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Experiment 8

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1. BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department Using the DECISION MAKER T27 read about the DECISION MAKER module in the TIMS User Manual Before plugging it in ensure that a The on board switch SW2 is switched to INT b The NRZ L waveform is selected with on board switch SW1 upper rear of board This configures the DECISION MAKER to accept bipolar non return to zero waveforms as you have from the analog output of the SEQUENCE GENERATOR T28 change the oscilloscope triggering and display an eye pattern T29 patch the DECISION MAKER into the system including the Z MOD connection to the oscilloscope T30 rotate the front panel DECISION POINT control knob of the DECISION MAKER There should be a highlighted point moving across both oscilloscope traces If the spot cannot be seen try varying the oscilloscope intensity control If still no spot seek help from your Laboratory Manager As a last resort refer to the TIMS User Manual and the oscilloscope User Manual There are settings on the DECISION MAKER circuit board to suit most oscilloscopes and once set by the Laboratory Manager they require no further adjustment note make sure a TTL bit clock is connected to B CLK in T31 locate the high intensity spot on the oscilloscope display This shows the sampling instant BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel
2. fier Figure 12 COMPARATOR and DECISION MAKER as regenerators Signal regeneration First patch up the complete system according to the block diagram of Figure 11 shown modeled in Figure 12 note that for selecting the best sampling instant the eye pattern is preferred The oscilloscope is triggered externally by the bit clock to check visually that the recovered sequence is error free you need the snapshot The oscilloscope is triggered externally by the SYNC pulse of the SEQUENCE GENERATOR Ideally for visual evaluation the first of these requires a long sequence and the second a short sequence For convenience however you may find it acceptable to use short sequence for both observations T20 before plugging in the SEQUENCE GENERATOR module select the minimum length sequence with the on board switch SW2 both toggles UP T21 model the complete system illustrated in Figure 12 except for the DECISION MAKER BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department T22 set the AUDIO OSCILLATOR to about 2 kHz This will suit the DECISION MAKER which has been designed for operation with clock rates of this order T23 ensure the oscilloscope is triggering on the SYNC signal from the SEQUENCE GENERATOR Check the sequence on CH1 A Using the COMPARATOR T24 set the TUNEABLE LPF to its widest bandwidth Check the signal on CH1 B is roughly of the sam
3. A sliding window correlator is used This was introduced in pervious experiment 3 A means of measuring the errors after alignment The error signal comes from an X OR gate There is one pulse per error The counter counts these pulses over a period set by a gate which may be left open for a known number of bit clock periods Detailed description Having examined the overall operation of the basic system and gained an idea of the purpose of each element we proceed now to show more of the specifics you will need when modeling with TIMS So Figure 6 has been expanded into Figure 7 below The detector is the DECISION MAKER module introduced in this experiment The LINE CODE ENCODER and LINE CODE DECODER modules were introduced in pervious experiment aust to detector t n retimed threshold bet clock stolen bit clock TRANSMITTER CHANNEL RECEIVER INSTRUMENTATION Figure 7 Block diagram of system in more detail BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department The extra detail in Figure 7 includes 1 Provision for transforming the data before transmission using any one of a number of line codes In this experiment we use the NRZ L code which provides level shift and amplitude scaling to suit the analog channel 2 Bit clock generation Because the line coder requires quarter bit period timing information it is driven by a master clock at four times the bit clo
4. BER in EE 460L Noisy University of Nevada Las Vegas Experiment 6 Channel ECE Department PREPARATION 1 The Noisy Channel Model Since TIMS is about modeling communication systems it is not surprising that it can model a communications channel Two types of channels are frequently required namely low pass and bandpass Low pass or Baseband channels A low pass channel by definition should have a bandwidth extending from DC to some upper frequency limit Thus it would have the characteristics of a low pass filter A speech channel is often referred to as a low pass channel although it does not necessarily extend down to DC More commonly it is called a baseband channel Bandpass channels A bandpass channel by definition should have a bandwidth covering a range of frequencies not including DC Thus it would have the characteristics of a bandpass filter Typically its bandwidth is often much less than an octave but this restriction is not mandatory Such a channel has been called narrow band Strictly an analog voice channel is a bandpass channel rather than low pass as suggested above since it does not extend down to DC So the distinction between baseband and bandpass channels can be blurred on occasion Designers of active circuits often prefer bandpass channels since there is no need to be concerned with the minimization of DC offsets Simplification The above description is an oversimplificati
5. Q10 once sequence alignment is attained the sliding window correlator is disabled Explain why alignment is not lost even if the noise level is raised until the BER increases to unacceptably high levels QII why were you advised to use a long sequence when counting errors Q12 explain the principle of what you were doing when adjusting the DC at the input to the DECISION MAKER 33 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department APPENDIX ERROR COUNTING UTILITIES module A full description of this module is available in the TIMS Advanced Module User Manual This should be essential reading before the module is used Before use it is necessary to check the settings of the on board switches SW1 and SW2 and the jumper J1 Briefly the module consists of two sub systems X OR gate This has two modes 1 pulse mode with a clock signal connected Acts as a gated sub system Somewhere near the middle of each clock pulse it makes an X OR decision regarding the two TTL inputs Its output is a TTL HI if they are different otherwise a LO In the present application it compares each bit of the regenerated received signal with a reference generator Differences which represent errors are counted by the FREQUENCY COUNTER in COUNT mode 2 normal mode with no clock input gate timing pulse This clocked sub system on receipt of a trigger pulse manual or electronic o
6. The shape of a binary sequence waveform is affected by transmission through a noisy bandlimited channel Since the aim of a transmission system is to deliver a perfect copy of the input to the output the received sequence needs to be restored to its original shape or regenerated The regeneration process is performed by what we will call a detector You should refer to your text book to see the wide range of circuits which has been devised for performing this regeneration process from the simple amplitude limiter or the comparator to extremely sophisticated and intelligent schemes As an example of moderate wave shaping refer to Figure 4 which shows the wave shape changes suffered by a sequence after passage through a bandlimited channel BIPOLAR INPUT SEQUENCE BANDLIMITED OUTPUT SEQUENCE showing delay BIT CLOCK time Figure 4 Waveforms before and after moderate bandlimiting If the bandlimited bipolar output waveform of Figure 4 is connected to a comparator whose reference is zero volts then the output will be HI say V volt whilst the waveform is above zero volts and LO say V volt when it is below Visual inspection leads us to believe that it is unlikely the comparator would make any mistakes Examination of the eye pattern of the same waveform would confirm this If there was noise and perhaps further bandlimiting the comparator would eventually start to fail But other more sophisticated circuits
7. operating on the same waveform might still succeed in regenerating a perfect copy of the original BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department To be fair to the comparator it is competent to decide whether the signal is above a HI or below a LO a reference Provided the signal to noise ratio SNR is not too low then this will be during the HI and LO bits As the SNR reduces the comparator output may not match favorably the input especially as regards pulse width Finally its performance as a detector will deteriorate and extra pulses or bits will appear as indicated in Figure 5 below But note that it is still operating faithfully as a comparator message INPUT ll 1 bandimited and delayed message bandlimbed GQ message PLUS noise comparatoe OUTPUT waveform 3 as input extra bit Figure 5 Deterioration of comparator output with noise Additional circuitry using the bit clock as a guide could be implemented to restore the bit width of the regenerated sequence This and more has been done with the TIMS DECISION MAKER The DECISION MAKER is fed a copy of the corrupted sequence and also a copy of the bit clock It examines the incoming sequence at a specified instant within each bit period This best sampling instant may be chosen by you after inspection of the eye pattern With additional external circuitry this could be autom
8. position 1 Record the chosen input voltage amplitude T3 switch back to the chosen filter and record the output voltage amplitude and the frequency BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department T4 tune to the next frequency Check that the input amplitude has remained constant adjust if necessary with the BUFFER AMPLIFIER Record the output voltage amplitude and the measurement frequency T5 repeat the previous Task until the full frequency range has been covered T6 The measurements have been recorded The next step is usually to display them graphically This you might like to do using your favorite software graphics package But it is also instructive at least once in your career to make a plot by hand since instead of some software deciding upon the axis ranges you will need to make this decision yourself Conventional engineering practice is to plot amplitude in decibels on a linear scale and to use a logarithmic frequency scale Why A decibel amplitude scale requires that a reference voltage be chosen This will be your recorded input voltage Since the response curve is shown as a ratio there is no way of telling what this voltage was from most response plots so it is good practice to note it somewhere on the graph make a graph of your results Choose your scales wisely Compare with the theoretical response Signal to noise ratio This n
9. signals look like with random noise added T7 set up the arrangement of Figure 10 above Use the channel model of Figure 3 In this experiment use a BASEBAND CHANNEL FILTERS module select say filter 3 You are now going to set up independent levels of signal and noise as recorded by the WIDEBAND TRUE RMS METER and then predict the meter reading when they are present together After bandlimiting there will be only a small rms noise voltage available so this will be set up first T8 reduce to zero the amplitude of the sinusoidal signal into the channel using the G gain control of the INPUT ADDER T9 set the front panel attenuator of the NOISE GENERATOR to maximum output T10 adjust the gain control g of the INPUT ADDER to maximum Adjust the G control of the OUTPUT ADDER for about 1 volt rms Record the reading The level of signal into the BASEBAND CHANNEL FILTERS module may exceed the TIMS ANALOG REFERENCE LEVEL and be close to overloading it but we need as BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department much noise out as possible If you suspect overloading then reduce the noise 2 dB with the attenuator and check that the expected change is reflected by the rms meter reading If not use the INPUT ADDER to reduce the level a little and check again Before commencing the experiment proper have a look at the noise alone first wideband then filt
10. 5 volts What is its rms value Q4 what would an rms meter read if connected to a square wave a Alternating between 0 and 5 volt b Alternating between 5 volt Q5 the measuring instrument used in this experiment was an rms volt meter Could you derive a conversion factor so that the instrument could be used as a direct reading relative power meter Q6 suppose a meter is reading 1 volt rms on a pure tone Wideband noise is now added until the meter reading increases by 10 a What would be the signal to noise ratio in dB b What would the rms volt meter read on noise alone 32 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department This answer is meant to show that measuring small changes to signal to noise ratios is difficult unless the signal to noise ratio is already small Do you agree How small Q7 wideband white noise is passed through a low pass filter to a meter If the filter bandwidth is decreased by one half what would be the change of the reading of the meter if a It responds to power answer in dB b It is a true rms volt meter give the percent change Q8 explain how you might measure or at least demonstrate the existence of a time delay through any of the filters Q9 familiarize yourself with the terms timing jitter and baseline wander Explain via the eye pattern how these affect the satisfactory operation of the detection device
11. EBAND TRUE RMS METER will read when the noise is reconnected T16 replace the noise patch cord into the INPUTADDER Record what the meter reads T17 calculate and record the signal to noise ratio in dB T18 measure the signal plus noise then the noise alone and calculate the SNR in dB Compare with the result of the previous Task T19 increase the signal level thus changing the SNR Measure both S N and S N N and predict each from the measurement of the other Repeat for different SNR 2 Decision Maker You will examine the decision process applied to a noisy bandlimited bipolar waveform using two methods a basic COMPARATOR un gated and the TIMS DECISION MAKER module In next section you will count actual errors but for now evaluation will be by visual comparison of the device output and the original sequence Test signal The test signal will come from a SEQUENCE GENERATOR via a noisy bandlimited channel The bandlimiting of the channel will be adjustable This is illustrated in block form in Figure 11 below SEQUENCE GENERATOR sequence oul NOISY CHANNEL COMPARATOR Figure 11 System to demonstrate sequence regeneration BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department A model of the block diagram of Figure 11 is shown in Figure 12 below It uses the macro CHANNEL MODEL module Use Tunable LPF module as band limiting channel UMUTES ODEL
12. ECE Department T32 adjust the front panel control of the DECISION MAKER so that the sampling instant is positioned at the best decision point within the bit period i e where the vertical eye opening is greatest T33 the regenerated sequence will be displayed on CH2 B Compare it with the input sequence on CH1 A There will of course be a time offset between the two due to the delay introduced by the filter and the regeneration process itself Note that the DECISION MAKER provides a new bit clock aligned with the regenerated sequence This is essential for later processing of the regenerated sequence e g by the error counter see later You will now make some more demanding tests of the DECISION MAKER Changing from one display to the other involves a little more than changing the connection to the ext trig of the oscilloscope as generally the sweep speed needs some slight adjustment Ideally also the snapshot needs a short sequence and the eye a long sequence But for these tests a short sequence should be acceptable for both What you will have to do each time you make a test is 1 Lower the bandwidth a little 2 Re position the sampling instant 3 Check the sequences for equality You can develop your own visual scheme for comparing sequences T34 now carry out some observations of the DECISION MAKER output as you did for the COMPARATOR output as either or both the channel bandwidth is varied and noise is adde
13. ERATOR RESET Confirm that the two sequences already synchronized are now aligned If you want to see the sliding window correlator at work again press the reset on the instrumentation SEQUENCE GENERATOR and alignment will be lost Re align by repeating the last Task T4 5 set the FREQUENCY COUNTER to its COUNT mode and patch it into the system complete with the gate signal from the ERROR COUNTING UTILITIES module T4 6 switch the gate of the ERROR COUNTING UTILITIES with the PULSE COUNT switch to be active for 10 bit clock periods Make a mental calculation to estimate how long that will be T4 7 to make an error count a Reset the FREQUENCY COUNTER b Start the error count by pressing the TRIG button of the ERROR COUNTING UTILITIES module 25 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department The active LED on the ERROR COUNTING UTILITIES module will light and remain alight until 90 of the count is completed when it will blink before finally extinguishing indicating the count has concluded With no noise there should be no errors But warning every time a count is initiated one count will be recorded immediately This is a confidence count to reassure you the system is active especially for those cases when the actual errors are minimal It does not represent an error and should always be subtracted from the final count Despite the abov
14. TER will measure the amplitude of the output voltage the FREQUENCY COUNTER will indicate the frequency of measurement the OSCILLOSCOPE will monitor the output waveform With TIMS there is unlikely to be any overloading of the filter if analog signals remain below the TIMS ANALOG REFERENCE LEVEL but it is always a good idea in a less controlled situation to keep a constant check that the analog system is operating in a linear manner not to big and not too small an input signal This is not immediately obvious by looking at the WIDEBAND TRUE RMS METER reading alone see Tutorial Q2 Note that the oscilloscope is externally triggered from the constant amplitude source of the input signal The measuring procedure is TI decide upon a frequency range and the approximate frequency increments to be made over this range A preliminary sweep is useful It could locate the corner frequency and the frequency increments you choose near the corner where the amplitude frequency change is fastest could be closer together T2 set the AUDIO OSCILLATOR frequency to the low end of the sweep range Set the filter input voltage to a convenient value using the BUFFER AMPLIFIER A round figure is often chosen to make subsequent calculations easier say 1 volt rms Note that the input voltage can be read without the need to change patching leads by switching the front panel switch on the BASEBAND CHANNEL FILTERS module to the straight through condition
15. ated or made adaptive The DECISION MAKER makes its decision at the instant you have specified within each bit period and outputs either a HI or a LO It makes each HI or LO last for a bit period It also outputs a new bit clock shifted in time relative to the input bit clock so that the new clock is aligned with the regenerated bit stream The sampling instant specified by you in this experiment is set by a front panel control and is indicated on the oscilloscope waveform by a high intensity spot The appropriate waveform to be viewing when selecting the sampling instant is the eye pattern For more details about the DECISION MAKER module refer to the TIMS User Manual You will have an opportunity to become acquainted with it in this experiment Please note that the circuitry of the DECISION MAKER has been optimized for a clock rate in the region of 2 kHz so it is unwise to use it at clock rates too far removed from this BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department 3 BER Measurement in the Noisy Channel This experiment serves as an introduction to bit error rate BER measurement It models a digital communication system transmitting binary data over a noisy bandlimited channel A complete instrumentation setup is included that allows measurement of BER as a function of signal to noise ratio SNR Many variations of this system are possible and the measurement of t
16. ck rate The timing information is obtained by dividing the master clock by four within the LINE CODE ENCODER This divided by four version of the master clock becomes the system bit clock 3 Provision for adding noise to the channel via the adder on the input side of the bandlimiting channel 4 Inclusion of an ADDER on the output side of the channel This restores the polarity change introduced by the input ADDER for line codes which are polarity sensitive It also provides an opportunity to fine trim the DC level to match the threshold of the DECISION MAKER 5 A decoder for the line code 6 Instrumentation for SNR adjustment not shown and BER measurement Theoretical predictions Bit error probability Pp is a function of E No For matched filter reception of bi polar baseband signaling it has been shown that You will measure not Pp but BER and not E No but SNR Figure 8 shows theoretical predictions based on eqn 1 above BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department Figure 8 Theoretical expectations BER versus SNR for bi polar signaling 10 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department EXPERIMENT 1 Filter amplitude response These days even the most modest laboratory is equipped with computer controlled apparatus which makes the measurement of a filter response in a fe
17. d Get some idea of the performance of the DECISION MAKER as compared with that of the simple COMPARATOR as a regenerator ERROR COUNTING UTILITIES module This is the first time the pulse counting capabilities of the ERROR COUNTING UTILITIES module have been used A complete description of the characteristics and behavior of the module can be obtained from the TIMS Advanced Modules User Manual 20 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department 3 BER Measurement The system to be modeled is shown in Figure 13 It will be patched up systematically section by section according to the scheme detailed below It has not been cluttered by showing oscilloscope connections You should set up the SCOPE SELECTOR for maximum usage of the facility for toggling between the A and B options for each channel Remember that during testing and afterwards the oscilloscope triggering comes from the SYNC output from the transmitter SEQUENCE GENERATOR for snapshots the bit clock for eye patterns data out stolen bit clock TRANSMITTER NOISY CHANNEL RECEIVER INSTRUMENTATION Figure 13 TIMS model of Figure 7 1 0 Transmitter T1 1 patch the transmitter according to Figure 13 from a SEQUENCE GENERATOR set to a short sequence both toggles of SW2 on circuit board UP a LINE CODE ENCODER using NRZ L and the MASTER SIGNALS module Note that the LINE CODE ENCODER accep
18. e error rate warning if alignment is ever lost the noise must be removed before attempting re alignment T5 10 set the SNR to say 10 dB and set the decision instant with the aid of an eye pattern 28 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department 6 0 DC threshold adjustment The effect of any DC threshold of the DECISION MAKER must be offset with DC introduced by the OUTPUT ADDER Two methods are suggested 1 After setting up as above add a small DC to the signal from the channel If the error count can be reduced then adjust for the smallest count 2 Set the DC output from the channel to 25 mV This is the threshold level of the DECISION MAKER in NRZ L mode Now implement one or the other method of threshold adjustment Method 1 T6 1 set the PULSE COUNT on the DECISION MAKER to 10 and press the TRIG button Adjust the noise level with the attenuator so that errors are accumulating at about 10 per second watch the second last digit T6 2 rotate the VARIABLE DC level about 45 anti clockwise Advance the g control of the OUTPUT ADDER about 20 The error rate should increase T6 3 slowly reduce the DC offset voltage magnitude rotate the VARIABLE DC control clockwise towards zero The error rate should slowly reduce then increase Return to the lowest rate and stay there This is an important adjustment It takes some practice At all time
19. e gain of the DC threshold adjustment path through the OUTPUT ADDER to Zero 22 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department T2 6 check that the amplitude of the signal out of the CHANNEL MODEL to is around 2 volts peak to peak If not adjust it with the OUTPUT ADDER gain control G The gain through the channel is now unity T2 7 confirm that the signal at the OUTPUT ADDER although of different shape and further delayed is clearly related to the input sequence When tracing the sequence through the system notice that there is a polarity inversion introduced by the INPUT ADDER of the channel and a second inversion introduced by the OUTPUT ADDER 3 0 Receiver The receiver consists of the DECISION MAKER and LINE CODE DECODER modules T3 1 before plugging in the DECISION MAKER a Switch the on board switch SW2 to INT DECISION POINT can now be adjusted with the front panel control b Select the expected line code with the on board rotary switch SW1 upper rear of board For this experiment it is NRZ L T3 2 patch up the DECISION MAKER including the Z MOD output to the oscilloscope It is assumed that the Z MOD adjustments have been made on the circuit board to suit your oscilloscope T3 3 trigger the oscilloscope from the bit clock and obtain an eye pattern at the channel output Adjust the sampling instant with the DECISION MAKER front pan
20. e shape as shown in Figure 4 The COMPARATOR should have no trouble regenerating this sequence T25 check the COMPARATOR output against the original sequence by looking at CH1 A and CH2 A simultaneously Satisfy yourself that regeneration is acceptable T26 now carry out some observations of the COMPARATOR output as either or both the channel bandwidth is varied and noise is added Get some appreciation of the limitations of the COMPARATOR as a regenerator Signal levels No mention was made above about the signal levels at the various interfaces You should be experienced enough now to realize how important these are Although this may seem to be a digital Style experiment most of the modules are processing analog level signals So the signal levels at their yellow interfaces should be adjusted appropriately That is they should be at about the TIMS ANALOG REFERENCE LEVEL or 4 volt peak to peak Check back that this was achieved DC threshold You can investigate the purpose of the DC threshold adjustment provided in the CHANNEL MODEL Slowly reduce the channel output amplitude whilst monitoring the COMPARATOR output Eventually as the signal amplitude is progressively decreased the COMPARATOR output will be all HI or all LO Fine adjustment of the DC level from the channel 1 will re position the sequence with respect to the COMPARATOR reference level nominally zero or ground and allow operation for even smaller input levels
21. e single confidence count you may wish to make a further check of the error counting facility before using noise T4 8 if the ERROR COUNTING UTILITIES GATE is still open press the instrumentation SEQUENCE GENERATOR reset button else press the TRIG to open the GATE The sequences should now be out of alignment The counter will start counting and continue counting errors until the GATE shuts It will record a count of between 2 and 10 with the PULSE COUNT switch set to make 10 counts You will record a different count each time this is repeated Why would this be You have just completed a major setting up procedure If it was achieved without any problems you are to be congratulated Although TIMS itself will behave reliably it is easy to make patching errors and their discovery and rectification is all part of the learning process 5 0 Error counting with noise preparation T5 1 increase the message sequence length of both SEQUENCE GENERATOR modules both toggles of SW2 DOWN 26 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department T5 2 re establish sequence alignment by pressing all the reset buttons in order input to output then momentarily connect the X OR output to the instrumentation SEQUENCE GENERATOR RESET input Adding noise principle It is now time to add the noise to the signal Noise must be introduced before bandlimiting since the channel bandlim
22. el control to the center of the eye Remember that some fine adjustment of the intensity control of the oscilloscope will probably be necessary to easily identify the bright spot at the sampling instant T3 4 trigger the oscilloscope from the SYNC output of the transmitter SEQUENCE GENERATOR Check that the reconstructed analog output from the DECISION 23 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department MAKER is a delayed version of but otherwise the same shape as that at the channel input T3 5 refer to the DECISION MAKER in the TIMS User Manual for threshold level information This varies according to the code in use For the NRZ L code the threshold is approximately 25 mV Thus the input signal amplitude must either swamp any possible DC threshold or if small must be adjusted to straddle it There is provision in the model the OUTPUT ADDER for this it will be checked in the next Section For now confirm that the output waveform is centered approximately about zero volts T3 6 patch up the LINE CODE DECODER selecting the NRZ L output T3 7 press the reset on the LINE CODE DECODER Check that the TTL output sequence is identical except for a delay with that at the transmitter SEQUENCE GENERATOR output Do not proceed unless these two TTL signals are identical 4 0 BER measurement instrumentation The transmission system is now fully set up You will now proceed
23. ered T11 switch the BASEBAND CHANNEL FILTERS module to the straight through connection switch position 1 Look at the noise on the oscilloscope T12 switch the BASEBAND CHANNEL FILTERS module to any or all of the low pass characteristics Look at the noise on the oscilloscope Probably you saw what you expected when the channel was not bandlimiting the noise an approximation to wideband white noise But when the noise was severely bandlimited there is quite a large change For example a The amplitude dropped significantly Knowing the filter bandwidth you could make an estimate of the noise bandwidth before bandlimiting b The appearance of the noise in the time domain changed quite significantly You might like to repeat the last two tasks using different sweep speeds and having a closer look at the noise under these two different conditions Record your observations When satisfied T13 reduce to zero the amplitude of the noise into the channel by removing its patch cord from the INPUT ADDER thus not disturbing the ADDER adjustment T14 set the AUDIO OSCILLATOR to any convenient frequency within the passband of the channel Adjust the gain G of the INPUT ADDER until the WIDEBAND TRUE RMS METER reads the same value as it did earlier for the noise level BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department T15 turn to your note book and calculate what the WID
24. ext part of the experiment will introduce you to some of the problems and techniques of signal to noise ratio measurements The maximum output amplitude available from the NOISE GENERATOR is about the TIMS ANALOG REFERENCE LEVEL when measured over a wide bandwidth that is wide in the TIMS environment or say about 1 MHz This means that as soon as the noise is bandlimited as it will be in this experiment the rms value will drop significantly You will measure both S N i e SNR and S N N and compare calculations of one from a measurement of the other The un calibrated gain control of the ADDER is used for the adjustment of noise level to give a specific SNR The TIMS NOISE GENERATOR module has a calibrated attenuator which allows the noise level to be changed in small calibrated steps BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department Within the test set up you will use the macro CHANNEL MODEL module already defined It is shown embedded in the test setup in Figure 10 below gt CHANNEL MODEL en leat er NOISE ext trig Figure 10 Measurement of signal to noise ratio As in the filter response measurement the oscilloscope is not essential but certainly good practice in an analog environment It is used to monitor waveforms as a check that overload is not occurring The oscilloscope display will also give you an appreciation of what
25. he performance of each of these can form the subject of separate experiments In this experiment the system is configured in its most elementary form Other experiments can add different forms of message coding line coding different channel characteristics bit clock regeneration and so forth A simplified block diagram of the basic system is shown in Figure 6 below REFERENCE MESSAGE l l stolen bit clock TRANSMITTER CHANNEL RECEIVER INSTRUMENTATION Figure 6 Block diagram of system The system can be divided into four sections Transmitter At the transmitter is the originating message sequence from a pseudo random binary sequence PRBS generator driven by a system bit clock Channel The channel has provision for changing its bandlimiting characteristic and the addition of noise or other sources of interference BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department Receiver The receiver detector regenerates the transmitted message sequence It uses a stolen bit clock BER instrumentation The instrumentation consists of the following elements 1 A sequence generator identical to that used at the transmitter It is clocked by the system bit clock stolen in this case This sequence becomes the reference against which to compare the received sequence 2 A means of aligning the instrumentation sequence generator with the received sequence
26. igned with the incoming sequence carried out with no noise present a high SNR e conditions for a known reference SNR are recorded e channel bandwidth is recorded e g which filter of the BASEBAND CHANNEL FILTERS module is in use 30 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department T6 5 measure BER according to the procedure in Task T4 7 Record the measurement and the conditions under which it was made Compare results with counts over short and long periods T6 6 decrease the noise level by one increment of the NOISE GENERATOR front panel attenuator Go to the previous Task Loop as many times as appropriate T6 7 plot BER versus SNR Relate your results to expectations 31 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department TUTORIAL QUESTIONS Q1 an analog channel is overloaded with a single sine wave test signal Is this always immediately obvious if examined with an oscilloscope Is it obvious with a A single measurement using a voltmeter b Two or more measurements with a voltmeter Explain you answers to a and b Q2 suppose an rms voltmeter reads 1 volt at the output of a noisy channel when the signal is removed from the input What would it read if the bandwidth was halved State any assumptions which were necessary for this answer Q3 a sinusoidal waveform has a peak to peak amplitude of
27. iting filters are required to bandlimit the noise as well The noise from the NOISE GENERATOR is wideband Its peak amplitude must not overload an analog module so its output has been restricted to 4 volt peak to peak the TIMS ANALOG REFERENCE LEVEL As soon as it is bandlimited this amplitude is reduced Amplification cannot be used to bring it up to a convenient level until after bandlimiting But by this time the signal has been added so that is not possible So the only way to obtain a small signal to noise ratio relatively high noise is to reduce the signal level This is done with the INPUT ADDER To set the noise level a Remove the signal from the channel input b Add as much noise as is available to implement the worst SNR possible by maximizing the gain through the INPUT ADDER and setting the attenuator of the NOISE GENERATOR for maximum noise output The SNR can later be increased less noise with this attenuator c Measure the noise level into the DECISION MAKER with the WIDEBAND TRUE RMS METER Then remove the noise replace the signal and adjust it to the same level d Replace the noise The SNR is 0 dB The system is now set up for the worst conditions under which measurements are to be made From now on the SNR will be improved in calibrated steps of the NOISE GENERATOR attenuator and BER measurements recorded The above steps will now be implemented 27 BER in EE 460L Noisy University
28. of Nevada Las Vegas Experiment 8 Channel ECE Department Adding noise practice T5 3 patch both the oscilloscope and the WIDEBAND TRUE RMS METER to observe the signal at the output of the channel 15 4 reduce the signal amplitude to zero with the G gain control of the INPUT ADDER T5 5 set the attenuator of the NOISE GENERATOR for maximum output Increase the noise level into the channel with the INPUT ADDER to maximum Record the reading of the rms meter N volt rms amplitude T5 6 remove the noise by unplugging the patch cord from the INPUTADDER T5 7 introduce some signal with the G control of the channel INPUT ADDER until the rms meter is reading the same as the previous noise reading Record this reading S volt rms amplitude T5 8 replace the noise Do not disturb the INPUT ADDER gain settings from now on T5 9 check the signal level at the channel output Use the G gain control of the OUTPUT ADDER to raise the input level to the DECISION MAKER to the TIMS ANALOG REFERENCE LEVEL 4 V peak to peak is allowable although there may be insufficient gain in the ADDER The SNR is now set up to a reference value 10log dB and this will be 0 dB However you may have your own reasons for selecting some other ratio but it needs to result in many errors From now on you can only reduce the noise using the calibrated attenuator of the NOISE GENERATOR This will increase the SNR which will in turn reduce th
29. on of a practical system It has concentrated all the bandlimiting in the channel and introduced no intentional pulse shaping In practice the bandlimiting and pulse shaping is distributed between filters in the transmitter and the receiver and the channel BER in EE 460L Noisy University of Nevada Las Vegas Experiment 6 Channel ECE Department itself The transmitter and receiver filters are designed knowing the characteristics of the channel The signal reaches the detector having the desired characteristics Noise Whole books have been written about the analysis measurement and optimization of signal to noise ratio SNR SNR is usually quoted as a power ratio expressed in decibels But remember the measuring instrument in this experiment is an rms voltmeter not a power meter Although in a measurement situation it is the magnitude of the ratio S N which is commonly sought it is more often S N N which is available In other words in a non laboratory environment if the signal is present then so is the noise the signal is not available only In this and most other laboratory environments the noise is under our control and can be removed if necessary So that S N rather than S N N can be measure directly For high SNRs there is little difference between the two measures The noisy channel model A representative noisy bandlimited channel model is shown in block diagram form in Figu
30. re 1 Band limitation is implemented by any appropriate filter The noise is added before the filter so that it becomes bandlimited by the same filter that band limits the signal If this is not acceptable then the adder can be moved to the output of the filter or perhaps the noise can have its own bandlimiting filter noise source calibrated attenuator Figure 1 Channel model block diagram BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department Controllable amounts of random noise from the noise source can be inserted into the channel model using the calibrated attenuator This is non signal dependent noise For low pass channels low pass filters are used For bandpass channels bandpass filters are used Signal dependent noise is typically introduced by channel non linearities and includes intermodulation noise between different signals sharing the channel cross talk Unless expressly stated otherwise in TIMS experiments signal dependent noise is considered negligible That is the systems must be operated under linear conditions Diagrammatic representation In patching diagrams if it is necessary to save space the noisy channel will be represented by the block illustrated in Figure 2 below H CHANNEL MODEL Figure 2 Macro CHANNEL MODEL module Note it is illustrated as a channel model module Please do not look for a physical TIMS module when patching
31. s set the error rate with the noise source attenuator so it is about 10 errors per second Concentrate on the second last and then the last digit as the minimum is approached 29 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department Method 2 T6 4 remove both inputs from the INPUT ADDER Using both the VARIABLE DC control and the OUTPUT ADDER g control set the DC level at the input to the DECISION MAKER 25 mV use the WIDEBAND TRUE RMS METER Replace the inputs to the INPUT ADDER Measuring the BER Everything is now set up for some serious measurements It is assumed that e both SEQUENCE GENERATORS are set for long sequences both toggles of the on board switch SW2 are DOWN e line code NRZ L has been patched for this experiment on the LINE CODE ENCODER and LINE CODE DECODER e line code NRZ L has been selected with SW1 on the DECISION MAKER board e all reset button have been pushed in turn from input to output e levels throughout the system have been set correctly typically with SNR 0 dB with max noise from the NOISE GENERATOR e DC threshold at the DECISION MAKER has been accounted for e signal into the DECISION MAKER is ideally at the TIMS ANALOG REFERENCE LEVEL e the DECISION POINT of the DECISION MAKER has been set up using an eye pattern with moderate noise present say an SNR of 10 dB e the SEQUENCE GENERATOR at the receiver has been al
32. to verify its overall operation The BER measurement instrumentation system is used to generate an identical sequence to that transmitted and aligned with that from the receiver detector These two sequences will be compared bit by bit and any disagreements counted The count is made over a pre determined number of bit clock periods and so the bit error rate BER may be calculated You will record the BER for various levels of noise and compare with theoretical expectations T4 1 patch up according to Figure 13 Note the instrumentation receiver SEQUENCE GENERATOR uses the LINE CODE DECODER strobe as its bit clock Trigger the oscilloscope for a snapshot Check that there is a short sequence coming from the instrumentation SEQUENCE GENERATOR output 24 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department T4 2 see the Appendix to this experiment for a short description of the ERROR COUNTING UTILITIES module including on board jumper and switch settings Plug it in Check that the line from the X OR output to the instrumentation SEQUENCE GENERATOR RESET is open T4 3 observe the two inputs to the X OR gate simultaneously It is unlikely that they are aligned but they should be synchronized Your good work is about to be rewarded with the sight of the two sequences snapping into alignment T4 4 momentarily close the line from the X OR output to the instrumentation SEQUENCE GEN
33. ts the master clock which is the 8 333 kHz 21 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department TTL sample clock from the MASTER SIGNALS module and divides it by four to produce the 2 083 kHz system bit clock for the SEQUENCE GENERATOR T1 2 press the reset on the LINE CODE ENCODER Check on CH1 A that a short TTL sequence has been generated by the SEQUENCE GENERATOR T1 3 simultaneously with the previous observation on CH1 A check the NRZ L output of the LINE CODE ENCODER on CH2 A Relative to the TTL on CH1 A it will be delayed half a bit period This is the signal being transmitted to the channel Confirm the code format 2 0 Channel model The macro CHANNEL MODEL module is shown modeled in Figure 3 T2 1 patch up the channel according to Figure 4 and insert it into the position shown in Figure 13 Use Baseband Channel filter module in the channel macro model 12 2 set the front panel attenuator of the NOISE GENERATOR to maximum output but reduce the channel noise to zero by rotating the INPUT ADDER gain control g fully anti clockwise T2 3 adjust the amplitude of the signal into the BASEBAND CHANNEL FILTERS module to near the TIMS ANALOG REFERENCE LEVEL say 2 volt peak to peak with the INPUT ADDER gain control G This level will need resetting when noise is added 12 4 select channel 3 of the BASEBAND CHANNEL FILTERS module T2 5 set th
34. up a system with this macro module included This macro module is modeled with five real TIMS modules namely 1 An INPUT ADDER module 2 A NOISE GENERATOR module 3 A bandlimiting module For example it could be a Any single filter module such as a TUNEABLE LPF for a baseband channel b A BASEBAND CHANNEL FILTERS module in which case it contains three filters as well as a direct through connection Any of these four paths may be selected by a front panel switch Each path has a gain of unity This module can be used in a baseband BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department channel The filters all have the same slot bandwidth 40 dB at 4 kHz but differing passband widths and phase characteristics c A 100 kHz CHANNEL FILTERS module in which case it contains two filters as well as a direct through connection Any of these three paths may be selected by a front panel switch Each path has a gain of unity This module can be used in a bandpass channel 4 An OUTPUT ADDER module not shown in Figure 1 to compensate for any accumulated DC offsets or to match the DECISION MAKER module threshold 5 A source of DC from the VARIABLE DC module This is a fixed module so does not require a slot in the system frame Thus the CHANNEL MODEL is built according to the patching diagram illustrated in Figure 3 below and noting item 5 above requires four slots in a s
35. utputs a pulse of length number of clock periods determined by the front panel switch PULSE COUNT the toggles of the on board switch SW2 and jumper J1 In this experiment the trigger pulse is initiated by the front panel TRIG push button The GATE output pulse a LO selected by toggle 2 of the on board switch SW1 is used to activate the FREQUENCY COUNTER in COUNT mode 34 BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department on board settings for this eo ef a a Pe SW2 ON torright__ PULSE COUNT switch settings times unity 35
36. w seconds and provides the output result in great detail At the very least this is in the form of amplitude phase and group delay responses in both soft and hard copy It is instructive however to make at least one such measurement using what might be called first principles In this experiment you will make a measurement of the amplitude versus frequency response of one of the BASEBAND CHANNEL FILTERS A typical measurement arrangement is illustrated in Figure 9 below Figure 9 Measurement of filter amplitude response In the arrangement of Figure 9 the audio oscillator provides the input to the filter at the TIMS ANALOG REFERENCE LEVEL and over a frequency range suitable for the filter being measured the BUFFER allows fine adjustment of the signal amplitude into the filter It is always convenient to make the measurement with a constant amplitude signal at the input to the device being measured The TIMS AUDIO OSCILLATOR output amplitude is reasonably constant as the frequency changes but should be monitored in this sort of measurement situation BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department the filter can be selected from the three in the module by the front panel switch positions 2 3 and 4 Each has a gain in the passband of around unity Remember there is a straight through path switch position 1 the WIDEBAND TRUE RMS ME
37. ystem unit ot ee eee ae ees eee eee e454 INPUT and OUTPUT and noise level DC threshold adjust adjust Figure 3 Details of the macro CHANNEL MODEL module Channel gain Typically in a TIMS model the gain through the channel would be set to unity This requires that the upper gain control G of both ADDER modules be set to unity Both the BASEBAND CHANNEL FILTER module and the 100 kHz CHANNEL FILTER module have fixed gains of unity If the TUNEABLE LPF is used then its adjustable gain must also be set to about unity However in particular instances these gains may be set otherwise Noise level The noise level is adjusted by both the lower gain control g of the INPUT ADDER and the front panel calibrated attenuator of the NOISE GENERATOR module Typically the gain would be set to zero g fully anti clockwise until noise is required Then the general noise level is set by g and changes of precise magnitude introduced by the calibrated attenuator BER in EE 460L Noisy University of Nevada Las Vegas Experiment 8 Channel ECE Department Theory often suggests to us the means of making small improvements to SNR in a particular system Although small they can be of value especially when combined with other small improvements implemented elsewhere An improvement of 6 dB in received SNR can mean a doubling of the range for reception from a satellite for example 2 Detection with the Decision Maker

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