Channel Analyzer Functional Block Diagram An FFT analyzer extracts the frequency spectrum of a time domain signal Fourier s theorem states that any waveform in the time domain can be represented by a weighted sum of sines and cosines The same waveform can then be represented in the frequency domain as a pair of amplitude and phase values at each component frequency The FFT transforms digital samples from the time domain into the frequency domain Each frequency component is the dot product also 6 2 ni com Chapter 6 Advanced Concepts kn
38. power of 2 This requirement is because the input and output converters are driven by the same DDS circuitry on the device The hardware synchronization between input and output is convenient for performing stimulus response tests If you make subsequent calls to Get National Instruments Corporation 5 NI DSA Software User Manual Appendix A NI DSA Software User Manual Common Questions HW Sampling Rate and Get HW Update Rate you will see that the ratio between these two parameters is a power of 2 What is the maximum sine output frequency I can obtain using NI DSA The maximum output frequency you can generate with NI DSA is 23 kHz This limit is governed by the maximum output sampling rate of 51 2 kS s Because the digital to analog converters on the NI 455X DSA devices feature sharp roll off anti imaging filters you can generate a spectrally pure sine tone up to 23 kHz with minimal quantization noise so when using the NI 455X devices you do not need to worry about the output signal becoming choppy or noisy when approaching the Nyquist frequency Why does NI DSA sometimes produce a slightly different output frequency than the one I request The sine signal generated by NI DSA always consists of 4 096 data points This buffer must contain an integer number of cycles of the sine wave to avoid adding unwanted noise to the signal Also the physical sample rate on the device is tied to the analog input rate Under some conditi
39. settings DSASession 0 FFTSize FFTWindow 100 0 0 Configure averaging parameters NIDSA configure base fft averaging gDSASession 0 AvgMode AvgWeighting NumberOfAvg VI FALSE Zoom FFT Example Configuring Zoom FFT analyzer NIDSA configure zoom fft engine gDSASession CHANNEL 0 0 0 1 NIDSA configure zoom fft span gDSASession CHANNEL 1 zoomSpan NIDSA configure zoom frequencies gDSASession CHANNEL 1 centerFrequency NIDSA configure zoom fft settings gDSASession CHANNEL number of frequency bins lines window timeIncr phaseSuppress NIDSA configure zoom fft averaging gDSASession CHANNEL AvgMode weightingMode numberOfAvg overloadReject Notes on Examples Routing You can route signals to two base analyzers and two zoom analyzers with Route Base FFT and Route Zoom FFT Each analyzer operates independently of the others and any input can be routed to any analyzer or combination of analyzers For example you can route one input to all four analyzers and perform four different analyses on the same input Note If you configure your instrument to perform a 4 channel FFT Route Zoom generates an error 4 channel FFTs can be performed in baseband only NI DSA Software User Manual 3 6 ni com Chapter 3 FFT Mode Programming Configuring the FFT Engine Each measurement type parameter of Configure Base FFT Engine automatically sets the preceding parameter The time parameter sets the
40. several milliseconds in some cases During this time data accumulates in the FIFO for future retrieval With a larger FIFO longer latencies can be tolerated In the case of analog output a FIFO permits faster update rates because the waveform data can be stored on the FIFO ahead of time This again reduces the effect of latencies associated with getting the data from system memory to the DAQ device a type of signal conditioning that allows you to attenuate unwanted portions of the signal you are trying to measure finite impulse response a non recursive digital filter with linear phase an ADC whose output code is determined in a single step by a bank of comparators and encoding logic signal sources with voltage signals that are not connected to an absolute reference or system ground Also called nonreferenced signal sources Some common example of floating signal sources are batteries transformers or thermocouples sample rate feet the factor by which a signal is amplified sometimes expressed in decibels a measure of deviation of the gain of an amplifier from the ideal gain G 8 ni com GND grounded measurement system H h half power bandwidth handshaked digital I O hardware hardware triggering hysteresis Hz IC IMD National Instruments Corporation G 9 Glossary ground See SE hour the frequency range over which a circuit maintains a level of at least 3 dB with respect to th
41. 0 National Instruments Corporation 5 5 NI DSA Software User Manual Chapter 5 Octave Analysis Add On Mode Programming Level measurements 1 Level measurements 2 Level measurements 3 When new measurement is ready it needs to be read Read octave measurements Returns Band power measurements needed to build the octave graph Total band power measurements Get octave frequencies Exact center frequencies Nominal center frequencies Refer to the Exact and Preferred Frequencies section of Chapter 6 Advanced Concepts for more information about these frequencies The fractional octave graph is an XY graph The X axis is nominal or exact center frequencies selectable The Y axis is the band power measurements NI DSA Software User Manual 5 6 ni com Chapter 5 Octave Analysis Add On Mode Programming LabVIEW Examples input select NIDSA Check New i DSA Read Octave NIDSA Get Octave Measurement vi E Measurements vi Frequencies vi Figure 5 4 Reading the Octave Measurement C Examples int CVICALLBACK TimerCB int panel int control int event void callbackData int eventDatal int eventData2 ViStatus NewMeasurement NewMeasurement 1 when a new measurement is ready ViInt32 NumberOfBands ViInt32 FilterSettled ViReal32 TotalBandPwr National Instruments Corporation 5 7 NI DSA Software User Manual Chapter 5 Octave Analysis
42. 2 1 3 octave 20 kHz 4 1 3 octave 10 kHz Configuration Limits NI 4551 Table 6 5 shows different configurations for which real time processing on the NI 4551 is possible and illustrates trade offs that you can make The table is not exhaustive but a guide other configurations are possible For this table the following assumptions are made the sampling rate is the default rate 51 2 kHz and both input channels are processed ni com Chapter 6 Advanced Concepts Table 6 5 Real Time Octave Analysis Configuration Guide for NI 4551 Fractional Level Highest Octave Number Measurements Fractional Fractional Averaging of Level Averaging Weighting Octave Center Octave Mode Measurements Mode Mode Frequency 1 1 Impulse 8 Impulse Eq A 16 kHz 1 3 Impulse 8 Leq A 12 5 kHz 1 3 Impulse 8 Leq A 16 kHz 1 3 Impulse 2 Leq A 20 kHz 1 3 Fast 8 Leq A 20 kHz 1 12 Impulse 0 None 5 339 Hz ANSI 5 496 Hz IEC 1 12 Fast 0 A 5 339 Hz ANSD 5 496 Hz IEC 1 12 Fast 8 Leq A 4 490 Hz ANSI 4 621 Hz IEC 5 National Instruments Corporation As Table 6 5 shows the NI 4551 can perform a 1 1 octave analysis in real time while applying the most computationally intensive averaging and weighting impulse and A respectively In addition it can simultaneously perform up to eight level measurements with impulse eq averaging For 1 3 octave analysis with impulse averaging you can gain a higher
43. 26 278 27 048 62 27 841 28 656 61 29 496 30 360 60 31 250 32 166 59 33 108 34 078 58 35 077 36 105 57 37 163 38 252 56 39 373 40 526 55 41 714 42 936 54 44 194 45 489 53 46 822 48 194 52 49 606 51 060 51 52 556 54 096 50 55 681 57 313 49 58 992 60 721 48 62 500 64 331 47 66 216 68 157 46 70 154 72 210 45 74 325 76 503 44 78 745 81 052 43 83 427 85 872 4 12 ni com Appendix A Common Questions Table A 1 Exact Center Frequencies for 1 12 Octave Analysis Continued National Instruments Corporation dus ANSI Standard Exact IEC Standard Exact octave Center Frequencies Hz Center Frequencies Hz 42 88 388 90 978 41 93 644 96 388 40 99 213 102 120 39 105 112 108 192 38 111 362 114 626 37 117 984 121 441 36 125 000 128 663 35 132 433 136 313 34 140 308 144 419 33 148 651 153 007 32 157 490 162 105 31 166 855 171 744 30 176 78 181 96 29 187 29 192 78 28 198 43 204 24 27 210 22 216 38 26 222 72 229 25 25 235 97 242 88 24 250 00 257 33 23 264 87 272 63 22 280 62 288 84 21 297 30 306 01 20 314 98 324 21 19 333 71 343 49 4 13 NI DSA Software User Manual Appendix A Common Questions Table A 1 Exact Center Frequencies for 1 12 Octave Analysis Continued NI DSA Software Us
44. 5096 as much as the most recent data in calculating the sound level The time constant for slow exponential mode is 1 s Impulse mode features a 35 ms time constant for rising signals and 1500 ms for falling signals Custom exponential mode allows you to define your own time constant The equivalent continuous level mode employs linear averaging over a finite block of time All time domain data in this block is weighted equally in computing the level If you choose to employ the equivalent continuous level mode make a call to Configure OLM Linear Averaging to specify the duration of the averaging block Impulse equivalent averaging mode actually combines two types of averaging The first step is to find a level value using the impulse exponential This value is then linearly averaged with past impulse exponential results As with equivalent continuous mode you specify the length of the linear average time block in Configure OLM Linear Averaging Peak averaging mode is not truly an averaging operation Instead it computes the level by recording the largest instantaneous level measurement since the beginning of the acquisition National Instruments Corporation 6 31 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual When acquiring level values you should first call Get Level Length to get the number of level measurements currently available Next make a call to Read Level Measurements to produce an a
45. A Software User Manual Celsius calibration DAC pin or wire lead to which you apply or from which you read the analog or digital signal Analog signals can be single ended or differential For digital signals you group channels to form ports Ports usually consist of either four or eight digital channels a condition for starting or stopping clocks clipping occurs when an input signal exceeds the input range of the amplifier hardware component that controls timing for reading from or writing to groups complementary metal oxide semiconductor common mode rejection ratio a measure of an instrument s ability to reject interference from a common mode signal usually expressed in decibels the smallest detectable change in an input voltage of a DAQ device the input range over which a circuit can handle a common mode signal the mathematical average voltage relative to the computer s ground of the signals from a differential input any voltage present at the instrumentation amplifier inputs with respect to amplifier ground the range of a parameter for which compensating adjustment can be made a method of triggering in which you simulate an analog trigger using software Also called software triggering device that transforms a signal from one form to another For example analog to digital converters ADCs for analog input digital to analog converters DACs for analog output digital input or output ports and counter tim
46. Add On Mode Programming ViReal64 linAvgTime ViReal64 ElapsedAvgTime ViString UnitLabel ViReal32 NominalCenterFreq 120 ViReal32 ExactCenterFreq 120 ViReal32 BandPowerMeas 120 switch event case EVENT TIMER TICK Check if a new measurement is ready NIDSA check new measurement gDSASession amp NewMeasurement if NewMeasurement return 0 Read the new measurement NIDSA get octave number of bands gDSASession amp NumberOfBands NIDSA get octave frequencies gDSASession ExactCenterFreq NominalCenterFreq NIDSA read octave measurements gDSASession 0 DBON amp FilterSettled amp ElapsedAvgTime amp linAvgTime amp TotalBandPwr BandPowerMeas UnitLabel Displays Octave graph DeleteGraphPlot mainpanel MAINPANEL GRAPH 1 VAL IMMEDIATE DRAW PlotXY mainpanel MAINPANEL GRAPH NominalCenterFreq BandPowerMeas NumberOfBands VAL FLOAT VAL FLOAT VAL VERTICAL BAR VAL EMPTY SQUARE VAL SOLID 1 VAL RED NI DSA Software User Manual 5 8 ni com Chapter 5 Octave Analysis Add On Mode Programming return VI SUCCESS Step 4 Control UseGet OLM Status National Instruments Corporation 5 9 NI DSA Software User Manual Advanced Concepts FFT Analyzer This chapter contains information useful for understanding the different types of dynamic signal analysis and the algorithms used The FFT analyzer mode performs frequency domain measurements by
47. Computer Based Instruments NI DSA Software User Manual Version 1 2 b f NATIONAL _ September 2001 Edition P INSTRUMENTS Part Number 370380A 01 Worldwide Technical Support and Product Information ni com National Instruments Corporate Headquarters 11500 North Mopac Expressway Austin Texas 78759 3504 USA Tel 512 683 0100 Worldwide Offices Australia 03 9879 5166 Austria 0662 45 79 90 0 Belgium 02 757 00 20 Brazil 011 284 5011 Canada Calgary 403 274 9391 Canada Montreal 514 288 5722 Canada Ottawa 613 233 5949 Canada Qu bec 514 694 8521 Canada Toronto 905 785 0085 China Shanghai 021 6555 7838 China ShenZhen 0755 3904939 Czech Republic 02 2423 5774 Denmark 45 76 26 00 Finland 09 725 725 11 France 01 48 14 24 24 Germany 089 741 31 30 Greece 30 1 42 96 427 Hong Kong 2645 3186 India 91805275406 Israel 03 6120092 Italy 02 413091 Japan 03 5472 2970 Korea 02 596 7456 Malaysia 603 9596711 Mexico 001 800 010 0793 Netherlands 0348 433466 New Zealand 09 914 0488 Norway 32 27 73 00 Poland 0 22 528 94 06 Portugal 351 1 726 9011 Russia 095 2387139 Singapore 2265886 Slovenia 386 3 425 4200 South Africa 11 805 8197 Spain 91 640 0085 Sweden 08 587 895 00 Switzerland 056 200 51 51 Taiwan 02 2528 7227 United Kingdom 01635 523545 For further support information see the Technical Support Resources appendix To comment on the documentation send e mail to techpubs ni com Copyright 2001 National
48. DAO 1 VI TRUE VI FALSE amp vi Notes on Examples The resource name is the device number assigned by Measurement amp Automation Explorer MAX If you have only one DSA instrument installed in your system the device number is Device 1 by default NI DSA Software User Manual 2 2 ni com Chapter 2 Programming with NI DSA If you have multiple DSA instruments installed launch MAX and open Devices and Interfaces to find your device number DSASession is a handle returned by Initialize Step 2 Configure Use Configure DSA Mode to download the code for FFT swept sine or octave analyzer mode to the digital signal processor DSP Then configure the analog front end with the following functions Set Input Voltage Range from 10 mV to 42 V in 10 dB increments To let the instrument automatically select the best range use Set Input Auto Range Set Input Coupling select DC or AC coupling for inputs UN Caution If you are using ICP type sensors select AC coupling to protect your instrument from potential damage ICP type sensors can cause large DC offset voltages to occur on the signal inputs e Configure Trigger sets the parameters for analog triggering Refer to the NI 4551 4552 User Manual Chapter 3 Hardware Overview for more information about configuring your analog input channels 5 Note Unless otherwise noted you can use the functions in this section and most NI DSA functions to apply parameters
49. FT Settings sets the size of the time record apply a window set time increment and phase suppression Configure Zoom FFT Averaging sets the averaging mode weighting number and overload rejection 3 4 ni com Chapter 3 FFT Mode Programming LabVIEW Examples NIDSA NIDSA Set NIDSA NIDSA Configure Classical Configure Configure NIDSA Route FFT Baseband Base FFT Base FFT Base FFT vi Engine vi Span vi Settings vi Averaging vi IMIDSR a ponononon A SE SEP Bocconi maz 20000 00 1024 Figure 3 3 Configuring Baseband FFT NIDSA Configure NIDSA Zoom Configure VN om MOSM ss NIDSA Route ZoomFFT Spani requencies vi ery Configure ZoomFFT vi Engine vi 1000 00 12000 00 Settings vi Zoom FFT Averaging vi IDSR E E9200 em 2008 e Figure 3 4 Configuring Zoom FFT C Examples Examples for baseband FFT and zoom FFT are included Baseband FFT Example Configure Baseband FFT Analyzer Route channel i to analyzer 0 NIDSA route base fft DSASession 0 InputChannel National Instruments Corporation 3 5 NI DSA Software User Manual Chapter 3 FFT Mode Programming NIDSA configure base fft engine gDSASession 0 VI FALSE VI TRUE VI FALSE VI FALSE Set the frequency span for Baseband FFT analyzer NIDSA set classical baseband span DSASession 0 BaseBandSpan Set FFT resolution and windowing NIDSA configure base fft
50. Hz 1 024 points of decimated data corresponds to a time duration of twice that of the highest span so the frequency resolution is improved to 100 Hz This process of doubling the time record and halving the span and resolution can be repeated for each additional decimate by 2 filter stage Zoom Span Characteristics Zoom spans must be a power of 2 of a classical baseband span If you want a 12 800 Hz zoom span you can use a 12 800 Hz baseband span in classical mode 100 200 400 800 lines or 12 800 Hz divided by any power of 2 as long as the resulting span lies within the baseband span Use the following formula to calculate the classical baseband span required for the zoom span you need required classical baseband span zoom span x 2 A 1 600 Hz zoom span is allowed with a 12 800 Hz classical span because the following is true 12 800 1 600 x 2 A 1 400 Hz zoom span on the other hand is not compatible with a 12 800 Hz baseband span since no power of 2 of 1 400 is equal to 12 800 If you are using an extended baseband span an additional calculation is necessary to determine the correct baseband span for a particular zoom span The formula for this case is required extended baseband span zoom span extended lines classical lines x 2 If you need a zoom span of 12 800 you need an extended baseband span of 15 200 Hz 32 400 Hz or 64 800 Hz for example The 475 400 factor converts the classical zoom span to
51. ITHOUT LIMITATION THE APPROPRIATE DESIGN PROCESS AND SAFETY LEVEL OF SUCH SYSTEM OR APPLICATION Conventions lt gt gt Gx bold ICP italic monospace The following conventions are used in this manual Angle brackets that contain numbers separated by an ellipsis represent a range of values associated with a bit or signal name for example DBIO lt 3 0 gt Square brackets enclose optional items for example response The symbol leads you through nested menu items and dialog box options to a final action The sequence File Page Setup Options directs you to pull down the File menu select the Page Setup item and select Options from the last dialog box The symbol indicates that the following text applies only to a specific product a specific operating system or a specific software version This icon denotes a tip which alerts you to advisory information This icon denotes a note which alerts you to important information This icon denotes a caution which advises you of precautions to take to avoid injury data loss or a system crash Bold text denotes items that you must select or click on in the software such as menu items and dialog box options Bold text also denotes parameter names ICP is an abbreviation for Integrated Circuit Piezoelectric ICP type products operate using a constant current source and return the output signal in the form of voltage modulation on the same
52. Instruments Corporation 6 7 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual Classical and Extended FFT Size Although the superior antialiasing performance of the NI 45XX allows a larger alias free bandwidth and thus more spectral lines sometimes a user wants to make measurements at bandwidths that have traditionally been used in spectral analysis Table 6 1 gives the number of frequency lines for standard FFT sizes using the extended and classical FFT spans Table 6 1 Number of Frequency Lines for Classical and Extended Spans FFT Block Size Extended FFT Span Classical FFT Span 128 samples 59 lines 50 lines 256 samples 118 lines 100 lines 512 samples 237 lines 200 lines 1 024 samples 1 kS 475 lines 400 lines 2 048 samples 2 kS 950 lines 800 lines Baseband Spans A baseband is a frequency span for a measurement that starts at DC and extends to the maximum alias free span of the device which depends on the sampling rate For an FFT block size of 1 024 and a sampling rate of 204 8 kS s the final spectrum contains 476 frequency bins with the first one representing 200Hz 2 to 200Hz 2 the second one 200Hz 2 to 3x200HZ 2 and so on until the 476 which will be centered around 95 kHz Due to the delta sigma converter s inherent oversampling filtering and antialiasing protection the minimum sampling rate allowed is 5 kS s which corresponds to a minimu
53. Like linear averaging the averaging process stops at the end of the specified time duration While impulse linear averaging is in progress the time for which the averaging process has elapsed is returned in the linear averaging seconds output of Read Octave Measurements and Read Level Measurements There is a one to one correspondence between the time returned and the measured values read from these functions In auto restart mode the time returned increments in intervals of the duration specified in Configure OLM Averaging Peak Averaging In peak or peak hold averaging the largest measured energy value is held The formula for peak averaging is as follows max y k 1 x k Where x k is the new measurement y k is the new average and y k 1 is the previous average As with exponential averaging the averaging process continues indefinitely Strictly speaking peak hold is not really a form of averaging because successive measurements are not mathematically averaged However as with other averaging processes it combines the results of several measurements into one final measurement and could thus be considered a kind of averaging Equal Confidence Averaging Equal confidence averaging is available only for octave measurements It is similar to exponential averaging the difference being that the time constant is dependent on the bandwidth of the filter The time constant for each band is such that the results ar
54. Measurement Close Figure 3 2 Zoom FFT Programming Flowchart National Instruments Corporation 3 3 NI DSA Software User Manual Chapter 3 FFT Mode Programming Step 2 Configure the FFT Analyzer The first step in configuring your FFT measurement is to route the digitized input signal to one or more analyzers using Route Base and Route Zoom To configure a baseband FFT use the following Configure Base FFT Engine sets the type of measurements the analyzer makes using the following Time acquires a time waveform FFT acquires a time waveform then performs FFT Auto acquires time waveform performs FFT and auto power measurement Cross acquires time waveform performs FFT auto power and cross power measurements Set Classical Baseband Span sets the classical baseband span Configure Base FFT Settings set the size of the time record apply a window sets time increment and phase suppression Configure Base FFT Averaging sets the averaging mode weighting number and overload rejection To set up a zoom FFT use the following NI DSA Software User Manual Configure Zoom FFT Engine sets the type of measurements the analyzer makes using the same parameters as Configure Base FFT Engine Configure Zoom FFT Span sets the zoom span and the lock to frequency Configure Zoom Frequencies set the start center and end frequencies Configure Zoom F
55. a combination of hardware and or software elements typically used with a PC that has the functionality of a classic stand alone instrument 2 a LabVIEW software module VI which consists of a front panel user interface and a block diagram program NI DSA Software User Manual Glossary W waveform multiple voltage readings taken at a specific sampling rate word the standard number of bits that a processor or memory manipulates at one time Microprocessors typically use 8 16 or 32 bit words working voltage the highest voltage that should be applied to a product in normal use normally well under the breakdown voltage for safety margin Z zero overhead looping the ability of a high performance processor to repeat instructions without requiring time to branch to the beginning of the instructions zero wait state memory memory fast enough that the processor does not have to wait during any reads and writes to the memory NI DSA Software User Manual G 18 ni com
56. an is determined by the difference between the start and end frequencies The real time zoom approach is to digitally filter and down sample the acquired time record This technique along with heterodyning the signal to center the frequency spectrum on a frequency other than DC provides a zoomed spectrum Ideally you could center the narrow span around any frequency below the baseband span Using decimating filters alone all spans must begin at 0 Hz DC If you frequency shift or heterodyne the input signal before the decimation stages the resulting span is centered at the modulation frequency allowing you to center your span in any frequency as long as the narrow span stays within the baseband span Heterodyning is used by the analyzer to compute zoomed spans those starting at frequencies other than DC The signal processor can heterodyne and filter in real time to provide a time record at all spans and center frequencies All of the signal processing computations including modulation digital filtering and decimation and computing the FFT are done is less time than it takes to acquire the data so the NI 45XX can process every input sample in measuring the signal spectrum In the heterodyning process the input points are multiplied by a complex unit vector cos wt and sin wt to yield a real and imaginary time record National Instruments Corporation 6 9 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Soft
57. an a lower one In general impulse exponential averaging requires more processing than the other exponential averaging modes Linear averaging and peak hold averaging are the least computationally intensive of the averaging modes Number of level measurements The higher the number of level measurements per channel the more processing time required e Weighting A B or C weighting A weighting is most computationally intensive and C weighting the least e Simultaneous streaming to disk If your instrument is configured for simultaneous streaming to disk there is less processor time available for performing octave and level measurements Sometimes you must eliminate one or more of these factors to get real time performance For example you can usually get real time fractional octave analysis at higher center frequencies by choosing fewer channels and limiting or eliminating simultaneous level measurements or streaming I O National Instruments Corporation 6 37 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual The maximum center frequencies for octave analysis with no averaging or level measurements are shown in Table 6 4 Table 6 4 Maximum Center Frequencies for Octave Analysis Number of Octave Analysis Maximum Center Channels Type Frequency per Channel 1 1 1 16 kHz 2 1 1 octave 16 kHz 4 1 1 octave 8 kHz 1 1 3 octave 20 kHz
58. and unweighted level measurements This is because for each channel only one set of octave measurements is returned but it is possible to have up to eight simultaneous level measurements Architecture of the Fractional Octave Analyzer Figure 6 8 shows the architecture of the fractional octave analyzer The 1 3 octave analyzer is shown it has three bandpass filters per octave The full octave 1 1 octave analyzer has only one bandpass filter per octave Similarly the 1 12 octave analyzer has 12 bandpass filters per octave 6 24 ni com Chapter 6 Advanced Concepts Averaged Power Input Time Average Domain Signal L Averaged Power Stage 1 Sampled at fs BPF r 3 a Averaged Power Y gt Average LPF 2 1 Decimator Sampling gt _ gt Averaged Power I L J Average frequency fs 2 L Averaged Power Stage 2 Average Averaged Power BPF Average y LPF 2 1 Decimator Sampling frequency 5 2091 gt Averaged Power 4 gt Average Averaged Power Stage N J9 Average 1 Y Averaged Power gt Average Figure 6 8 Fractional Octave Analyzer Architecture The input time domain signal is sample
59. asurement to return from the following choices Real part Imaginary part Magnitude Magnitude squared Phase Unwrapped phase dbUnits dB off linear units dBon dBmon pkrmsUnits Peak RMS Peak to peak 3 10 ni com Chapter 3 FFT Mode Programming phaseUnits Degrees Radians x0 x axis value of the first data point returned in the y axis measurement array dx x axis increment for data points returned in the y axis measurement array Tip Keep in mind that the units for x0 and dx parameters are seconds for time domain measurements and hertz for FFTs NI4552 If you configure an NI 4552 for 4 channel time FFT or auto power measurements you may also use these type parameters National Instruments Corporation Time 2 Time 3 Windowed time 2 Windowed time 3 Base FFT 2 Base FFT 3 Base auto power 2 Base auto power 3 3 11 NI DSA Software User Manual Chapter 3 FFT Mode Programming LabVIEW Example NIDSA Get Measurement NIDSA Read EAT vi ro NIDSA Check New Measurement vi E Base Auto Power D v Figure 3 5 Read FFT Measurements C Example When reading FFT measurements ideally you need to use a timer in order to read measurements periodically This Function is called periodically timer tick int CVICALLBACK TimerCB int panel int control int event void callbackData int eventDatal int eventData2 V
60. aveform Discontinuity After Windowing Some of the windows available in NI DSA are e Uniform e Hanning e Blackman Harris standard 4 term and 7 term National Instruments Corporation 6 13 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual e Hlattop e Kaiser e sidelobe e Force e Exponential Forced exponential For more information about using windowing functions refer to NI Application Note 041 The Fundamentals of FFT Based Signal Analysis and Measurement FFT Averaging Averaging successive measurements tends to improve measurement accuracy Averaging is usually performed on measurement results or on individual spectra but not directly on the time record Linear and exponential averaging are available in RMS vector and peak hold modes Linear averaging combines N spectral records with equal weighting When the number of averages has been completed the analyzer stops averaging and presents the averaged results Exponential averaging provides a continuous weighted average and emphasizes new spectral data more than old Both methods of averaging are performed using this formula New Average New Spectra x 1 N Old Average x N 1 N where N is the number of averages for linear averaging or is computed from the time constant for exponential averaging RMS averaging computes the weighted mean of the sum of squared values RMS averaging reduces fluctuations
61. by comparing the response or output of the filter with its input This process of feeding into a DSA both the signal sent to the filter and the response obtained from it and comparing the amplitude levels of each frequency component is known as a frequency response analysis When you analyze two simultaneously sampled channels the properties of each channel are not particularly important but the relationship between them are For example the exact levels of a filter s input and output signals are not critical it is the ratio of output to input that determines the filter s response at a given frequency A dual channel analyzer computes an instantaneous windowed FFT for each input channel This instantaneous FFT can be averaged and processed to generate other measurements The average auto power spectrum of channel x is computed by multiplying each FFT component by its complex conjugate The average cross power spectrum S y is a dual channel measurement computed by multiplying each FFT component of channel x by the complex conjugate of the corresponding component from channel y National Instruments Corporation 6 5 NI DSA Software User Manual Chapter 6 Advanced Concepts SQUE TUS Dm The complex frequency response function H f is calculated from measured spectra using the following formula Here channel x represents a stimulus signal sent to the unit under test while channel y represents the response
62. ch data is processed as it is acquired instead of being accumulated and processed at a later time a measure in LSB of the linearity of an ADC It includes all non linearity and quantization errors It does not include offset and gain errors of the circuitry feeding the ADC the smallest signal increment that can be detected by a measurement system Resolution can be expressed in bits in proportions or in percent of full scale For example a system has 12 bit resolution one part in 4 096 resolution and 0 0244 of full scale a technique whereby a device is signaled not to use its local memory while the memory is in use from the bus an acknowledge by a destination that signifies that the cycle did not complete and should be repeated a flat cable in which the conductors are side by side the difference in time between the 10 and 90 points of a system s step response root mean square the square root of the average value of the square of the instantaneous signal amplitude a measure of signal amplitude seconds samples NI DSA Software User Manual Glossary Sensor settling time signal conditioning SNR software trigger software triggering source impedance SS S s synchronous system noise system RAM TC T H NI DSA Software User Manual a device that responds to a physical stimulus heat light sound pressure motion flow and so on and produces a corresponding electrical signal th
63. cs The valid type values for reading swept sine measurements are as listed Swept spectrum 0 reads complex measurement of stimulus source level fundamental only Swept spectrum 1 reads complex measurement of response level fundamental only Swept frequency axis reads the list of frequencies the analyzer has been sweeping Swept frequency frequency response transfer function Swept cross power spectrum treads cross power spectrum of stimulus and response Swept THD reads the total harmonic distortion measurement calculated using the second harmonic through the harmonic specified by Maximum THD Harmonic in Configure Swept Sine Harmonics Swept SINAD reads signal to noise distortion 1 D Noise Swept harmonic 1 reads the frequency response of the source frequency harmonic specified in the first element of the five element harmonic mapping array created by Configure Swept Sine Harmonics Swept harmonic 2 same as swept harmonic 1 using the 2 element in the harmonic mapping array Swept harmonic 3 same as swept harmonic 1 using the 3 element in the harmonic mapping array 4 6 ni com Chapter 4 Swept Sine Mode Programming e Swept harmonic 4 same as swept harmonic 1 using the 4 element in the harmonic mapping array e Swept harmonic 5 same as swept harmonic 1 using the 5 element in the harmonic mapping array e Swept RMS squared reads the total
64. d filter cuts off at 20 kHz reducing the sample rate by 2 again but not the number of points The new record then has twice the original duration and half of the original span resulting in a a 40kHz span centered around 40kHz The time record duration is twice the duration of the full span time record The sample rate is one fourth of the baseband rate Further iterations of this process can reduce the span and provide better resolution for analysis If you pass 2 048 samples for a 2 KS FFT at 204 8 kHz through this decimating filter the output will be 1 024 samples at 102 4 kHz Both records correspond to a 10 ms duration time record and so the frequency resolution or bin width is 1 10 ms 100 Hz Once again the length of the time record always determines the frequency resolution of the spectrum To make sure the decimating filter is fast and memory efficient the alias free range for narrow spans is reduced from 475 lines to the classical 400 lines per 1 024 point FFT to allow the use of a faster and less sharp filter thereby improving performance As a result of this reduction in range the maximum alias free frequency span is 80 kHz for zoom instead of the 95 kHz available for baseband The resulting span is 400 1 024 x f so that our span for one stage of decimation is 400 1 024 x f 2 40 kHz 6 10 ni com Chapter 6 Advanced Concepts Performing a 1 024 point FFT at the highest span yields a frequency resolution of 200
65. d at a sampling frequency f The Band Pass Filters BPFs in stage 1 corresponding to the highest octave filter the signal directly at the sampling frequency The signal is then passed through a digital low pass filter LPF to remove higher frequencies then to a 2 1 decimator which effectively reduces the sampling frequency to half f 2 That is for every two samples input one sample is output The lowpass filtered and decimated signal is then passed to the BPF s corresponding to the next highest octave in stage 2 The process is repeated for a maximum of 10 stages of BPF s or a maximum of nine stages of lowpass filtering and decimation National Instruments Corporation 6 25 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual The time domain samples at the output of each BPF are squared then averaged according to the averaging mode selected and the averaged output of each BPF is returned as the measurement Third octave analysis provides a maximum of 30 results 10 octaves x 3 BPFs octave 1 1 octave analysis provides a maximum of 10 results 10 octaves x 1 BPF octave and 1 12 octave analysis provides a maximum of 120 results 10 octaves x 12 BPFs octave If you analyze a frequency range of less than 10 octaves you get less than the maximum number of results If you analyze a frequency range of more than 10 octaves the analyzer only returns the number of results for 10 octaves Exact
66. d is reset automatically when the overload condition passes Check status returns non sticky overload status The overload information returned by Check Status is bit encoded as shown in Table 6 8 Table 6 8 Overload Detection Bit Encoding Channels with Value Returned by Bit Pattern Overload Check Status LSBs Condition 0 0000 None 1 0001 1 2 0010 2 3 0011 1 and 2 4 0100 3 6 42 ni com Chapter 6 Advanced Concepts Table 6 8 Overload Detection Bit Encoding Continued Channels with Value Returned by Bit Pattern Overload Check Status LSBs Condition 5 0101 and 3 6 0110 2 and 3 7 0111 1 2 and 3 8 1000 4 9 1001 1 and 4 A 1010 2 and 4 B 1011 1 2 and 4 C 1100 3and4 D 1101 1 3 and 4 E 1110 2 3 and 4 F 1111 1 2 3 and 4 National Instruments Corporation NI DSA Software User Manual Common Questions FFT Mode FFT Analysis Questions What is the maximum number of bins lines for a baseband FFT measurement using NI DSA The maximum number of lines is 800 for a classical span and 950 for an extended span What is the difference between a classical baseband span and an extended baseband span A classical span is one with a ratio of one frequency line for every 2 56 data samples In an ideal analyzer with perfect brick wall antialiasing the number of useful frequency lines would be exactly
67. d the answers you need contact your local office or National Instruments corporate Phone numbers for our worldwide offices are listed at the front of this manual NI DSA Software User Manual B 2 ni com Glossary Prefix Meaning Value p pico 10 22 n nano 10 9 u micro 10 6 m milli 10 3 k kilo 103 M mega 106 G giga 10 t tera 1022 Numbers Symbols negative of or minus Q ohm per percent positive of or plus 45V 5 VDC source signal A A amperes AC alternating current AC coupled allowing the transmission of AC signals while blocking DC signals National Instruments Corporation G 1 NI DSA Software User Manual Glossary ACH A D ADC ADC resolution AIGND alias amplification amplitude flatness antialiasing filter anti imaging filter AOGND ASIC asynchronous NI DSA Software User Manual analog input channel signal analog to digital analog to digital converter an electronic device often an integrated circuit that converts an analog voltage to a digital number the size of the discrete steps in the ADC s input to output transfer function therefore the smallest voltage difference an ADC can discriminate with a single measurement analog input ground signal a false lower frequency component that appears in sampled data acquired at too low a sampling rate a type of signal conditioning that
68. device are not affected by differences in ground potentials the voltage that an isolated circuit can normally withstand usually specified from input to input and or from any input to the amplifier output or to the computer bus G 10 ni com kbytes s kS Kword L LabVIEW latched digital I O library linearity linearization low frequency corner LSB National Instruments Corporation G 11 Glossary kilo the standard metric prefix for 1 000 or 10 used with units of measure such as volts hertz and meters kilo the prefix for 1 024 or 210 used with B in quantifying data or computer memory a unit for data transfer that means 1 024 bytes s 1 000 samples 1 024 words of memory laboratory virtual instrument engineering workbench a type of digital acquisition generation where a device or module accepts or transfers data after a digital pulse has been received Also called handshaked digital I O a file containing compiled object modules each comprised of one of more functions that can be linked to other object modules that make use of these functions NIDAQMSC LIB is a library that contains NI DAQ functions The NI DAQ function set is broken down into object modules so that only the object modules that are relevant to your application are linked in while those object modules that are not relevant are not linked the adherence of device response to the equation R KS where response S stimu
69. dwidth where the settling time is in seconds and the bandwidth is in hertz Hz National Instruments Corporation NI DSA Software User Manual Chapter 6 Advanced Concepts The bandwidth for each band can be calculated by the formula bandwidth exact center frequency x 21 27 10 where n 1 for full octave 3 for 1 3 octave and 12 for 1 12 octave and the exact center frequency for each band is returned in the exact center frequencies returned from Get Octave Frequencies The lower frequency bands require more settling time because of their smaller bandwidth You can find out if the octave filters have settled by checking the settled parameter returned by Read Octave Measurements If both octave and level measurements are configured in Configure OLM Engine the averaging process is started only after all the octave filters have settled If only level measurements are configured then the averaging process is started immediately Overload Detection NI DSA Software User Manual Two types of overload detection flags sticky or non sticky are used by the NI DSA software A sticky overload flag is set when an overload is detected on a particular channel remains set after the overload condition disappears and is not reset until you reconfigure or restart averaging on that channel Get OLM Status returns a sticky overload status A non sticky overload flag is set when an overload is detected on any input channel an
70. e Peak hold Level measurements Equivalent continuous level Slow Fast Impulse Impulse Eq Peak Custom In the optional octave analyzer mode the data are passed through a parallel bank of filters averaged if needed then transferred to the host memory Source Mode NI 4551 Only With your NI 4551 and NI DSA in source mode you can generate signals of the following types e Sine up to two tones e Chirp e Noise PRN white noise pink noise and band limited noise e Arbitrary the NI 4551 generates an output based on the data up to 4 096 samples you place in the output buffer NI DSA Software User Manual 1 4 ni com Programming with NI DSA This chapter provides instructions for programming with NI DSA and specific programming examples which you can modify for your application Note This manual uses a naming convention to jointly refer to LabVIEW VIs and C language function calls For example Read Measurement when shown in this font refers both to the LabVIEW NI DSA Read Measurements VI and to the C function NIDSA read measurement Basic Concepts In general programs built with NI DSA always perform the same basic types of operations In this section the operations are described as steps in general terms In later sections specific instructions for individual operating modes are given Step 1 Initialize Establishes communication with the DSA instrument and
71. e DC signal component What is real time operation for a 455X DSA device When referring to a NI 455X DSA device real time operation simply means that all time domain data points are used in the frequency domain calculations you request Another way to express this is that the analyzers on the DSA device can keep up with the incoming time domain data points If real time operation is not maintained the analyzers skip some time domain data points because they cannot perform their computations fast enough to include them all Failure to maintain real time operation does not generate an error or software failure How do I verify that I am maintaining real time operation Make a call to Check Status after performing a read operation to verify that you are running in real time This function returns an array of status values the first element in this array is the real time flag A value of indicates that real time has been maintained and a value of zero indicates that it has not If your software does not make frequent calls to Read Measurement you may not transfer all of your frequency domain measurements from the device analyzer to your host PC This condition does not constitute a failure to maintain real time and does not assert the real time warning flag in Check Status Why does my time domain data sometimes look incorrect in FFT analyzer mode When you use NI DSA Read Measurement to read in time domain data it important to set th
72. e View parameter on the function to Real Viewing the time domain data as magnitude values produces incorrect results Also you should not use the dB On variable for time domain data On the other hand most frequency domain applications call for the magnitude or phase view options dB on is often helpful for visualizing the magnitude response What are the benefits of capture mode Capture mode allows you to stream time domain data points directly to a buffer in host RAM Even if you do not read the time domain data from the driver in real time you do not lose any of the time domain data The most A 2 ni com Appendix A Common Questions common application of capture mode is to stream the whole time domain record to disk Capture mode is very similar to standard circular buffered NI DAQ functions in LabVIEW You should use Configure Capture Buffer to reserve this memory space Like standard DAQ functions Capture mode generates an error if this buffer overflows Read from the buffer periodically to prevent an overflow from occurring What do the terms overlap and time increment mean Overlap is a percentage ranging from 0 100 It indicates the fraction of time domain data that is shared between two adjacent FFTs For instance consider two FFTs each of which is 0 1 second in length Assume the first FFT covers the block of time from T 5 00 seconds to T 5 10 seconds The second FFT covers the block of time from T 5 02 to T 5
73. e amount of time required for a voltage to reach its final value within specified limits the manipulation of signals to prepare them for digitizing signal to noise ratio the ratio of the overall rms signal level to the rms noise level expressed in decibels a programmed event that triggers an event such as data acquisition a method of triggering in which you simulate an analog trigger using software Also called conditional retrieval a parameter of signal sources that reflects current driving ability of voltage sources lower is better and the voltage driving ability of current sources higher is better simultaneous sampling a property of a system in which each input or output channel is digitized or updated at the same instant samples per second used to express the rate at which a board samples an analog signal 1 hardware a property of an event that is synchronized to a reference clock 2 software a property of a function that begins an operation and returns only when the operation is complete a measure of the amount of noise seen by an analog circuit or an ADC when the analog inputs are grounded RAM installed on a personal computer and used by the operating system as contrasted with onboard RAM terminal count the highest value of a counter track and hold a circuit that tracks an analog voltage and holds the value on command G 16 ni com THD THD N throughput rate transducer transduce
74. e maximum voltage allowed e Wait on trigger indicates that the DSA instrument is waiting for a trigger condition to be met Refer to the NI 4551 4552 User Manual Chapter 3 Hardware Overview for more information about triggering e Linear averaging status e Get base FFT average complete e Get zoom FFT average complete Error handling National Instruments Corporation 3 13 NI DSA Software User Manual Chapter 3 FFT Mode Programming e Error message Reset Using Capture Mode to Acquire Time Domain Data NI DSA Software User Manual When you are using the FFT mode of NI DSA you can obtain time domain waveforms from Read Measurement This function returns time domain blocks whose size scales with the number of FFT lines you are calculating For instance a 400 line classical or 475 line extended FFT corresponds to 1 024 time domain data points For many applications this type of acquisition is sufficient However if your FFT analyzer falls out of real time you lose one or more of these time domain blocks In other words the time domain data set contains a gap Capture mode provides an alternative method that guarantees gapless time domain data Capture mode is most often used to stream data to disk The API for capture mode is quite similar to the standard NI DAQ operations in LabVIEW When using capture mode call Configure Capture Buffer before the acquisition begins This call sets aside a buffer space in RAM to
75. e nominal level a type of digital acquisition generation where a device or module accepts or transfers data after a digital pulse has been received Also called latched digital I O the physical components of a computer system such as the circuit boards plug in boards chassis enclosures peripherals and cables aform of triggering where you set the start time of an acquisition and gather data at a known position in time relative to a trigger signal the lag between making a change and the effect of the change hertz cycles per second Specifically refers to the repetition frequency of a waveform integrated circuit intermodulation distortion the ratio in decibels of the total rms signal level of harmonic sum and difference distortion products to the overall rms signal level The test signal is two sine waves added together according to the following standards SMPTE A 60 Hz sine wave and a 7 kHz sine wave added in a 4 1 amplitude ratio DIN A 250 Hz sine wave and an 8 kHz sine wave added in a 4 1 amplitude ratio CCIF A 14 kHz sine wave and a 15 kHz sine wave added in a 1 1 amplitude ratio inches NI DSA Software User Manual Glossary INL input bias current input impedance input offset current instrument driver instrumentation amplifier integrating ADC interrupt interrupt level I O isolation isolation voltage NI DSA Software User Manual integral nonlinearity a measure in
76. e response and stimulus The coherence function provides an estimate of the quality of a frequency response measurement In general higher Y values indicate better measurements Coherence is calculated as follows SA Baseband and Zoom Frequency Spans As explained in the FFT Analyzer section the NI 45XX offers zoom and baseband analyzers Some of the specifications and uses of these analyzers and the relationship between them are explained here Alias Free Bandwidth The alias free bandwidth is a specification of the usable frequency range of the instrument at a given sample rate Signals at or below the alias free bandwidth is measured at the specified accuracy To prevent aliasing the analog signal applied to a digitizer can not contain frequency components that are above one half of the sample rate f or f 2 Such a filter can not be implemented a realizable filter begins attenuating below f 2 and does not sufficiently attenuate until some point above f 2 which can result in aliasing of signals slightly below 2 For this reason the alias free bandwidth of the instrument will be somewhat less than f 2 The oversampling delta sigma converters used on the NI 4551 and NI 4552 contain integral antialiasing filters that provide an alias free bandwidth of 46 496 of the sampling frequency At the maximum sampling rate of 204 8 kS s the NI 45XX alias free bandwidth 1s 95 kS s 204 8 kS s x 46 4 95 kS s National
77. e same as the source frequency To put it another way a swept sine measurement is just a direct form of Fourier Transform at each frequency point This is why the Integration Time must correspond to an exact number of cycles The longer the integration time the narrower the detection bandwidth at the source frequency However the measurement takes longer to perform Sweep Frequency and Auto Resolution Start and stop frequencies specify the measurement frequency span and can be specified within the range shown here 0 lt f lt f 2 Note Sweeping through very low frequencies 1 Hz for example will result in excessive integration times National Instruments Corporation 6 17 NI DSA Software User Manual Chapter 6 5 NI DSA Software User Manual Advanced Concepts The number of points results in the minimum frequency resolution of a swept sine measurement number of points can be set from 1 to 2 048 The points can be in a linear or logarithmic progression In some cases it is desirable to sweep over a wide frequency range while still detecting narrow transition in the frequency response function An example might be a narrow notch filter To measure such a system a large Number of Points are needed to obtain fine frequency resolution This effectively increases the measurement time In order to resolve this problem auto resolution mode can be used In auto resolution mode fewer measurements will be made in frequency ran
78. e to make frequency response measurements when you need more accuracy control and dynamic range than FFT based frequency response measurements provide 3 Note Swept sine analyzer mode requires a signal source so this mode is supported only on the NI 4551 for PCI A swept sine analyzer measures the response of a system one frequency at a time based on a sequence of frequency points that you can specify At each point the source an analog output channel generates a sine wave at a constant frequency The input channels measure only at this frequency After going through the specified sequence of frequency points the measurements are complete In addition the auto ranging auto level and auto resolution features of NI DSA can automatically adjust the input gain output amplitude and step size to optimize the measurements for a particular device under test Octave Analyzer Mode Optional Use octave analyzer mode to analyze the frequency content of signals in a way that is analogous to human hearing Octave analysis is required by many government standards Features of the octave analyzer mode include Full 1 3 and 1 12 octave measurements e 25 600 or 51 200 Samples sec e A B or C weighting e EC ANSI compliant Averaging modes Linear Exponential e Slow Fast National Instruments Corporation 1 8 NI DSA Software User Manual Chapter 1 Introduction to NI DSA Impulse e Custom Equal confidenc
79. e within the confidence level in dB specified in the confidence level input of Configure Octave Averaging ofthe correct mean with a probability of 6896 The probability that the results are within twice the confidence level of the correct mean is 9696 National Instruments Corporation 6 35 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual For example if you specify a confidence level of 1 dB and the correct mean is 10 dB there is a 68 confidence that the measured value is between 9 11 dB and a 9646 confidence that the measured value is between 8 12 dB Table 6 3 shows the modes that are available for each of the octave or level measurements Table 6 3 Averaging Modes for Octave and Level Measurements Averaging Mode Octave Level None Y Linear Y Exponential slow Y Y Exponential fast Y Y Exponential custom Y Y Exponential impulse Y Y Impulse linear Y Peak hold Y Equal confidence A B and C Weighting The weighting curves correspond to the response of the human ear at different levels of sound pressure Specifically A B and C weighting correspond to the response at levels of 40 dB 70 dB and 100 dB respectively Considerations for Octave and Level Measurements The NI 45XX uses an embedded digital signal processor to do most octave and level processing in real time that is all samples from
80. eband span of any base channel causes the sampling rate to change affecting the span of all other channels Classical spans are as follows e Up to 800 lines in 0 80 KHz span for baseband e Up to 1 600 line in 0 80 KHz span for zoom analyzer National Instruments Corporation 3 7 NI DSA Software User Manual Chapter 3 FFT Mode Programming Acceptable extended spans are Up to 800 lines in 0 95 KHz span for baseband Up to 1 600 line in 0 95 KHz span for zoom analyzer 3 Note NI DSA automatically sets the hardware sampling rate to optimize antialiasing and real time performance based on the span you select NI DSA Software User Manual Configuring the FFT Settings Size of the buffer is directly related to the FFT resolution 1 024 pts 400 lines for baseband 1 024 points 800 lines for zoom Window Window to apply to the time domain signal applied to reduce spectral leakage Time increment Related to overlapping overlapping 100 time increment Phase suppression Phase suppression sets the amplitude threshold magnitude squared above which the phase of a particular frequency component is computed This feature is used to disregard the phase information for frequency components with negligible amplitudes i e noise Setting the value of Phase Suppress equal to 0 0 causes the phase to be computed for all frequency components no matter how small their magnitude The value assigned to Phase Suppress must have u
81. eight related bits of data an eight bit binary number Also used to denote the amount of memory required to store one byte of data the range of frequencies present in a signal or the range of frequencies to which a measuring device can respond a memory address that serves as the starting address for programmable registers All other addresses are located by adding to the base address a number system with a base of 2 a signal range that includes both positive and negative values for example 5 V to 5 V a type of coaxial signal connector a lowpass filter having a very flat passband a very sudden sharp transition region and high rejection in the stopband temporary storage for acquired or generated data software a high speed data transfer in which the address of the data is sent followed by back to back data words while a physical signal is asserted the group of conductors that interconnect individual circuitry in a computer Typically a bus is the expansion vehicle to which I O or other devices are connected Examples of PC buses are the ISA and PCI bus a type of a plug in board or controller with the ability to read and write devices on the computer bus NI DSA Software User Manual Glossary C C CalDAC channel circuit trigger clip clock CMOS CMRR code width common mode range common mode signal common mode voltage compensation range conditional retrieval conversion device NI DS
82. ement Functional Block Diagram Usually the configuration functions are called one time before measurements are started then the status and read functions are repeated in a loop However you can program the operation of the instrument to meet the specific needs of your application National Instruments Corporation 6 29 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual The functions can also be classified according to the types of measurements they are used for In general a function used for both octave and level measurements has the abbreviation OLM in the function name A function used only for octave measurements has the word octave in its name and one used only for level measurements has level in its name The categories of functions classified this way are as follows e Global Instrument Wide Functions apply to the entire device for all channels and for both octave and level measurements Examples of global instrument wide functions include Set OLM Sampling Rate Configure Weighting Filter e Octave and Level Measurement Functions apply to both octave and level measurements The following are octave and level measurement functions Configure OLM Engine Configure OLM linear averaging Restart OLM averaging Get OLM status e Octave Measurement functions apply only to octave measurements and include the following Configure Octave Measurem
83. ence purposes the table shows the center frequencies up to over 20 kHz However the center frequencies are limited to 10 678 72 Hz ANSI or 10 991 63 Hz IEC when the sampling rate is 25 600 Hz and to 53 39 36 Hz ANSI or 5 495 81 Hz IEC when the sampling rate is 51 200 Hz A 10 ni com Appendix A Common Questions Table A 1 Exact Center Frequencies for 1 12 Octave Analysis Ter ANSI Standard Exact IEC Standard Exact octave Center Frequencies Hz Center Frequencies Hz 90 5 52 5 69 89 5 85 6 02 88 6 20 6 38 87 6 57 6 76 86 6 96 7 16 85 7 37 7 59 84 7 81 8 04 83 8 28 8 52 82 8 77 9 03 81 9 29 9 56 80 9 84 10 13 79 10 43 10 73 78 11 05 11 37 77 11 71 12 05 76 12 40 12 76 75 13 14 13 52 74 13 92 14 33 73 14 75 15 18 72 15 625 16 083 71 16 554 17 039 70 17 538 18 052 69 18 581 19 126 68 19 686 20 263 67 20 857 21 468 National Instruments Corporation A 11 NI DSA Software User Manual Appendix A Common Questions Table A 1 Exact Center Frequencies for 1 12 Octave Analysis Continued NI DSA Software User Manual aas ANSI Standard Exact IEC Standard Exact octave Center Frequencies Hz Center Frequencies Hz 66 22 097 22 745 65 23 411 24 097 64 24 803 25 530 63
84. ent gDSASession amp NewMeasurement if NewMeasurement return 0 Read Frequency axis values NIDSA get measurement length gDSASession SweptFregAxis 1 amp Length amp RealOrComplex NIDSA read measurement gDSASession SweptFreqAxis 0 Length 0 0 0 0 amp 0 amp gFreqAxis Read Frequency response NIDSA get measurement length gDSASession SweptFreqResp 1 amp respLength amp respRealOrComplex 4 8 ni com Chapter 4 Swept Sine Mode Programming NIDSA read measurement gDSASession SweptFreqResp 0 respLength 2 1 0 0 amp respxO amp respdx gFreqResp PlotXY mainpanel MAINPANEL GRAPH gFreqAxis gFregResp Length VAL FLOAT VAL FLOAT VAL THIN LINE VAL EMPTY SQUARE VAL SOLID 1 VAL RED break return VI_SUCCESS Step 4 Control Checks the status of the ongoing measurement and error handling Status functions return the status of the instrument such as whether or not an overload has occurred at the input is the instrument operating in real time or the time that has elapsed since averaging was begun Example Get OLM Status Check Status returns status values for up to seven conditions Some of the key status indicators are listed here National Instruments Corporation Real time indicates whether the onboard DSP can keep up with the rate of data acquisition Main error indicates an error on the DSA device Input overload status
85. ents Configure Octave Averaging Configure Octave Weighting Get Octave Frequencies Get Octave Number of Bands Level Measurement functions apply only to level measurements and include the following functions Configure Level Measurements Get Level Labels Get Level Length Read Level Measurements Level Measurements Sound pressure level measurements are defined by two main parameters The first of these is the type of frequency weighting A B C or none The second parameter sets the averaging scheme To define the level averaging mode you should call Configure Level Measurements 6 30 ni com Chapter 6 Advanced Concepts Seven averaging schemes are available e F Fast Exponential e S Slow Exponential e IQ Impulse Exponential e C Custom Exponential e EQ E quivalent Continuous Level e EQ Impulse Equivalent Continuous Level e P Peak The parameters for a level measurement are often designated as LXY where X is the weighting filter type and Y is the averaging type For instance LAEQ indicates an A weighted Equivalent Continuous Level measurement The four exponential averaging modes weigh recent data more heavily than older data in calculating the level The time weighting curve is an exponential decay with a definite time constant For instance the fast exponential mode has a 125 ms time constant This means that data from 125 ms in the past is weighted
86. ents refer to Chapter 3 FFT Mode Programming e To use NI DSA for acquiring time domain signals refer to Chapter 3 FFT Mode Programming the Using Capture Mode to Acquire Time Domain Data section e Ifyou are using an NI 4551 to generate a signal refer to Chapter 4 Swept Sine Mode Programming the Source Mode NI 4551 Only section e Forswept sine measurements refer to Chapter 4 Swept Sine Mode Programming Step 2 Configure the Swept Sine Analyzer section e For octave and level measurements refer to Chapter 5 Octave Analysis Add On Mode Programming Read functions are different for each measurement mode For mode specific programming instructions refer to Chapter 3 FFT Mode Programming Chapter 4 Swept Sine Mode Programming and Chapter 5 Octave Analysis Add On Mode Programming 2 4 ni com Step 4 Control Step 5 Close Chapter 2 Programming with NI DSA Control functions are different for each measurement mode but generally start stop or change the operation of your DSA instrument For example you might call Restart OLM Averaging to restart averaging for a particular analyzer channel Refer to your online help for more information about control functions Any application you write with NI DSA should have a close function LabVIEW Example dsa session error in no error Figure 2 3 Close C Example NIDSA close DSAsession National Instruments Corpora
87. er Manual das ANSI Standard Exact IEC Standard Exact octave Center Frequencies Hz Center Frequencies Hz 18 353 55 363 91 17 374 58 385 55 16 396 85 408 48 15 420 45 432 77 4 445 45 458 50 13 471 94 485 77 12 500 514 65 11 529 73 545 25 10 561 23 577 68 9 594 60 612 03 8 629 96 648 42 7 667 42 686 98 6 707 11 727 83 5 749 15 711 11 4 793 70 816 96 3 840 9 865 54 2 890 90 917 00 1 943 87 971 53 0 1 000 1 029 30 1 1 059 46 1 090 51 2 1 122 46 1 155 35 3 1 189 21 1 224 05 4 1 259 92 1 296 84 5 1 334 84 1 373 95 A 14 ni com Appendix A Common Questions Table A 1 Exact Center Frequencies for 1 12 Octave Analysis Continued National Instruments Corporation Mas ANSI Standard Exact IEC Standard Exact octave Center Frequencies Hz Center Frequencies Hz 6 1 414 21 1 455 65 7 1498 31 1 542 21 8 1 587 40 1 633 92 9 1 681 79 1 731 07 10 1 781 80 1 834 01 11 1 887 75 1 943 06 12 2 000 2 058 60 13 2 118 93 2 181 02 14 2 244 92 2 310 71 15 2 378 41 2 448 11 16 2 519 84 2 593 68 17 2 669 68 2 747 91 18 2 828 43 2 911 31 19 2 996 61 3 084 42 20 3 174 80 3 267 83 21 3 363 59 3 462 15 22 3 563 59 3 668 02 23 3 775 5 3 886 13 24 4 000 4 117 21 25 4 237 85 4 362 03 26 4 489 85 4 621 41 27 4 756 83 4 896 21 28 5 039 68 5 187 36 29 5 339 36 5 495 81 A 15 NI DSA Software User Manual Appendix A Common Questi
88. er channel basis Configure weighting filter enables A B or C weighting Configure Octave Measurements configures the type of octave analysis to perform as well as the low and high center frequencies Set the following parameters Octave type 1 1 1 3 or 1 12 Lo center frequency center frequency Compliance ANSI IEC 5 3 NI DSA Software User Manual Chapter 5 Octave Analysis Add On Mode Programming e Configure Octave averaging sets the averaging mode time constant and confidence level Average Type 0 None 1 Linear band power outputs are equally weighted and averaged over the specified integration time 2 Exponential band power outputs are exponentially averaged with new filter data weighted more than older data 3 Equal confidence the time constant for each band power output is individually set so the results have a 68 probability of being within confidence level of the true mean for every band power level 4 Peak hold band power outputs are set at the peak output from each band filter Exponential mode selects the time constant to use for exponential averaging 0 Fast 125 ms 1 Slow 1 000 ms 2 Impulse 35 ms time constant stage followed by a peak detector with a 1 500 ms time constant decay rate 4 Custom use Time Constant value Time Constant customized time constant value Confidence Level Note To set linear averaging integration time use Co
89. ers are conversion devices G 4 ni com conversion time counter timer coupling crosstalk current drive capability current sinking current sourcing D A DAC DACxOUT daisy chain DAQ dB DC National Instruments Corporation G 5 Glossary the time required in an analog input or output system from the moment a channel is interrogated such as with a read instruction to the moment that accurate data is available a circuit that counts external pulses or clock pulses timing the manner in which a signal is connected from one location to another an unwanted signal on one channel due to an input on a different channel the amount of current a digital or analog output channel is capable of sourcing or sinking while still operating within voltage range specifications the ability of a DAQ board to dissipate current for analog or digital output signals the ability of a DAQ board to supply current for analog or digital output signals digital to analog digital to analog converter an electronic device often an integrated circuit that converts a digital number into a corresponding analog voltage or current analog channel x output signal a method of propagating signals along a bus in which the devices are prioritized on the basis of their position on the bus data acquisition 1 collecting and measuring electrical signals from sensors transducers and test probes or fixtures and inputting them to a
90. est possible span that is greater than or equal to the top of the zoom span However 800 Hz violates the device minimum span so the smallest acceptable value also that fulfills the power of 2 requirement is 3 200 Hz Thus you call Set Classical Baseband Span with a span value of 3 200 Hz The NI DSA shipping examples for performing zoom FFTs in LabVIEW and LabWINDOWS CVI demonstrate a quick log based algorithm to calculate the appropriate baseband span for any given zoom span Can I use a zoom FFT and set the lower frequency span limit at 0 Hz Yes This operation is essentially a high resolution baseband analysis although you still program it in zoom FFT mode The minimum span for a baseband analyzer on the NI 455X DSA devices is just under 1 953 7 Hz meeting the minimum sampling rate of 5 kHz However this minimum span width limitation does not apply to zoom mode Using zoom allows A 4 ni com Appendix A Common Questions you to increase the frequency resolution of your FFT both by increasing the number of lines to 1 600 and by using a smaller upper frequency limit If you use the zoom FFT analyzer with a lower limit of 0 Hz the zoom algorithm skips the heterodyning step to shift frequencies but still performs decimation and filtering This feature provides a slight improvement in the efficiency of the algorithm for case of a 0 Hz lower limit What are the zoom time and zoom win time measurements The zoom time measurement prov
91. f e 0 off default e on e 2 one shot type sets the type of output signal to generate The valid values for type and the function to use to further define the outputs as follows e 0 Default Sine dual tone Use Configure Sine Source next e 1 Chirp Use Configure Chirp Source next e 2 Noise Use Configure Noise Source next e 3 Arbitrary Use Configure Arbitrary Source next Configure Sine source Freq 1 Frequency of the first tone Amp 1 Amplitude of the first tone Freq 2 Frequency of the second tone 2 Amplitude of the second tone DC Offset DC offset in volts of the dual tone signal output If the source type is noise you can also use the Noise Type and Band Limited Noise parameters Refer to the NI DSA online help for more information 4 12 ni com Chapter 4 Swept Sine Mode Programming In order to configure an Arbitrary source the following operations are requested e Configure Arbitrary Source Amp Amplitude of the output signal Mode Scale buffer by Amp buffer needs to be normalized 0 output value 1 Configure Arbitrary Buffer Enter the values of the arbitrary waveform max buffer size 4 096 points The update rate of your output is determined by the scan rate set for the inputs If you change the input rate the update rate is the same as the scan rate if the scan rate is less or equal than 51 2 kHz one half the scan rate if the inp
92. ftware User Manual Does the zoom FFT reduce acquisition time For any given frequency resolution you must acquire data for the same time interval whether you are using baseband or zoom analysis If you use either a zoom FFT or a baseband FFT the time block of data required to perform a single FFT is 1 Df where Df is the frequency resolution In the example above you require 1 0 625 Hz 1 6 seconds of data for an FFT When using zoom FFTs should I set the baseband span as well as the zoom span Yes Set the classical baseband span to an appropriate value when performing a zoom analysis The reason for this setting is that the baseband span settings control the sampling rate on the DSA hardware For any particular zoom span choose the smallest classical baseband span that meets the following three criteria 1 The classical baseband span is equal to or greater than the maximum frequency of interest in the zoom span 2 The classical baseband span is equal to or greater than the device minimum of 1 953 7 Hz This minimum span corresponds to a sampling rate of 5 kS s 3 Theratio between the baseband span width and zoom span width is a power of 2 For example if you set a zoom span to examine frequencies between 600 Hz and 800 Hz then the zoom span width is 200 Hz because you need a baseband span that produces a power of 2 when divided by 200 Hz Possible values include 200 Hz 400 Hz 800 Hz and so on yet 800 Hz is the low
93. ges where the response is fairly flat Auto resolution mode makes more measurements where response is a strong function of frequency Auto resolution can dramatically save measurement time without losing resolution Since points may be skipped the number of points actually measured may far less than that specified Auto resolution is specified by these three parameters Faster threshold Slower Threshold and Maximum Step Size Auto resolution mode examines the measurements of successive frequency points If the change of the most recent measurement relative to the previous one is within the Fast Threshold for both channels the sweep will take larger steps by skipping frequency points Each time this threshold is met the step size will be increased by one until the Maximum Step Size is reached If the change of the measurements is more than the Slower Threshold for either channel then the analyzer discards the newest measurement and measures the point immediately after the last point that was within the thresholds If measurements differ by more than the Faster Threshold on either channel but less than the Slower Threshold on both channels the analyzer maintains the present sweep speed by continuing to skip the same number of points Input Auto Ranging With auto ranging on the instrument can automatically adjust the input range based on the measured signal levels in an attempt to measure all inputs at full scale This has a great i
94. half the number of samples a 2 048 point FFT would yield 1 024 frequency lines Most analyzers have filters that significantly attenuate frequencies near integer multiples so a 2 048 point FFT actually produces 800 useful frequency lines With NI DSA you can choose FFTs with classical spans of 50 100 200 400 or 800 lines The NI 455X devices feature anti alias filters with a very sharp roll off Because of this feature the devices can push closer to ideal behavior and provide more useful lines without requiring additional samples This feature is called the extended span Extended span FFTs provide 59 118 237 475 or 950 frequency lines without the need for additional data points higher sampling rates or a longer acquisition time The number of frequency bins in an FFT depends on the number of time domain points you analyze To increase the frequency resolution of an FFT you must sample over a longer time period Why does a 400 line baseband FFT return an array with 401 data points By definition a 400 line FFT contains information on the spectral content of a signal at 400 nonzero frequencies The analyzer also returns the DC National Instruments Corporation A 1 NI DSA Software User Manual Appendix A Common Questions NI DSA Software User Manual content of the signal providing a total of 401 amplitude values An x line FFT always returns an array of length x 1 and the first element of this array is the amplitude of th
95. here are two types of linear averaging available one shot and auto restart In one shot mode linear averaging is performed on all measurements for the specified duration then the averaging stops All of the averages calculated before the final average are also available 6 32 ni com Chapter 6 Advanced Concepts In auto restart mode as in one shot mode linear averaging is done for a specified duration then the average is reset and averaging in performed on the next set of measurements for the same duration This process continues indefinitely In auto restart mode the results are available only for the end of each averaging period that is each result is the linear average of the measurements for the specified duration One shot and auto restart modes are illustrated in Figure 6 11 One shot Linear Averaging Duration 0 Start Stop A Auto Restart Linear Averaging Duration Duration Duration q gt lt gt lt 0 Start 1st Result 2nd Result 3rd Result Figure 6 11 One Shot and Auto Restart Linear Averaging While linear averaging is in progress the elapsed time for the current averaging process is returned as the linear averaging seconds parameter of Read Octave Measurements and Read Level Measurements There is a one to one correspondence between the time returned and the measured values read from these functions In one shot mode the value returned is the actual value in seconds in auto restart m
96. iStatus NewMeasurement Vilnt32 Length Vilnt32 RealOrComplex ViReal32 x0 ViReal32 dx ViReal32 ActualFS switch event case EVENT TIMER TICK Check if a new measurement is ready NIDSA check new measurement DSASession amp NewMeasurement if NewMeasurement return 0 NI DSA Software User Manual 3 12 ni com Chapter 3 FFT Mode Programming Read the new measurement NIDSA get measurement length DSASession BaseFFTO FREO LINES CLASSICAL amp Length amp RealOrComplex Read the measurement performed by Analyzer BaseFFTO Get Magnitude in dB Use RMS units and Degrees NIDSA read measurement DSASession BaseFFTO 0 Length MAGVIEW DBON RMSUNIT DEGREES amp x0 amp dx gFFTMeasurement break Step 4 Control Checks the status of ongoing measurement and error handling Status functions return the status of the instrument such as whether or not an overload has occurred at the input whether the instrument is operating in real time or if the time that has elapsed since averaging was begun Example Get OLM Status Check Status returns status values for up to seven conditions The primary status indicators are as follows e Real time indicates whether the onboard DSP can keep up with the rate of data acquisition e Main error indicates an error on the DSA device e Input overload status indicates an overload condition on an input channel The input level exceeded th
97. ides the most recent time domain data after it has been heterodyned filtered and decimated by the zoom analyzer The zoom win time shows the same calculation including the effect of the time domain smoothing window These are very specialized measurements that you should use only if you need to perform additional processing on the heterodyned data in software For most applications requiring time domain data access you should use either the time measurement or employ capture mode Source Mode What types of analog output signals can I generate using NI DSA NI DSA includes functions to generate standard white noise pink noise band limited noise periodic white noise PRN single or dual tone sine and chirp signals Furthermore you can use NI DSA to define and generate an arbitrary analog output signal It is important to notice that the data for this signal is stored entirely within the 4 096 point FIFO memory on the board Because of this constraint the number of point in your arbitrary data array must be a power of two equal to or less than 4 096 If you use the NI 455X in NI DAQ compatibility mode you can store a much longer analog output data array in the RAM of the host computer What is the relationship between the input and output sampling rates on the NI 455X device If you are performing simultaneous analog input and output with your NI 455X device the ratio between the input and output sampling rates must always be a
98. in the data but does not reduce the actual noise floor Vector averaging uses the complex FFT spectrum The real part is averaged separately from the imaginary part This can reduce the noise floor for random signals because they are not phase coherent from one time record to the next For single channel measurements vector averaging requires a trigger so that the real and imaginary parts of the signal add in phase instead of canceling randomly 6 14 ni com Chapter 6 Advanced Concepts Peak Hold averaging retains the peak of the spectral magnitudes on a frequency bin by bin basis The peak values are stored in the original complex form so that all complex forms of the data can be displayed Each new spectral record for RMS and Vector averaging can be weighted using either Linear or Exponential weighting Swept Sine Unlike the FFT which measures all the frequencies simultaneously the swept sine analyzer measures one single frequency at a time Based on a specified sequence of frequency points the swept sine analyzer makes the measurements one by one At each point the source generates a sine wave and maintains a constant frequency The inputs measure only at this frequency After going through the sequence of frequency points the measurements are complete Figure 6 6 shows the typical application of swept sine analysis System n Under Test OUT Y OUT DAC1 OUT ACHO ACH BNC 2140 2142
99. indicates an overload condition on an input channel Wait on trigger indicates that your DSA instrument is waiting for a trigger condition to be met Refer to Chapter 3 Hardware Overview of the NI 4551 4552 User Manual for more information about triggering 4 9 NI DSA Software User Manual Chapter 4 Swept Sine Mode Programming LabVIEW Example C Example Get Swept Sine Status returns the status of the swept sine measurement with the following indicators Sweep state returns the state the current sweep in swept sine analyzer mode Q Sweeping sweep is in progress Paused sweep has been paused 2 Done sweep has completed Measurement error An overload or measurement error has occurred during the present sweep Overload An overload has occurred during the present sweep Control swept sine controls the swept sine analyzer mode Sweep control controls the swept sine analyzer mode Run runs resumes the sweep from the present frequency Pause pauses the sweep at the present frequency Restart restarts the sweep at the configured start frequency NIDSA Control Swept Sine vi Figure 4 4 Controlling the Swept Sine Measurement Controlling the Swept sine measurement NIDSA_controlsweptsine NI DSA Software User Manual DSASession SWEEP RESTART 1 4 10 ni com Chapter 4 Swept Sine Mode Programming Source Mode NI 4551 Only Source mode is useful fo
100. le takes longer at low frequencies 1 second at 1 Hz for example Is there a minimum frequency I can successfully test using NI DSA swept sine analysis The minimum test frequency for swept sine analysis is 0 1 Hz Can I measure the level of individual harmonics rather than THD using swept sine analysis Yes You can obtain up to five harmonics from the swept sine measurement To set up the swept sine harmonic analysis call Configure Swept Sine Harmonics Pass a value of 1 to Enable Harmonic Mapping The National Instruments Corporation A 7 NI DSA Software User Manual Appendix A Common Questions NI DSA Software User Manual Harmonic Mapping parameter is an array of five integers defining the harmonics of interest For example if the elements of this array are 2 3 5 9 20 the swept sine harmonic measurements returned are e Swept sine harmonic 1 2 harmonic e Swept sine harmonic 2 3 harmonic e Swept sine harmonic 3 5 harmonic e Swept sine harmonic 4 9 harmonic e Swept sine harmonic 5 20 harmonic The default value for the mapping is 2 3 4 5 6 giving you the first five harmonics after the fundamental frequency In some cases you may not obtain a valid reading for the higher harmonics at all test frequencies This error occur if the frequency of the harmonic exceeds 23 000 Hz For instance you cannot see the fourth or fifth harmonic when the excitation frequency exceeds 4 600s Hz NI DSA returns
101. lter 25 600 or 51 200 Measurement Read Octave Measurement Get Octave Frequencies Close Octave Plot XY Graph BandPower vs Center Frequency Figure 5 1 Octave Analysis Programming Flowchart for NI DSA 5 2 ni com Chapter 5 Octave Analysis Add On Mode Programming Step 2 Configure the Octave Analyzer Configuring Your Octave and Level Measurement To configure octave and level measurements use the following functions Set OLM Sampling rate set the hardware Sample Rate parameter to 25 600 or 51 200 S s The sample rate you select the number of input channels used and the number of additional real time measurements you make affect the maximum center frequencies for octave analysis Refer to the Considerations for Octave and Level Measurements section for more information on how your programming choices affect real time operation Configure OLM engine enables measurements using the following parameters Time acquires a time waveform Time is automatically set to true if any of level octave or FFT are true Level performs level measurement Octave performs octave measurement FFT performs baseband FFT measurement 5 Note You can set octave and level to TRUE in order to perform octave and level measurements at the same time National Instruments Corporation Configure Octave weighting enables or disables weighting on a p
102. lus and a constant a type of signal conditioning in which software linearizes the voltage levels from transducers so the voltages can be scaled to measure physical phenomena in an AC coupled circuit the frequency below which signals are attenuated by at least 3 dB least significant bit NI DSA Software User Manual Glossary Mbytes s memory buffer MITE MS MSB MTBF MTTR NC NI DAQ NIST noise nonlatched digital I O NI DSA Software User Manual meters 1 Mega the standard metric prefix for 1 million or 106 when used with units of measure such as volts and hertz 2 mega the prefix for 1 048 576 or 220 when used with B to quantify data or computer memory a unit for data transfer that means 1 048 576 bytes s See buffer MXTI Interface to Everything a custom ASIC designed by National Instruments that implements the PCI bus interface The MITE supports bus mastering for high speed data transfers over the PCI bus million samples most significant bit mean time between failure mean time to repair predicts downtime and how long it takes to fix a product normally closed or not connected National Instruments driver software for DAQ hardware National Institute of Standards and Technology an undesirable electrical signal Noise comes from external sources such as the AC power line motors generators transformers fluorescent lights soldering irons CRT displays computers electrical storm
103. m baseband span of 2 320 Hz 5 000 S s x 1 024 475 2 320Hz with the extended FFT size and a minimum of 1 953 Hz 5 000 x 1 024 400 1 953Hz with the classical size and because of the above mentioned reasons the maximum baseband span is 95 KHz In NI DSA the maximum FFT size is 2 kS 2 048 samples allowing up to 800 FFT lines classical or 950 lines extended frequency span To summarize the FFT resolution number of lines that you specify determines the length of the time domain record and number of samples to be acquired The reverse is also true 6 8 ni com Chapter 6 Advanced Concepts The sampling rate determines the frequency span The FFT resolution is the number of lines or bins and the frequency resolution is the frequency span divided by the number of lines Zoom Spans If you need better frequency resolution than the baseband analyzers can provide you need to use the narrow span capabilities of the zoom analyzers The Real Time Zoom FFT Process Time record length is directly related to frequency resolution Increasing the duration of the time record increases the frequency resolution To increase the duration of the time record you must either capture more points or lower the sampling rate A narrower span allows you to zoom in on a center frequency with a selected span thus improving your frequency resolution zoom span has a start frequency a center frequency and an end frequency The zoom sp
104. mpact on the dynamic range of the measurement Input auto ranging monitors the measurement of each point If overloading occurs during the sweep the analyzer increases the input range to the next higher level If the signal level drops below the threshold of the next lower range the analyzer decreases the input range accordingly Note Auto ranging may increase measurement times 6 18 ni com Chapter 6 Advanced Concepts Source Auto Level and Ramping There are five parameters associated with source auto level They are auto level channel ideal reference upper reference lower reference and maximum source level Source auto level adjusts the source amplitude to maintain a constant level called Ideal Reference at the channel 0 or channel 1 input The reason to use Source auto level is to improve the signal to noise ratio of the measured signal Suppose you want to measure the transform function of a device with an input range set to 1 0 V and the device has both gain and attenuation within the frequency span for example from 30dBV to 100dBV Without source auto level the source must maintain a constant level that is less than 30dBV to prevent overload across the whole frequency span When the sweep frequency reaches the point of greatest system attenuation the output signal level drops to 130dBV Such a signal level is still measurable but may not be optimum With source auto level the source tries to maintain an Ideal Refere
105. mples NI DSA Software User Manual Configuring Harmonics Measurements Optional Configure Swept Sine Harmonics configure the harmonics used for swept sine harmonic measurements e Maximum THD harmonic sets the highest harmonic number for THD computations For example set maximum THD harmonic to 3 to use only the second and third harmonics to compute THD e Enable harmonic mapping sets to TRUE to measure harmonics of the source frequency on the response Enable Harmonic Mapping has no effect on your THD measurement 4 4 ni com Chapter 4 Swept Sine Mode Programming e Harmonic mapping array if Enable Harmonic Mapping is TRUE you must set the five array elements to 32 bit integers corresponding to the five harmonics to be returned as Swept Sine Harmonic lt 1 5 gt For example if you set the elements of the Harmonic Mapping Array to 2 3 5 9 20 the measurements returned are as follows Swept sine harmonic 1 2 4 harmonic Swept sine harmonic 2 3 harmonic Swept sine harmonic 3 5 harmonic Swept sine harmonic 4 9 harmonic Swept sine harmonic 5 20 harmonic If Enable Harmonic Mapping is TRUE and you do not specify a Harmonic Mapping Array the default array 2 3 4 5 6 is used The maximum value for Maximum THD Harmonic and any element in the Harmonic Mapping Array is 64 Configuring Custom Frequencies Optional If sweep mode in Configure Swept Sine is set to custom freq
106. n of signal generation In this example a while loop is used to continue signal generation for 50 ms LabVIEW Example Figure 4 7 Generating the Signal NI DSA Software User Manual 4 14 ni com Octave Analysis Add On Mode Programming In many cases when dealing with sound measurement and analysis the final customer is the human ear and like most human senses the ear exhibits a response based on a logarithmic scale for both the level and the frequency So to produce results that are somewhere related to this human perception sound levels are expressed in decibels and frequency content measured with a logarithmic scale Many research efforts are currently focused on this field of psycho acoustics but octave band analysis remains the first choice technique Figure 5 1 illustrates the recommended programming flow for octave analysis with NI DSA National Instruments Corporation 5 1 NI DSA Software User Manual Chapter 5 Octave Analysis Add On Mode Programming NI DSA Software User Manual Initialize y y Configure Trigger Configure Octave Weighting Configure Weighting Filter Set OLM Sampling Rate Configure OLM Engine Check New Configure Octave Measurements Configure Octave Averaging Configure Mode Octave Set Input Voltage Range Set Input Coupling Configure Hardware Front End Configure Weighting Fi
107. nce Level of 1 0V at the output of the device under test The source level decreases at the point of gain and the source level increases at the point of attenuation until the Maximum Source Level reached In this example the output level of the device under test is 0 dBV to 100 dBV assuming the Maximum Source Level is reached instead of 0 dBV to 130 dBV Varying the source level narrows the range of the output signals avoiding overloads when there is gain and increasing the output signal to noise ratio when there is attenuation Source auto level requires the input auto ranging to be on because the inputs must track the changing source level Source auto level should only be used when performing transfer function measurements because the source level changes are not normalized Only the ratio of channel 1 to channel 0 is source level independent Source Ramping is the rate at which the source level changes With Source Ramping on the source level changes gradually at the ramping rate The settling time starts after the ramp Enabling Source Ramping increases the sweep time but avoids instantaneous source level changes which could cause the device under test to respond erratically National Instruments Corporation 6 19 NI DSA Software User Manual Chapter 6 Advanced Concepts Swept Sine Measurements NI DSA Software User Manual Spectrum The spectrum the measurement of a single channel over a sweep can be returned for
108. nfigure OLM Linear Averaging LabVIEW Examples NI DSA Software User Manual octave channel NIDSA NIDSA Configure Configure Weighting Octave Filter vi Weighting vi Figure 5 2 Configure Weighting Filter 5 4 ni com Chapter 5 Octave Analysis Add On Mode Programming channel select NIDSA Set OLM Sampling Rate vi NIDSA Configure NIDSA Configure Octave OLM Engine vi Measurements vi NIDSA Configure Octave Averaqina vi C Example Figure 5 3 Configure OLM Measurement Set the sampling frequency 25 6 or 51 2 kS sec NIDSA set olm sampling rate gDSASession SampleRate enables time Octave and level measurements NIDSA configure olm engine gDSASession 0 VI TRUE VI TRUE VI TRUE VI FALSE NIDSA configure octave measurements gDSASession OctaveType LoFreg HiFreq OCT COMPL ANSI amp NumberOfBands NIDSA configure octave averaging gDSASession 0 AvgType ExpMode TimeConstant 1 0 Step 3 Read Octave Measurement Before reading a measurement check to see if a new measurement is ready The following operations are typically enclosed in a loop In this example a while loop is used e Check new measurement returns 1 when a new block of data is ready a 0 if not Octave mode refer the octave description for further information Octave power 0 Octave power 1 Octave power 2 Octave power 3 Level measurements
109. ni com Appendix A Common Questions Integration time this value given in units of seconds defines how long each frequency measurement should actually take This is the time during which the DSA device generates an excitation reads the response and calculates the transfer function e Integration cycles this is controls how many excitation signal cycles should go into a particular frequency response calculation As with the actual settling period the calculation time at any frequency can be either settle time or Settle Cycles Frequency which ever is greater The total time for an entire swept sine measurement is the sum of the settling time and integration time used at every frequency under test There is no quick formula for determining the best values for the swept sine averaging parameters For the most part these value should be determined empirically start at large values for all parameters and reduce the values slowly until you see the final results of your frequency response test begin to change In general a longer settling period helps for measurements on systems whose response dies off slowly when excitation is ceased Examples include high Q resonant circuits or sound measurements in fairly reverberant environment A longer integration period helps if the unit under test contains some element of random noise this should average out over the course of several cycles of measurement Octave and Level Analyzer Mode
110. nical accuracy In the event that technical or typographical errors exist National Instruments reserves the right to make changes to subsequent editions of this document without prior notice to holders of this edition The reader should consult National Instruments if errors are suspected In no event shall National Instruments be liable for any damages arising out of or related to this document or the information contained in it EXCEPT AS SPECIFIED HEREIN NATIONAL INSTRUMENTS MAKES NO WARRANTIES EXPRESS OR IMPLIED AND SPECIFICALLY DISCLAIMS ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE CUSTOMER S RIGHT TO RECOVER DAMAGES CAUSED BY FAULT OR NEGLIGENCE ON THE PART OF NATIONAL INSTRUMENTS SHALL BE LIMITED TO THE AMOUNT THERETOFORE PAID BY THE CUSTOMER NATIONAL INSTRUMENTS WILL NOT BE LIABLE FOR DAMAGES RESULTING FROM LOSS OF DATA PROFITS USE OF PRODUCTS OR INCIDENTAL OR CONSEQUENTIAL DAMAGES EVEN IF ADVISED OF THE POSSIBILITY THEREOF This limitation of the liability of National Instruments will apply regardless of the form of action whether in contract or tort including negligence Any action against National Instruments must be brought within one year after the cause of action accrues National Instruments shall not be liable for any delay in performance due to causes beyond its reasonable control The warranty provided herein does not cover damages defects malfunctions or service failures caused by owner s failure to follow
111. nits of voltage squared For example to configure phase suppression for all frequency components with amplitudes less than 40 dBV 0 01V set Phase Suppress to 0 01 V 2 0 0001 V2 Averaging Averaging successive measurements tends to improve the accuracy of your measurements Use Configure Base FFT Averaging Or Configure Zoom FFT Averaging to set the following Mode Vector RMS Peak Hold Weighting Linear Exponential Number of averages 3 8 ni com Chapter 3 FFT Mode Programming Step 3 Read FFT Measurements Before you can read a measurement you need to know if it has been completed Check New Measurement returns a 1 if a new measurement is ready You must call Check New Measurement before reading the first measurement and before reading any subsequent measurement set Reading a New Measurement To read a new measurement use the following functions Get Measurement Length returns the number of data points to read based on the type and length of the measurement and the set of frequency lines you choose with freqLineSelector Variables for freqLineSelector are Valid lines returns alias free lines only Alllines returns all FFT lines Classical lines returns alias free classical lines Read Measurement treads the measurement from the specified analyzer channel The valid measurement type settings for Get Measurement Length and Read Measurement are limited to the
112. nt returns the RMS squared value of the measured signal The DC component of the signal is excluded from the RMS squared calculation NI DSA computes this measurement for channel 1 which is normally connected the to output of the system under test NI DSA Software User Manual 6 22 ni com Chapter 6 Advanced Concepts Octave Analyzer Add On In Octave analyzer mode your NI 45XX can perform fractional octave analysis and level measurements in real time The basic analyzer architecture features and operation are discussed in this section The NI DSA Real Time Octave Analysis RTOA software add on for NI DSA supports e Fractional octave analysis for 1 1 1 3 and 1 12 octave analysis with the following averaging modes Linear Exponential slow fast custom Impulse Peak Hold Equal Confidence Level measurements with the following averaging modes Linear Leq Exponential slow fast custom Impulse Impulse Eq Peak Hold e A B and C weighting With the RTOA you can do octave level or both octave and level measurements and have the additional option of specifying the weighting if required octave and level measurements are returned as RMS root mean square values The RMS value of a signal is a measure of the energy it contains In terms of vibration signals it can be viewed as the destructive ability of the signal Level measurements return the total amount of
113. o dB on the THD is returned as a dB scaled value which is calculated as shown in the following equation THD g 20 x log SINAD SINAD stands for Signal in Noise and Distortion It is the reciprocal of THD Noise In general SINAD is computed as shown here A EA D SINAD D Where A signal amplitude RMS N noise RMS and D distortion RMS NI DSA calculates SINAD as shown here SINAD 100 x E A Where E signal amplitude RMS A RMS value of the fundamental frequency NI DSA computes SINAD on channel 1 which is normally connected to the output of the system under test National Instruments Corporation 6 21 NI DSA Software User Manual Chapter 6 Advanced Concepts With Read Measurement parameter dB units set to dB on the SINAD is calculated as shown here SINAD 20 x log E A Swept Harmonics Up to five individual harmonic distortion measurements can be returned Each one is calculated as shown in the following formula HD 100 Am m XT Where A RMS value of the mth harmonic m is from 2 to 6 and A RMS value of the fundamental NI DSA computes swept harmonics on channel 1 which is normally connected to the output of the system under test With Read Measurement parameter dB units set to dB on the distortion of the individual harmonic is calculated as shown here HD 20 x1 Am m 7 X10g Ai RMS Squared This measureme
114. oard or a GPIB interface board the ratio of the largest signal level a circuit can handle to the smallest signal level it can handle usually taken to be the noise level normally expressed in decibels a device that converts linear or rotary displacement into digital or pulse signals The most popular type of encoder is the optical encoder which uses a rotating disk with alternating opaque areas a light source and a photodetector the condition or state of an analog or digital signal an onboard EEPROM that may contain device specific initialization and system boot functionality external digital trigger a voltage pulse from an external source that triggers an event such as A D conversion National Instruments Corporation G 7 NI DSA Software User Manual Glossary F false triggering FIFO filtering FIR flash ADC floating signal sources ft G gain gain accuracy NI DSA Software User Manual triggering that occurs at an unintended time first in first out memory buffer the first data stored is the first data sent to the acceptor FIFOs are often used on DAQ devices to temporarily store incoming or outgoing data until that data can be retrieved or output For example an analog input FIFO stores the results of A D conversions until the data can be retrieved into system memory a process that requires the servicing of interrupts and often the programming of the DMA controller This process can take
115. ode the time returned is a multiple of the duration parameter of Configure OLM Averaging iyi Note In one shot mode you can specify any time duration If the end of the time duration falls between sample intervals the closest sample is included in the duration In auto restart mode the time duration can only be an integer multiple of 10 milliseconds If any other duration is specified it is rounded off to the nearest 10 milliseconds National Instruments Corporation 6 33 NI DSA Software User Manual Chapter 6 Advanced Concepts NI DSA Software User Manual For level measurements the linear average is denoted as Leq equivalent sound power over the specified time duration Exponential Averaging Exponential averaging is a continuous averaging process that weighs both current as well as past data The amount of weight given to past data as compared to current data depends on the exponential time constant The higher the time constant the more weight given to past data whereas the lower the time constant the more weight is placed on the current data The formula for exponential averaging is as follows y k al x y k 1 a2 x x k Where x k is the new measurement y k is the new average y k 1 is the previous average al exp 1 0 time constant x sampling rate and a2 1 al The value of the time_constant depends on the type of exponential averaging selected The various types of exponential averaging available are
116. ons Table A 1 Exact Center Frequencies for 1 12 Octave Analysis Continued NI DSA Software User Manual das ANSI Standard Exact IEC Standard Exact octave Center Frequencies Hz Center Frequencies Hz 30 5 656 85 5 822 61 31 5 993 23 6 168 84 32 6 349 60 6 535 66 33 6 727 17 6 924 29 34 7 172 19 7 336 03 35 7 550 99 7 772 26 36 8 000 8 234 42 37 8 475 70 8 724 06 38 8 979 70 9 242 82 39 9 513 66 9 792 43 40 10 079 37 10 374 72 4 10 678 72 10 991 63 42 11 313 71 11 645 23 43 11 986 46 12 337 69 44 12 699 21 13 071 32 45 13 454 34 13 848 58 46 14 254 38 14 67206 47 15 101 99 15 545 51 48 16 000 16 468 84 49 16 951 41 17 448 12 50 17 957 39 18 485 64 51 19 027 31 19 584 86 52 20 158 74 20 749 43 53 21 357 44 21 983 26 A 16 ni com Technical Support Resources Web Support National Instruments Web support is your first stop for help in solving installation configuration and application problems and questions Online problem solving and diagnostic resources include frequently asked questions knowledge bases product specific troubleshooting wizards manuals drivers software updates and more Web support is available through the Technical Support section of ni com NI Developer Zone The NI Developer Zone at ni com zone is the essential resource for building measurement and automation systems At the NI Developer
117. ons these two constraints prevent NI DSA from generating the precise analog frequency you request If you are unsure about the frequency that is being generated at any time you can query the software for the actual analog frequency by calling Get Sine Source Settings In many cases the DSA device generates the requested frequency exactly and if it does not the discrepancy is rarely exceeds a few hertz There are also a few cases in which NI DSA is unable to coerce the output frequency to a nearby value and returns an error For example the baseband span is 12 000 Hz corresponding to a hardware sampling rate of 30 720 S s You request an output sine frequency at 16 000 Hz which requires an analog output sampling rate greater than 32 000 S s because of the Nyquist frequency 30 720 S s the input rate is not fast enough Doubling this output sampling rate to a value greater than 64 000 S s is also not feasible because the hardware supports a maximum output rate of 51 200 S s In this example the best solution is to increase the input baseband span to 16 000 Hz or greater speeding up the input sampling rate and allowing you to obtain higher output sampling rates A 6 ni com Appendix A Common Questions Swept Sine Mode What is the advantage to using swept sine analysis rather than a broadband FFT based frequency response Swept sine analysis provides two main advantages over a broadband frequency response measurement in which all f
118. ons those with EU in their names let you configure read and display measurements in eight standard units including Pa g and m s or in any custom unit you specify Configure Engineering Units e Configure EU EU Mode enables or disables engineering units for selected channels e Configure EU Label selects the label according to the type of sensor you are using to match the units you are using EU Label Select selects V Pa g m s in s m s in s or a custom label EULabelString if EU Label Select custom can contain the label text for example f s Configure EU Scale EU scale sets the scale factor for EU Scale Type depending on the sensitivity of your sensor scale type V EU EU V or dB EU V Configure EU dB Reference enters the reference level for your measurements For example assume you are using a microphone as your transducer to measure sound pressure which is usually expressed in Pa The sensitivity of the microphone is 20 mV Pa You call Configure EU Label to set the label to Pa Configure EU Scale to set the scale factor to 0 020 20 mV 0 020 V and the EU scale to V EU With these settings a 1 V signal from the microphone is presented as a sound pressure of 94 dB so you set the dB reference to 20 uPa 0 00002 Pa National Instruments Corporation 3 15 NI DSA Software User Manual Chapter 3 FFT Mode Programming LabVIEW Example ELI Parameters
119. oundaries as shown in Figure 6 3 Time Record Repetition Figure 6 3 Waveform Discontinuity at Record Boundary These artificial discontinuities were not originally present in your signal and result in a smearing or leakage of energy from your actual frequency to all other frequencies This phenomenon is known as spectral leakage The amount of leakage depends on the amplitude of the discontinuity with a larger discontinuity causing more leakage Because the amount of leakage is dependent on the amplitude of the discontinuity at the boundaries you can use windowing to reduce the size of the discontinuity and hence reduce spectral leakage Windowing consists of multiplying the time domain signal by another time domain waveform known as a window whose amplitude tapers gradually and smoothly toward zero at the edges The result is a windowed signal 6 12 ni com Chapter 6 Advanced Concepts with no or very small discontinuities and thus reduced spectral leakage There are many different types of windows The one you choose depends on your application Figure 6 4 illustrates a window applied to a single waveform record The record contains about 1 25 cycles of the sine wave Window Figure 6 4 Window Applied to Waveform Record Figure 6 5 illustrates two cycles of the windowed waveform with the discontinuity between records is no longer present Windowing Figure 6 5 W
120. own as the inner product or scalar product of the time domain signal with the complex exponential at the component frequency The discrete Fourier transform DFT describes the relationship between the time domain signal x n and the frequency domain signal X k N 1 2mnk X k x x n e 0 N 1 C n 0 The term FFT refers to any of several efficient algorithms for computing the DFT The FFT is significantly faster than the DFT and the performance advantage grows as N increases When k 0 the DC component is the dot product of x n with cos 0 jsin 0 or with 1 0 When 1 the first bin or frequency component is the dot product of x n with cos 2mm N jsin 2mn N Here 2 N is a single cycle of the cosine wave and 2 is a single cycle of the sine wave In general the real part of FFT bin k Re X 4 is the dot product of x n with cycles of the cosine wave and the imaginary part of bin k Im X k is the dot product of x n with k cycles of the sine wave The FFT output X k is complex containing real and imaginary components for each index k For real world applications the magnitude and phase of the frequency domain signal is of more interest than the complex output X k They can be computed using these formulae which are essentially a cartesian to polar coordinate conversion e Magnitude IX k JRe X k x Im X k X k x X k e Phase
121. pically removed by digital filters a plug in data acquisition board card or pad that can contain multiple channels and conversion devices Plug in boards PCMCIA cards and devices such as the DAQPad 1200 which connects to your computer parallel port are all examples of DAQ devices SCXI modules are distinct from devices with the exception of the SCXI 1200 which is a hybrid digital ground signal differential mode an analog input consisting of two terminals both of which are isolated from computer ground whose difference is measured a way you can configure your device to read signals in which you do not need to connect either input to a fixed reference such as the earth or a building ground See port a TTL level signal having two discrete levels a high and a low level digital input output direct memory access a method by which data can be transferred to from computer memory from to a device or memory on the bus while the processor does something else DMA is the fastest method of transferring data to from computer memory G 6 ni com DNL down counter drivers dynamic range E encoder event expansion ROM EXT TRIG external trigger Glossary differential nonlinearity a measure in least significant bit of the worst case deviation of code widths from their ideal value of 1 LSB performing frequency division on an internal signal software that controls a specific hardware device such as a DAQ b
122. potential differences and transients the amount of time required for the analog output voltage to reach its final value within specified limits the maximum rate of change of analog output voltage from one level to another the range of frequencies which a device can properly propagate or measure a type of handshaked latched digital I O in which internal counters generate the handshaked signal which in turn initiates a digital transfer Because counters output digital pulses at a constant rate this means you can generate and retrieve patterns at a constant rate because the handshaked signal is produced at a constant rate NI DSA Software User Manual Glossary PCI peak to peak PFI phase suppression Plug and Play devices port posttriggering potentiometer ppm pretriggering propagation propagation delay pts pulse trains pulsed output NI DSA Software User Manual Peripheral Component Interconnect a high performance expansion bus architecture originally developed by Intel to replace ISA and EISA It is achieving widespread acceptance as a standard for PCs and work stations it offers a theoretical maximum transfer rate of 132 Mbytes s a measure of signal amplitude the difference between the highest and lowest excursions of the signal programmable function input a feature in FFT mode that lets you set a threshold to effectively filter out noise from signal phase information devices that do no
123. r 476 including the DC bin It does not have 512 lines because the width of the anti alias filter transition band is non zero To provide alias protection at the Nyquist frequency f 2 the filter also rejects some of the frequencies below the Nyquist frequency 475 lines 204 800 S s _ 1024 5 DTE The spectrum obtained represents the frequency range from DC to 95 kHz with 475 bins spaced every 200 Hz as shown here _ span 95 kHz lines N 204 8 kHz 1024 475 6 4 ni com Chapter 6 Advanced Concepts Af span _ 95 kHz se ps oS Every frequency component of the FFT has a magnitude and phase The phase is relative to the start of the time record or relative to a cosine wave starting at the beginning of the time record Single channel phase measurements are stable only if the input signal is triggered Dual channel phase measurements compute phase differences between channels so that if the channels are simultaneously sampled triggering is usually not necessary Dual Channel FFT Analysis A typical application for dual channel analysis is measuring the frequency response or transfer function of a system To characterize the cutoff frequency of an analog low pass filter you could send signals of every frequency within the filter s range and measure how much each signal is attenuated A better way to do it is to send a signal that has all the frequency components in it and see how much each of them is attenuated
124. r excitation transfer rate trigger TTL U unipolar update update rate V VI National Instruments Corporation G 17 Glossary total harmonic distortion the ratio of the total rms signal due to harmonic distortion to the overall rms signal in decibel or a percentage signal to THD plus noise the ratio in decibels of the overall rms signal to the rms signal of harmonic distortion plus noise introduced the data measured in bytes s for a given continuous operation calculated to include software overhead See sensor a type of signal conditioning that uses external voltages and currents to excite the circuitry of a signal conditioning system into measuring physical phenomena the rate measured in bytes s at which data is moved from source to destination after software initialization and set up operations the maximum rate at which the hardware can operate any event that causes or starts some form of data capture transistor transistor logic a signal range that is always positive for example 0 to 10 V the output equivalent of a scan One or more analog or digital output samples Typically the number of output samples in an update is equal to the number of channels in the output group For example one pulse from the update clock produces one update which sends one new sample to every analog output channel in the group the number of output updates per second volts virtual instrument 1
125. r generating signals for frequency response measurements and so on NIDSA_Initialize Y NIDSA Set Output Voltage Range 100 1V or 10V y NIDSA_Set_Hardware_Update_Range 51200 Max Y NIDSA Configure Source Sine Chirp Noise or Arb A This Function Will Depend NIDSA Configure Sine Source on the Selection Made Above lt NIDSA Close Figure 4 5 Source Mode Flowchart Step 2 Configure Source e Set Output Voltage Range sets the desired output voltage range for the selected output channel outputSelect analog output channel select to be configured e 0 channel 0 e 1 channel 1 e configures all channels simultaneously hy Note Presently only one output channel 0 is supported National Instruments Corporation 4 11 NI DSA Software User Manual Chapter 4 Swept Sine Mode Programming NI DSA Software User Manual voltageRange voltage range for the selected output channel e 0 10 0V e x1 00V e 2 100mV e 3 OFF not supported e Set Hardware Update Rate sets the hardware update rate Default Value 51200 0 S s Valid Range 1 250 0 to 51 200 0 S s e Configure source configures the source output turns the output signal on or off and sets the desired output signal type Valid source signal types are Sine dual tone Chirp Noise and Arbitrary sourceOn turns the output source signal on or of
126. requencies are excited and measured simultaneously The first is a wider dynamic range NI DSA can automatically adjust the input gain and output attenuation to maximize the dynamic range of your measurements If your system s response at a particular frequency is very weak the driver uses a higher gain setting on input Likewise NI DSA can attenuate the output signal and reduce the input gain in when the response is strong to eliminate clipping The second advantage that swept sine provides is flexible measurements of signal distortion and nonlinearity quantifying nonlinear behavior is often important in audio applications Swept sine analysis allows you to measure distortion because it provides excitation at only a single discrete frequency at any given time For example assume you are exciting an amplifier with a 1 kHz tone The basic response of the amplifier can be measured at 1 kHz and the total harmonic distortion THD can be found by summing the response amplitudes at 2 kHz 3 kHz 4 kHz and so on This is not feasible with broadband excitation because there is no way to determine if a 2 KHz response signal is a harmonic from 1 kHz or if it is simply the linear response to excitation at 2 KHz When using swept sine analysis why does it take longer to test low frequencies than higher ones In order to measure the response at a particular frequency NI DSA must generate at least one complete sinusoidal cycle at that frequency This cyc
127. rray containing the level values You can also call Get Level Labels to generate a string array containing the corresponding until labels for the level measurements The octave analyzer engine includes RMS meters that allow you to perform level measurements on the unweighted or weighted time domain waveform Averaging Modes You can choose between several averaging modes for both octave and level measurements Averaging of octave measurements is performed on the filtered outputs of the bandpass filters whereas averaging for level measurements is performed on the input time domain data For both octave and level measurements averaging is done on the power magnitude squared value An explanation of the various types of averaging modes available are e None Jn this case no averaging is done The result of each measurement is made available without combining it with any other measurement Linear For linear averaging a weighted average is performed over the time duration specified in Configure OLM Averaging All measurements made over the time duration are weighted equally Linear Averaging The formula for linear averaging is as follows y k al x y k 1 a2 X x k where x k is the new measurement y k is the new average y k 1 is the previous average al k 1 k a2 1 k and k is an integer equal to the number of measurements averaged so far Linear averaging returns the arithmetic mean of the measurements T
128. s welders radio transmitters and internal sources such as semiconductors resistors and capacitors Noise corrupts signals you are trying to send or receive atype of digital acquisition generation where LabVIEW updates the digital lines or port states immediately or returns the digital value of an input line Also called immediate digital I O or non handshaking G 12 ni com nonreferenced signal sources NRSE Nyquist Frequency 0 onboard channels operating system optical isolation output settling time output slew rate P passband pattern generation National Instruments Corporation G 13 Glossary signal sources with voltage signals that are not connected to an absolute reference or system ground Also called floating signal sources Some common example of nonreferenced signal sources are batteries transformers or thermocouples nonreferenced single ended mode all measurements are made with respect to a common NRSE measurement system reference but the voltage at this reference can vary with respect to the measurement system ground one half of the sampling rate f channels provided by the plug in data acquisition board base level software that controls a computer runs programs interacts with users and communicates with installed hardware or peripheral devices the technique of using an optoelectric transmitter and receiver to transfer data without electrical continuity to eliminate high
129. set its power on state Step 2 Configure Selects the measurement mode and sets up the instrument to perform specific operations Step 3 Control Starts stops and checks the status of instrument operations Step 4 Read Transfers data from the instrument to the host computer Step 5 Close Terminates communication between the software and the instrument and deallocates system resources In addition to these operations there are operations performed using utility functions Utility functions perform auxiliary operations Examples of Utility functions include Reset Self Test National Instruments Corporation 2 1 NI DSA Software User Manual Chapter 2 Programming with NI DSA Revision Query Error Query Error Message Notes for C Programmers The constants used in the example programs are defined in the NI DSA header file nidsa h The C code snippets in this manual use these constants for clarity VISA data types defined in visatype h are also used Step 1 Initialize Establishes a connection between your instrument and your application by using Initialize Initialize returns session number which your application uses as a handle to communicate other function calls to the instrument You can also use Initialize to query the device name or to reset the instrument LabVIEW Example resource name reset device Cu Figure 2 1 Initialize C Example NIDSA init
130. sis using impulse averaging and A weighting the maximum center frequency can be as high as 2 378 Hz ANSI or 2 448 Hz IEC for real time processing If you sacrifice the weighting change to exponential fast averaging mode or both the maximum center frequency can be higher Frequency Ranges For each fractional octave mode Table 6 7 summarizes the lowest and highest center frequencies for a single channel at different sampling rates For both 1 1 octave and 1 3 octave the frequencies are specified as preferred frequencies from the corresponding ANSI and IEC standards For 1 12 octave they are specified in terms of the exact center frequencies Table 6 7 Octave Analyzer Frequency Ranges for a Single Channel Analysis Center Frequencies at Center Frequencies at Type f 25 6 kHz f 51 2 kHz 1 1 octave High 8 kHz High 16 kHz Low 16 Hz Low 31 5 Hz 1 3 octave High 10 kHz High 20 kHz Low 12 5 Hz Low 25 Hz 1 12 octave High 10 678 72 Hz ANSI Low 11 05 Hz ANSI High 10 991 63 Hz IEC Low 11 37 Hz IEC High 5 339 36 Hz ANSI Low 5 52 Hz ANSI High 5 495 81 Hz IEC Low 5 69 Hz IEC Settling Time The fractional octave filters require a certain amount of time before any valid measurement is available at the outputs of the filter This time is the settling time and is inversely proportional to the bandwidth of the filter according to the formula settling time 1 5 x ban
131. t require DIP switches or jumpers to configure resources on the devices also called switchless devices 1 a communications connection on a computer or a remote controller 2 a digital port consisting of four or eight lines of digital input and or output the technique used on a DAQ board to acquire a programmed number of samples after trigger conditions are met an electrical device the resistance of which can be manually adjusted used for manual adjustment of electrical circuits and as a transducer for linear or rotary position parts per million the technique used on a DAQ board to keep a continuous buffer filled with data so that when the trigger conditions are met the sample includes the data leading up to the trigger condition the transmission of a signal through a computer system the amount of time required for a signal to pass through a circuit points multiple pulses a form of counter signal generation by which a pulse is outputted when a counter reaches a certain value G 14 ni com Q quantization error quantizer real time relative accuracy resolution resource locking retry ribbon cable rise time rms National Instruments Corporation G 15 Glossary the inherent uncertainty in digitizing an analog value due to the finite resolution of the conversion process a device that maps a variable from a continuous distribution to a discrete distribution a property of an event or system in whi
132. the ADC are processed as they arrive at the processor and none are lost However it is possible to exceed the capacity of the processor to handle data in real time The ability of the processor to keep up with the incoming data depends on the following factors e Sampling frequency and number of input channels the default sampling rate of the NI 45XX in octave analyzer mode is 51 2 kHz You can use Set OLM Sampling Rate to set the sampling rate to one of 6 36 ni com Chapter 6 Advanced Concepts two values 25 6 kHz or 51 2 kHz The NI 4551 has two input channels and the NI 4552 has four input channels The higher the number of channels used the more difficult it is for the processor to keep up in real time With an NI 4552 the more input channels that you use for fractional octave analysis the lower the highest center frequency analyzed must be in order for real time processing to be possible The NI 4552 is unable to perform 1 3 octave analysis on all four channels at the default sampling rate even with the highest center frequency set to 4 kHz If you use three or more channels use Set OLM Sampling Rate to set the sampling rate to 25 6 kHz so you can process all four channels with a high center frequency of up to 10 kHz Fractional octave mode 1 1 1 3 or 1 12 frequency range and averaging mode 1 12 octave analysis requires the most processing and 1 1 octave the least A higher center frequency requires more processing th
133. the National Instruments installation operation or maintenance instructions owner s modification of the product owner s abuse misuse or negligent acts and power failure or surges fire flood accident actions of third parties or other events outside reasonable control Copyright Under the copyright laws this publication may not be reproduced or transmitted in any form electronic or mechanical including photocopying recording storing in an information retrieval system or translating in whole or in part without the prior written consent of National Instruments Corporation Trademarks LabVIEW National Instruments NI and ni com are trademarks of National Instruments Corporation ICP is a registered trademark of PCB Piezotronics Inc Other product and company names mentioned herein are trademarks or trade names of their respective companies WARNING REGARDING USE OF NATIONAL INSTRUMENTS PRODUCTS 1 NATIONAL INSTRUMENTS PRODUCTS ARE NOT DESIGNED WITH COMPONENTS AND TESTING FOR A LEVEL OF RELIABILITY SUITABLE FOR USE IN OR IN CONNECTION WITH SURGICAL IMPLANTS OR AS CRITICAL COMPONENTS IN ANY LIFE SUPPORT SYSTEMS WHOSE FAILURE TO PERFORM CAN REASONABLY BE EXPECTED TO CAUSE SIGNIFICANT INJURY TO A HUMAN 2 IN ANY APPLICATION INCLUDING THE ABOVE RELIABILITY OF OPERATION OF THE SOFTWARE PRODUCTS CAN BE IMPAIRED BY ADVERSE FACTORS INCLUDING BUT NOT LIMITED TO FLUCTUATIONS IN ELECTRICAL POWER SUPPLY COMPUTER
134. the Readme htm file that was installed with the instrument driver for last minute changes and updates 2 Choose the measurement mode that is best for your application For more information about NI DSA measurement modes refer to the NI DSA Measurement Modes section 3 Begin programming your DSA instrument Detailed programming instructions and examples for each of the measurement modes as shown in this list National Instruments Corporation For basic NI DSA programming instructions common to all NI DSA applications refer to Chapter 2 Programming with NI DSA For specific FFT mode programming instructions and examples refer to Chapter 3 FFT Mode Programming For programming examples and instructions for swept sine mode and source mode refer to Chapter 4 Swept Sine Mode Programming For instructions and examples of octave and level analyzer mode programming refer to Chapter 5 Octave Analysis Add On Mode Programming 1 1 NI DSA Software User Manual Chapter 1 Introduction to NI DSA Related Documentation e National Instruments Application Note 41 The Fundamentals of FFT Based Signal Analysis and Measurement NIDSA Help for C Programmers and NIDSA Help for Basic Programmers have detailed information about the NI DSA functions and parameters If you have installed NI DSA the default location of these help files is Start Programs National Instruments Ni dsa e Help for LabVIEW VIs is available
135. time and this frequency can be applied full scale With auto ranging swept sine measurements can be further optimized for each frequency according to the output level of the device under test so the signal level is as close to full scale as possible Optimizing the input range at each frequency rather than applying the frequency rich time domain data can extend the dynamic range of the measurement to beyond 140 dB For example if the transfer function has both gain and attenuation the auto level control will adjust the source level to compensate for the output of the system If the system under test has gain at one certain frequency the amplitude of the sweep source can be reduced automatically to prevent the output of the system from overloading the DSA input channel At the points with significant attenuation the source level can be increased to boost the system output level to provide a better signal to noise ratio Point by point optimization gives swept sine analysis a great advantage over FFT measurement Using auto resolution mode can greatly speed up your swept sine measurements Often you want to measure a frequency response with a large span and you want to preserve the frequency resolution With the FFT method the only option is to increase the size of the FFT This consumes memory and increases measurement time In auto resolution mode even though a large number of points is still needed to increase the frequency resolution the
136. tion 2 5 NI DSA Software User Manual FFT Mode Programming Figure 3 1 shows the recommended program flow for baseband FFT analysis Figure 3 2 shows the recommended program flow for zoom FFT analysis For a detailed discussion of FFT analysis refer to the Step 2 Configure the FFT Analyzer section National Instruments Corporation 3 1 NI DSA Software User Manual Chapter 3 FFT Mode Programming Initialize Y Configure Mode Set Input Voltage Range y y Configure Trigger Configure Base Engine Set Classical Base Band Configure Configure Base FFT Settings Configure Base Averaging Check New Measurement Get Measurement Length Read Measurement Close Configure Set Input Coupling Hardware Front End Parameters NI DSA Software User Manual Figure 3 1 Baseband FFT Programming Flowchart ni com Chapter 3 FFT Mode Programming Initialize Y Configure Mode Set Input Voltage Range y Configure Set Input Coupling Hardware Front End y Configure Trigger Configure Zoom FFT Engine y Configure Zoom FFT Span y Configure Zoom Frequencies Configure Zoom FFT Parameters Configure Zoom FFT Settings Configure Zoom FFT Averaging No Check New Measurement Get Measurement Length Read
137. to all input channels or to apply different parameters to one or any combination of input channels independently of the others LabVIEW Example input select Free Run NIDSA NIDSA Set Input NIDSA Set Input NIDSA Configure Configure Voltage Range vi Coupling vi Trigger vi DSA Mode vi Figure 2 2 Configure National Instruments Corporation 2 3 NI DSA Software User Manual Chapter 2 Programming with NI DSA C Example Configure Hardware settings Put hardware in FFT mode NIDSA configure dsa mode DSASession FFT MODE Set all channels for input range of 10 V NIDSA set input voltage range DSASession 1 VRANGE 10V NIDSA set input coupling DSASession 1 COUP AC NIDSA configure trigger DSASession FREERUN TriggerChannel Level Hysteresis Slope Step 3 Read NI DSA Software User Manual Delay Notes on Examples By default NI DSA returns all measurements in volts but you can have NI DSA return measurements in the engineering units EU that you choose Configure EU used with the other Configure EU functions lets you choose the units for your measurements and scale them accordingly More information about using engineering units refer to Chapter 3 FFT Mode Programming the Using Engineering Units section You have now configured your hardware for the input signal and are now ready to configure your measurements e To configure FFT measurem
138. uencies use Configure Swept Sine Custom to specify the sweep frequencies The parameters for this function are e Buffer size sets the size of the custom frequencies array up to 2 048 e Custom frequencies array specifies the frequencies to step through Step 3 Read Swept Sine Measurement Before you can read a measurement you need to know if it has been completed Check New Measurement returns a 1 if a new measurement is ready You must call Check New Measurement before reading the first measurement and before reading any subsequent measurement set Reading a New Measurement To read a new measurement use the following functions e Get Measurement Length returns the number of data points to read based on the type and length of the measurement and the set of frequency lines you choose with frqLineSelector National Instruments Corporation 4 5 NI DSA Software User Manual Chapter 4 Swept Sine Mode Programming NI DSA Software User Manual Values for frqLineSelector are Valid lines returns alias free lines only lines returns all FFT lines A Classical lines returns alias free classical lines Read Measurement treads the measurement from the specified analyzer channel The valid measurement Type settings for Get Measurement Length and Read Measurement are limited to the measurements that you specified with Configure Swept Sine Configure Swept Sine Average and Configure Swept Sine Harmoni
139. ure Swept Sine Configure Configure Swept Sine Average Swept Sine Parameters Configure Swept Sine Source Start Sweeping Control Swept Sine Restart Check New Measurement Get Measurement Length Get Swept Frequencies Read Measurement Get Measurement Length Get Frequency Response Read Measurement Get Swept Sine Status Plots Frequency Response vs Frequencies in a XY Graph Status Done Close Figure 4 1 Swept Sine Programming Flowchart ni com Chapter 4 Swept Sine Mode Programming Step 2 Configure the Swept Sine Analyzer Configuring Inputs Configure frequency characteristics of inputs to your device under test DUT with Configure Swept Sine The primary Configure Swept Sine parameters are described in the following list Sweep mode choose from the following Normal sweep from Start Frequency to Stop Frequency Auto Resolution automatically adjust sweep within Auto Max Step Fast Threshold dB and Slow Threshold dB parameters Custom frequencies use Configure Swept Sine Customto set sweep parameters Linlog set linear or logarithmic frequency steps Start frequency starting sweep frequency Stop frequency ending sweep frequency e Number of steps number of frequency steps in sweep Set the sine sweep settling time and integration time with Configure Swept Sine Average Configuring the Source
140. ut scan rate is between 51 2 kHz and 102 4 kHz and one fourth of the scan rate if it is between 102 4 kHz and 204 8 kHz Both of the outputs on the NI 4551 work as a single output if the device is used in DSA Instrument Mode through NI DSA Output channel 0 generates the specified output and channel generates the same signal with a 180 degree shift If you need to operate the two output channels of your NI 4551 independently program the instrument using NI DAQ LabVIEW Examples NIDSA Set NIDSA Set NIDSA Configure NIDSA Configure Output Voltage Hardware Source vi Sine Source vi Range vi Update Rate vi de Figure 4 6 Configuring the Source National Instruments Corporation 4 13 NI DSA Software User Manual Chapter 4 Swept Sine Mode Programming C Examples Set Output voltage range to 10 V NIDSA set output voltage range DSASession 0 OUTVRANGE 10V Set update rate to 51 2 kS s NIDSA set hw sampling rate DSASession 51200 Turn source ON and select a sine wave output signal NIDSA configure source DSASession SRC ON SINE SRC Configure sine source Note This function allows a dual tone generation To generate a single tone set Amplitude2 and Freq2 parameters to 0 NIDSA configure sine source DSASession Freql Amplitudel Freq2 Amplitude2 DCOffset Step 3 Generate Signal The preceding step actually starts the generation of the signal In this step you control the duratio
141. vel measurement Why can t I change the sampling frequency of the DSA device to arbitrary values when in octave mode To ensure compliance with existing ANSI and IEC standards the sampling frequency of the device can be set to either 51 200 Hz or 25 600 Hz when in octave mode by using Set OLM Sampling Rate It is not possible to change the sampling frequency to another value using Set Hardware Sampling Rate Youcan read the current value of the sampling frequency by using Get Hardware Sampling Rate How long does the signal take to propagate through the input section of the NI 4551 4552 The signal delay the time it takes for a signal entering an analog input channel to become available as digital data at the output of the delta sigma ADC is 42 sample periods regardless of the sample rate for example when using the octave mode and a sampling rate of 51 2 kS s In octave analyzer mode the sampling frequency is fixed at 25 600 Hz or 51 200 Hz The signal delay with a sample rate of 51 200 Hz is approximately 0 82 ms 42 S 51 200 S s 0 82 ms Thus it takes about 0 82 milliseconds for any change in the analog input signal to be available for processing by the fractional octave analyzer What are the center frequencies for 1 12 octave analysis The exact center frequencies depend on which standard ANSI or IEC you choose The ANSI and IEC standard center frequencies for 1 12 octave analysis are as shown in Table A 1 For refer
142. ware User Manual The original frequency span center is then shifted to DC or heterodyned this means that the upper half of the span stays positive and the lower half below the span center becomes a set of negative frequencies The result is that a span from 0 to 80kHz becomes a span of 40 to 40 kHz This data goes through a digital low pass filter which cuts off at 40 kHz this operation results in a 40 kHz usable span centered at 40 kHz The sampling rate needed is then only 102 4 kS s instead of the original 204 8 kS s hardware sampling rate defined by the selected baseband of 80kHz so only every other point of the original span is used At this point the original baseband span is represented by a complex time record with half as many points as the original one half the sampling rate and therefore same duration both the real and imaginary part of this complex record have half of the original sample rate and half of the original span If you keep the hardware sampling rate constant at 204 8 kHz the NI 45XX can digitally halve the effective sampling rate by passing the digital samples through one stage of a decimate by 2 lowpass filter Digital filtering and down sampling are used to narrow the heterodyned data by zooming around the heterodyne frequency the center of the original span So while the first filter reduces the sampling rate by half and the number of samples by splitting the time record in real and imaginary parts the secon

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