TDS3FFT FFT Application Module User Manual
Contents
1. channel instru ments Ch 1 through Ch 4 4 channel instruments and Ref 1 through Ref 4 all instru ments SetFFT Vert Sets the display vertical scale Scale to units Available scales are dBV RMS and Linear RMS Set FFT Sets which window function Window to Hanning Hamming Blackman Harris or Rectangular to apply to the source signal Refer to page 12 for more FFT window information FFT Source Key Points Push the side menu button to select the source m Using FFT slows down the oscilloscope s response time in Normal acquisition mode 10k record length m A waveform acquired in Normal acquisition mode has a lower noise floor and better frequency resolution than a waveform acquired in Fast Trigger mode Signals that have a DC component or offset can cause incorrect FFT waveform component magnitude values To minimize the DC component choose AC Coupling on the source signal To reduce random noise and aliased components in repetitive or single shot events set the oscilloscope acquisition mode to average over 16 or more samples Average mode attenuates signals not synchronized with the trigger Do not use the Average acquisition mode if the source signal contains frequencies of interest that are not synchronized with the trigger rate Do not use Peak Detect and Envelope modes with FFT Peak Detect and Envelope modes can add significant distortion to the FFT results For transient impulse one shot sign2. Advanced Test Equipment Rentals A ished 1981 www atecorp com 800 404 ATEC 2832 User Manual TDS3FFT FFT Application Module 071 0349 01 HA O E O TN 071034901 Copyright O Tektronix Inc All rights reserved Tektronix products are covered by U S and foreign patents issued and pending Information in this publication supercedes that in all previously published material Specifications and price change privileges reserved Tektronix Inc P O Box 500 Beaverton OR 97077 TEKTRONIX TEK TEKPROBE and Tek Secure are registered trademarks of Tektronix Inc DPX WaveAlert and e Scope are trademarks of Tektronix Inc WARRANTY SUMMARY Tektronix warrants that the products that it manufactures and sells will be free from defects in materials and workmanship for a period of one 1 year from the date of shipment from an authorized Tektronix distributor If a product proves defective within the respective period Tektronix will provide repair or replacement as described in the complete warranty statement To arrange for service or obtain a copy of the complete warranty statement please contact your nearest Tektronix sales and service office EXCEPT AS PROVIDED IN THIS SUMMARY OR THE APPLICABLE WARRANTY STATEMENT TEKTRONIX MAKES NO WARRANTY OF ANY KIND EXPRESS OR IMPLIED INCLUDING WITHOUT LIMITATION THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE IN NO EVENT SHALL TEKTRONIX BE LIABL
3. E FOR INDIRECT SPECIAL OR CONSEQUENTIAL DAMAGES Contacting Tektronix Product Support Service Support Toll free Number Postal Address Web Site For questions about using Tektronix measurement products call toll free in North America 1 800 833 9200 6 00 a m 5 00 p m Pacific time Orcontact us by e mail support tektronix com For product support outside of North America contact your local Tektronix distributor or sales office Tektronix offers a range of services including Extended Warranty Repair and Calibration services Contact your local Tektronix distributor or Sales office for details Fora listing of worldwide service centers visit our web site In North America 1 800 833 9200 An operator can direct your call Tektronix Inc Department or name if known P O Box 500 Beaverton OR 97077 USA www tektronix com Contents Safety Summary 000s 2 Installing the TDS3FFT Application Module 5 Introduction cece eee eee eee eee ees 5 FFT Features 0c ccc eee e eee eee eee eens 6 Displaying an FFT Waveform 7 FFT Math Menu cece eee eee teens 8 FFT Windows 0 cece cece eee eee eee 12 Aliasing 0 eee eee eee 14 FFT Examples iii ess massas e a me eens 16 Safety Summary To avoid potential hazards use this product only as specified While using this product you may need to access other parts of the system Read the General Safe
4. als set the oscilloscope to trigger on the transient pulse in order to center the pulse information in the waveform record FFT Vertical Scale Key Points E Push the side menu button to select a scale Available scales are dBV RMS and Linear RMS m Use the Vertical POSITION and SCALE knobs to vertically move and rescale the FFT waveform m To display FFT waveforms with a large dynamic range use the dBV RMS scale The dBV scale displays component magnitudes using a log scale expressed in dB relative to 1 Vrms where O dB 1 Ves Or in source waveform units such as amps for current measurements m To display FFT waveforms with a small dynamic range use the Linear RMS scale The Linear RMS scale lets you display and directly compare components with similar magnitude values Nyquist Frequency Key Point m To determine the Nyquist frequency push the ACQUIRE menu button This displays the current sample rate on the bottom right area of the screen The Nyquist frequency is one half of the sample rate For example if the sample rate is 25 0 MS s then the Nyquist frequency is 12 5 MHz 10 Zooming an FFT Display Use the Zoom button along with horizontal POSITION and SCALE controls to magnify FFT waveforms When you change the zoom factor the FFT waveform is horizontally magnified about the center vertical graticule and vertically magni fied about the math waveform marker Zooming does not affect the actual time base or tr
5. igger position settings NOTE FFT waveforms are calculated using the entire source waveform record Zooming in on a region of either the source or FFT waveform will not recalculate the FFT waveform for that region Measuring FFT Waveforms Using Cursors You can use cursors to take two measurements on FFT waveforms magnitude in dB or signal source units and frequency in Hz dB magnitude is referenced to 0 dB where 0 dB equals 1 Vrms Use horizontal cursors H Bars to measure magnitude and vertical cursors V Bars to measure frequency Magnitude cursors Frequency cursors 11 FFT Windows Applying a window function to the source waveform record changes the waveform so that the start and stop values are close to each other reducing FFT waveform discontinuities This results in an FFT waveform that more accurately represents the source signal frequency components The shape of the window determines how well it resolves frequency or magnitude information MARA Source waveform 12 Waveform data points x With windowing FFT Window Characteristics Best for measuring Blackman Best magnitude Predominantly single fre Harris worstatresolving quency waveforms to look frequencies for higher order harmonics Hamming Better frequency Sine periodic and narrow Hanning poorer magnitude band random noise accuracy than Rectangula
6. mathematically converts the standard time domain signal repetitive or single shot acquisition into its frequency components providing spectrum analysis capabilities Being able to quickly look at a signal s frequency components and spectrum shape is a powerful research and analysis tool FFT is an excellent troubleshooting aid for m Testing impulse response of filters and systems m Measuring harmonic content and distortion in systems m Identifying and locating noise and interference sources m Analyzing vibration m Analyzing harmonics in 50 and 60 Hz power lines FFT Features The FFT application module provides the following features FFT Windows Four FFT windows Rectangular Hamming Hanning and Blackman Harris let you match the optimum window to the signal you are analyzing The Rectangular window is best for nonperiodic events such as transients pulses and one shot acquisitions The Hamming Hanning and Blackman Harris windows are better for periodic signals Analyze Repetitive Single Shot and Stored Waveforms You can display an FFT waveform on any actively ac quired signal periodic or one shot the last acquired signal or any signal stored in reference memory dB or Linear RMS Scales The FFT vertical graticule can be set to either dB or Linear RMS A dB scale is useful when the frequency component magnitudes cover a wide dynamic range letting you show both lesser and greater magnitude frequenc
7. ncy 14 If the increased number of frequency components shown on the screen makes it difficult to measure individual components use the Zoom button to magnify the FFT waveform Use a filter on the source signal to bandwidth limit the signal to frequencies below that of the Nyquist frequency If the components you are interested in are below the built in bandwidth settings 20 MHz bandwidth for all oscilloscopes 150 MHz bandwidth for 300 MHz and 500 MHz oscilloscopes set the source signal bandwidth to the appropriate value Push the Vertical MENU button to access the source channel bandwidth menu 15 FFT Examples FFT Example 1 A pure sine wave can be input into an amplifier to measure distortion any amplifier distortion will introduce harmonics in the amplifier output Viewing the FFT of the output can determine if low level distortion is present You are using a 20 MHz signal as the amplifier test signal You would set the oscilloscope and FFT parame ters as listed in the table FFT Example 1 Settings Control Setting CH 1 Coupling AC Acquisition Mode Average 16 Horizontal Resolution Normal 10k points Horizontal SCALE 100 ns FFT Source Chi FFT VertScale dBV FFT Window Blackman Harris 16 y NANANA A A VVV VV EV VV VV WY 1 The first component at 20 MHz figure label 1 is the source signal fundamental frequency The FFT wave form also shows a seco
8. nd order harmonic at 40 MHz 2 and a fourth order harmonic at 80 MHz 3 The pres ence of components 2 and 3 indicate that the system is distorting the signal The even harmonics suggest a possible difference in signal gain on half of the signal cycle 17 FFT Example 2 Noise in mixed digital analog circuits can be easily observed with an oscilloscope However identifying the sources of the observed noise can be difficult The FFT waveform displays the frequency content of the noise you may then be able to associate those frequen cies with known system frequencies such as system clocks oscillators read write strobes display signals or switching power supplies The highest frequency on the example system is 40 MHz To analyze the example signal you would set the oscilloscope and FFT parameters as listed in the following table FFT Example 2 Settings Control Setting CH 1 Coupling AC Acquisition Mode Sample Horizontal Resolution Normal 10k points Horizontal SCALE 4 00 us Bandwidth 150 MHz FFT Source Chi FFT VertScale dBV FFT Window Hanning 18 Note the component at 31 MHz figure label 1 this coincides with a 31 MHz memory strobe signal in the example system There is also a frequency component at 62 MHz figure label 2 which is the second harmonic of the strobe signal 19 20
9. r Transients or bursts where Hamming has the signal levels before and slightly better fre quency resolution than Hanning after the event are signifi cantly different Rectangular Point by point multiply Window function Hanning Best frequency worst magnitude resolution This is essentially the same as no win dow Transients or bursts where the signal levels before and after the event are nearly equal Equal amplitude sine waves with frequencies that are very close Broad band random noise with a relatively slow varying spectrum Source waveform after windowing 13 Aliasing Problems occur when the oscilloscope acquires a signal containing frequency components that are greater than the Nyquist frequency 1 2 the sample rate The fre quency components that are above the Nyquist frequen cy are undersampled and appear to fold back around the right edge of the graticule showing as lower frequen cy components in the FFT waveform These incorrect components are called aliases Nyquist frequency 0 Hz 4 sample rate y Frequency y Aliased ane Actual frequencies Use the following methods to eliminate aliases m Increase the sample rate by adjusting the Horizontal SCALE to a faster frequency setting Since you increase the Nyquist frequency as you increase the horizontal frequency the aliased frequency components should appear at their proper freque
10. ty Summary in other system manuals for warnings and cautions related to operating the system Preventing Electrostatic Damage CAUTION Electrostatic discharge ESD can damage components in the oscilloscope and its accessories To prevent ESD observe these precautions when directed to do so Use a Ground Strap Wear a grounded antistatic wrist strap to discharge the static voltage from your body while installing or removing sensitive components Use a Safe Work Area Do not use any devices capable of generating or holding a static charge in the work area where you install or remove sensitive components Avoid handling sensitive components in areas that have a floor or benchtop surface capable of generating a static charge Handle Components Carefully Do not slide sensitive components over any surface Do not touch exposed connector pins Handle sensitive components as little as possible Transport and Store Carefully Transport and store sensitive components in a static protected bag or container Manual Storage The oscilloscope front cover has a convenient place to store this manual Installing the TDS3FFT Application Module Refer to the TDS3000 amp TDS3000B Series Application Module Installation Instructions for instructions on installing and testing the application module Introduction The FFT application module adds FFT Fast Fourier Transform measurement capabilities to your oscillo scope The FFT process
11. y components on the same display A Linear scale is useful when the frequency component magni tudes are all close in value allowing direct comparison of their magnitudes Time Signals and FFT Waveforms Displayed Together The time signals and FFT waveforms can be shown together on the display The time signal highlights the problem the FFT waveform helps you determine the cause of the problem Displaying an FFT Waveform 1 Set the source signal Vertical SCALE so that the signal peaks do not go off screen Off screen signal peaks can result in FFT waveform errors 2 Set the Horizontal SCALE control to show five or more cycles of the source signal Showing more cycles means the FFT waveform shows more frequency components provides better frequency resolution and reduces aliasing If the signal is a single shot transient signal make sure that the entire signal transient event and ringing or noise is displayed and centered on the screen 3 Push the Vertical MATH button to show the math menu 4 Push the FFT screen button to show the FFT side menu 5 Select a signal source You can do an FFT on any channel or any stored reference waveform 6 Select the appropriate vertical scale and FFT window 7 Use zoom controls and the cursors to magnify and measure the FFT waveform FFT Math Menu Bottom Side Description FFT Set FFT Sets the FFT signal source Source to Valid input sources are Ch 1 and Ch 2 2
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