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Practical Notes

1. P 2 4 8 Elec321 Practical Notes Note and explain the effect of varying the amplitudes both equal and frequency of the modulation inputs 13 Squarelaw Detection Make X2 Y2 Z9 0 X1 Y1 AM signal using the method of section 9 ii with Ac 5 Am 5M lt 5 The input is of the form Vj 5 cos t 1 M cos pt and the output is Vo Vvi2z 10 25 cos t 1 m cos Opt 2 125 1 cos 2uct 1 2M cos mt 2 M2 2 m2 cos 2m This includes terms at dc which are easily removed and terms at high frequency e g 20 20 Wm etc which may also be removed The only terms in the region of the modulation frequency m are Vof 25 M cos mt 0625 M2 cos Amt This method does produce an output at the modulation frequency m i e it acts as a detector H owever it also produces second order distortion of the detected output which becomes very significant for full modulation with m 1 Check these conclusions in practice Keep the AM signal circuit for the next section 14 Synchronous Demodulation Make X9 Y Z7 0 X 1 AM waveform as in section 13 Y1 10 cosot i e in phase with the carrier The output is Vo 5 cos ict 1 mcos pt 25 1 c0s 20t 1 Mcos pt This has only one term near m namely after filtering out dc and high frequency components P 2 4 9 Elec321 Practical Notes V of 25 M cos Omt so that we have distortion free detection capability The trouble is thatY 1 the c
2. Check the operation of the Decision Maker using eyepatterns simply trigger the CRO off the 2kHz dock instead of SYNC for the purpose Reducethe channel bandwidth and note the deterioration in shape of the bit stream note also that its delay also varies so that the Decision Point needs to be varied for best results Add more noise to the bit stream and note that the safe vertical eye height is reduced in fact with 22 dB of noise the eye doses with the recommended settings Observe the behaviour of the Decision Maker on a longer timebase observing the original X TTL bit stream and the recovered bit stream while again triggering off the SYNC of the Sequence Generator Check that some Decision Point settings produce errors in the output Recovered bit stream 2kHz clock 7 Use the Error Counting Utilities to count sudh errors Error Counting Frequency from Comparator a ues Counter B A B TL Original bit stream ACER from X TTL CLK Twin Pulse Twin Pulse Generator Generator CLK Q2 CLK 2 Start with a bit stream near perfect using the widest LPF setting and 0 dB of noise Initially disconnect Q2 from the CLK input of the Error Counting Utilities ObserveA B ASB and note that although one bit stream is a good copy of the other if the Decision Point is reasonable the exdusiveOR indicates lots of errors because of ther rdative delay Observe A B and the Q2 output of the second Twin
3. set on COUNTS red button e First check that the clock is at 2 KHZ using the CRO Press the red TRIG button of the Pulse Counter of the Error Counting Utilities to start the 10 pulse gate about 50 sec and start counting Notes Ignore the first pulse it is sourious An Active light indicates that the count is proceeding One simple check is to disconnect theA input to the ex OR this should give 50000 errors Now measure the bit error rate as a function of SNR for successive 2dB increases in noise levd Compare with theory ELEC321 Practical Notes Practical Session 12 LINE CODES Study of various line codes for favourable properties such as easy extraction of clock minimal spectral width no dc component immunity to inversion error detection capability Reading Lecture Notes 20 Couch 144 163 Roden 208 213 Schwartz 192 355 Extra reading may be found in the laboratory handout TIMS AMSI User Manual 25 31 TIMS AMSI Ideas for Experiments 14 16 Record the waveforms for the various line codes for the full 32 bits of data in the sequence This will be easier if you use the x10 timebase scale and decalibrate it so that the transitions of the waveforms occur on main graticules of the CRO screen some will occur with a half division delay A good scheme is to use the back of ordinary graph paper for the waveforms put your sheet on something white so that the grid lines are easily visible Make sure t
4. Amplitude V 1 Initial Phase deg 90 FM Deviation Hz V 1000 Provide a sinewave source at 1 kHz to modulate this carrier and set it to produce a frequency deviation of 2 kHz e Check the frequency deviation To do this you need to compare lots of half periods of the signal The circuit below will produce a suitable timebase for this purpose It generates a narrow pulse soon after the input the FM signal crosses through Zero and this triggers a sweep of 1V msec until the next zero crossing of the input The numbered boxes are constants gt lt and and A are standard icons The 1 S block is an integrator Blocks Integration resetintegrator leave its various parameters equal to 0 Just for this section use Simulation Properties Frequency 40960000 Hz 4096 MHz End at 0 002 sec Plot the signal y against this timebase x and calculate the extreme frequencies from this plot Signal analogue Output P 9 10 18 ELEC321 Practical Notes e Check the spectrum Use Simulation Properties Frequency 4096000 Hz End at 0 008 sec Add a spectrum analyser to the screen Comm Operators Spectrum Real Triggered 32 k Rectangular KHz dBm Hz 1 ohm Add a new plot block to the screen to display the spectrum External Trigger X Y Plot X Axis 4 Label the plot and the axes Add an Impulse at t 0 Comm Signal Sources Impulse to trigger the spectral analysis Connect three outputs
5. CRO cursors to its peaks and troughs ii Repeat i except using the signal generated in section A i iii Now similarly check the Fourier components of a square wave To avoid the need for two external generators use a 20Vpp sine wave at a fixed 25 kHz and make the input a 20V pp square wave of variable frequency Verify that you only get an appreciable output from the low pass filter if the square wave frequency is at an odd submultiple of the sine wave frequency Check that the amplitude series is as expected going to at least the eleventh harmonic 8 Suppressed Carrier M odulation D SBSC We continue to multiply sine waves but interpret the results differently To reduce the length of equations and because they don t really matter we omit 1 99 Make X2 Y 22 0 X1 10 cosa t fc 4kHz y1 10 cos Ot fm 50Hz The output is out 10 cos ct x cos mt 5 cOS m t 5 cos c m t The first line of the expression for out shows how the gain for the frequency the carrier is modulated at frequency mp the modulation The second shows that the output contains two Fourier components the sidebands Vary the input frequencies to see the effect on the carrier and the envelope N ote particularly the zero crossings of the envelope for comparison with the next section Check for carrier reversal around these zero crossings Vary the input amplitudes to see the effect on the envelope Explain your ob
6. Pulse Generator With both delays at a maximum and a reasonable if not optimum setting of the Decision Point the Q2 pulses should only occur when A B is LOW This means that the Q2 pulses occur when the original and recovered waveforms are equal howevey if the Decision M aker wasin error a Q2 pulse would occur when A B L Connect the Q2 pulseto the CLK input of the Error Counting Utilities You should now only get pulses from A B when an error really occurs P 11 13 5 ELEC321 Practical Notes Provided that you can keep the adjustments so that there is a time when the original and recovered waveforms are equal and can adjust the Q2 timing to g amp sampling pulses then you are in a position to make real bit eror rate measurements Measure the bit error rate as a function of SNR Put the LPF to its widest setting the Decision Point is then not too critical Ensure as above that you are ready to measure real errors dueto noise Monitor A B and increase the noise 2dB steps until you start to see only an occasional error on the CRO Start with the noise 2 dB lower Check that thesignal alone is 2 00 VRMS Set the Pulse Counter section of the Error Counting Utilities to x1 one measurement will therefore take about 50 seconds The measurement routine is e Check and record thesignal in VRMS by disconnecting the noise from the Adder e Similarly measure the noise signal in VRMS e Reset the Frequency Counter
7. Stop band attenuation Approx 40 dB P 6 8 17 Elec321 Practical Notes Procedure Dual Analog Switch 100kHz Caner ae ot Data Clock 8 3kHz Sequence Generator Fig 1 ASK Modulator Utilities ASK Rectifier d In Fig 2 ASK Demodulator Start with the simple ASK system of Figs 1 2 Initially use a 100kHz Master signal for the carrier and the 83kHz Master signal for the data clock Use a sequence length of 32 trigger the CRO off the SYNC of the Sequence Generator and put the CRO timebase on x10 MAGnification to ease the task of checking the various waveforms Set the REFerence level of the Comparator about half way up the input pulse Record appropriate waveforms and check the data output against the X data input Now try asimpler detector using the Diode LPF of the Utilities module rather than the Rectifier 60kH z LPF N ote the effect of varying the REFerence input to the Comparator Next try band limiting the transmission using a bandpass 100kKHz Channel Filter Examine particularly the first few and last few cycles of a transmitted pulse to estimate the filter properties Or measure them using the signal generator P 6 8 18 Elec321 Practical Notes Using the signal generator rather than the 8kHz Master signal as the data clock analogue dial 2V pp check how high the data rate may be both with and without the 100kH z Channel Filter What limits the data rate We ll look more fully i
8. This is not necessarily just what you want Give each stream a different Initial State e Setup in phase and quadrature carriers Say 1V at 16kHz e Generate a 16 QAM signal to transmit the four data signals Use multiplication x at times by a constant and at others by a carrier and addition To be more realistic you may liketo filter the data signals say low pass at 8 kHz before modulation to avoid excessive bandwidth e Using synchronous demodulation including filtering and squaring recover each of the four data signals from the 16 QAM signal Use a filter at say 8 KHz with 512 taps to remove the carrie frequency components discuss your choice of cutoff frequency Squaring may use a gt icon with appropriate comparison values constants 1 The signals with amplitude 2 are the simplest just look for a voltage of greater than say 0 5 so recover them the ones with amplitude 1 may then be derived but you may not have time to sort them out 00 14 31 Ol YA 3 1 10 Y 4 3 1 1 IL Y 4 3 2 2 1 P 9 10 22 ELEC321 Practical Notes Practical Sessions 11 13 ADVANCED SYSTEMS WITH TIMS You will cover one topic in each of these three weeks If the number of students is not too large you will all do Session 11 in Week 11 and so on However if numbers rise unexpectedly we will not have enough equipment and the Sessions will be done in a different order for different groups There w
9. blue to the light green input Run the simulation again and note the difference Drag the wire to an empty spot on the diagram and release the mouse button to remove the wire Now reconnect the wire from the generator to the blue plot input Run the simulation again Right click release the plot block to change its properties Under Axis change the Y scale limits to 2 and 2 and the X scale limits to 0 5 and O msec use Time Scaling Label the Y axis and the plot block Title will label the whole block while Subtitle may be used to identify individual waveforms Left click OK and check the effect note that no simulation run was needed Run the simulation again and note that the axes lose your settings In the plot block properties under Options choose Fixed Bounds change the axes as above and run a simulation noting that the Y axis does not change this time but the X axis does to display all samples Move a block to some other spot by moving the cursor over the block arrowed cross and left click dragging the block elsewhere N ote how the wire moves with the block Use shift left click to select the sine wave block and type lt Delete gt to remove it and the attached wire Recover it but not the wire with Edit Undo Note that you need to click elsewhere to deselect P 9 10 4 ELEC321 Practical Notes Add a 4kHz sine wave generator 1 V 90 and a summing junction 2 to the diagram Make connections to add the
10. icons control the simulation Producer blocks That is blocks that produce signals starting with 1 a constant Consumer blocks That is blocks that accept signals starting with 0 display and plot Annotation blocks Starting with label Arithmetic blocks Starting with abs Boolean blocks Starting with gt Do a quick survey of all the aboveicons P 9 10 3 ELEC321 Practical Notes Now start using CommSim Left click release the sinusoid icon Locate the mouse at a convenient point and left click release again to add a sine wave generator to the diagram Do the same for aploticon Right click release the sine wave generator to set up its properties Give it an amplitude of 1 at 1000 Hz and label it appropriately Movethe cursor over the output lead of the sine wave generator until it becomes an upright arrow Left click drag a wire from there to the dark blue input arrowhead 2nd down of the plot block Move the cursor to an edge of the plot block double arrow and left click drag the edges in turn to make the block a more appropriate size and shape Click the green arrowhead icon to run a simulation Edit the properties of the sine wave generator so that it starts with a phase of 90 at t 0 Check by running a simulation Unfortunately phase itself cannot be specified so if the frequency is changed the phase will not be 90 Move the cursor to the input of the plot block and drag the wire from the
11. the display over the full frequency range of specification 15 The Rest pp 7 9 16 17 19 21 22 Unless you have time to spare you may delay investigation of the other mainly digital facilities of TIMS until a later session P 5 2 Elec321 Practical Notes Practical Sessions 6 8 1 Coherent D emodulation Reading Schwartz 4 5 4 6 4 8 Lecture N otes 6 4 6 5 7 4 7 7 7 11 8 3 8 4 8 6 8 8 N otes A signal modulated on to a carrier may be recovered by multiplying by the carrier as this translates the signal back to the baseband But we may not be given a carrier signal and may have to use a local one Must it be the exact right frequency If so is its phase important You should generate an AM signal and check the above You may liketo try using a phase locked loop see below to extract an appropriate carrier from the AM signal Then generate a SSBSC signal and check how well the method works Generate the modulated carriers using the Master signals with fe 100 kHz fm 208 kHz Report fully including theory and comment on the significance of these results for real world reception Amplitude M odulation M ethod 1 Ideas p 14 The equation to be modelled is y t V x 1 m coso mt coso t A suitable model is V k AM V 9k 1 V5k cst oso t 252 where Vik ERS Voko V Vk P 6 8 1 Elec321 Practical Notes This mode allows separate control of the amplitude of the sid
12. 0 5 for the power Use Comm Estimators Variance Blocks Arithmetic pow and 0 in var display Variance rs mean ms Signal e Quantise the compressed signal e Expand this signal and plot its error e Determine the rms error e Repeat for several other signal sizes Say 002 005 0 2 05 and 0 95 volts L V is out of range P 9 10 13 ELEC321 Practical Notes 3 Eye patterns Use Simulation Properties Frequency 2000000000 Hz 2 GHz End at 5 usec e Setup asquare wave Comm Signal Sources Rectangular Pulses Pulse Frequency 5000000 Hz 5 MHz Duty Cycle 50 High Level 1 Low Level 1 e Put it through a lowpass filter Add a FIR filter to the screen Comment on the parameters suggested below Use Number of Taps 2000 Cutoff Freq 1 30000000 Hz Filter Type Lowpass Window Type Rectangular Put the square wave through the filter and plot the input and the output separately e Avoiding the filter transient plot the eye pattern Plot the filtered signal in another Plot block set as follows X Upper Bound 0 1 MicroSeconds Retrace Enabled Start Time 1 End Time 5 Interval 0 1 e Add noise to the original signal and filter the result Comm Signal Sources Noise Set at 300000000000 Deg Kelvin 3x1011 from 50 ohms Click and drag a summing junction Add the noise and the signal Put this through the filter e Plot the eye pattern of this filtered noisy signal e Repeat for several degrees of n
13. 4 10 Phase Shifter pp 13 14 Briefly check its operation at both low and high frequencies 11 Multiplier p 12 i Use it to make a DSBSC signal peaking at 2 V with fe 100 kHz fm 2 kHz Are all envelope peaks equal balance If the inputs are in turn connected to ground rather than the signal is the output zero feedthrough Have the X and Y inputs equal feedthrough at 100 kHz In sections ii and iii take particular care with triggering of the CRO ii Reduce the modulation input to make the output about 1 Vpp Add a 2Vpp in phase carrier to check that you get AM You may need to slightly phase shift one version of the carrier to get best results in this and the next part ii Add a quadrature carrier instead and check that you get PM Revert to AM ready for the next section 12 Utilities Module p 22 Set up the AM waveform as in section 11 ii Connect this signal to the DIODE LPF and discuss the output It may be helpful to view input and output together triggering the CRO off the message Keep this circuit ready for the next section 13 60kHz Lowpass Filter p 26 i Check this filter using the VCO output as a test signal iil Apply the AM waveform as in section 11 ii to this filter and check that you get zero output No component near f 14 Quadrature Phase Splitter p 15 Apply a signal from the audio oscillator to both inputs and apply the two outputs to the X Y inputs of the CRO Check
14. Choose Landscape and Fit Diagrams to Page Check that all is well with File Print Preview then save and print your diagram You should choose a resolution of 600 rather than 300 dots per inch Incidentally if at some time you need more than one plot block a good idea is to get one just as you want it then Edit Copy and Paste or Ctrl C and Ctrl V it to get more of just the same size and properties But use a new block for your first spectrum perhaps Now do asimple exercise without detailed instructions The aim is to demonstrate that if cosine waves at frequencies f and f are multiplied together the resultant is the sum of cosine waves at frequencies f1 f2 and f1 f2 2cosm t cos zt COS 1 7 t cos 07 t f1 4 kHz and f2 5 kHz are appropriate values Include blocks to perform the multiplication and the sum mentioned and compare the results Plot plenty of waveforms using no more than 2 per plot dark blue and orange and making the truth of the proposition very clear to see Make sure that your name and the date time are included on the printout page Now add adifferent layer of proof by determining the spectrum of the product signal Change the Simulation Properties Range to run for 8 msec at 4 096 MHz Choose a spectrum analyser block using Comm Operators Spectrum Real and add it to the diagram P 9 10 5 ELEC321 Practical Notes Right click release it to set up values of Trigger Mode Trig
15. ELEC321 Communication Systems Practical Notes Writing Practical Reports Introduction Even at this late stage of their studies many students produce unsatisfactory practical reports these notes set out some general guidelines to writing practical reports as well as giving information which is specific to ELEC321 What are reports for Practical reports can have a variety of aims and audiences For example you may be required to test a particular piece of commercial equipment and report on its suitability to replace current equipment You may be required to measure the properties of a given circuit and check whether they meet the manufacturer s specifications Even these two examples illustrate two important things to guide your report what is its purpose and who will read it Aim of ELEC321 reports For ELEC321 the answers are clear Your lecturer will read the report which should demonstrate that you understand the relevant theory can apply it to real circuits have made measurements as accurately as your equipment permits and have checked that within these limits the behaviour of the circuit agrees with the theory If it does not then you should find out why which of the above aspects have you got wrong Indeed a convincing treatment of such discrepancies can make the difference between an excellent report and an average one Have you got it right Before you claim that the circuit behaviour does agree with the theory make sur
16. a signal chosen to demonstrate aliasing Change the input frequency to 28 kHz and record the altered plots P 9 10 12 ELEC321 Practical Notes 2 Quantisation noise and companding Use Simulation Properties Frequency 4096000 Hz End at 0 001 sec e Setup asignal Sinusoid Frequency Hz 1000 Amplitude 0 1 Label Signal 0 1 V e Compress the original signal using p 255 Comm Operators Compander gets you a ready to use U 255 compresser expander Set Compander Properties Compress Max Value 1 Law u Value 255 Connect the signal to it Display the input and output probably on one plot You may need to vary the axis scales to display various features of the signals e Expand the compressed signal and plot its error Get a u 255 expander as above except using Expand rather than Compress Connect the signal and plot the output e Quantise the original signal to 8 bit accuracy One possible method is as follows Or try Comm Operators A D Converter Signal Output 0 0078125 Quantise The constant 1 is a half bit offset to ensure that the analogue value is rounded not just truncated Get the truncating quantiser with Blocks Nonlinear quantize and specify the Resolution shown for amp bit precision for 1V signals e Plotits error Invert X the original signal and add this to the companded signal e Determine the rms error This is the square root of the variance as shown below specify
17. adrature carrier Attempt synchronous detection using an off frequency carrier P 9 10 8 6 e gt o gt N e gt o gt o o Q9 e o o o ELEC321 Practical Notes FM generation and detection Generate AM by adding a DSBSC signal to a carrier Make a small change to produce narrowband FM Check the amplitude modulation and phase deviation of this signal Set up a wideband FM signal Check the frequency deviation Check the spectrum Demodulate the FM signal by differentiation rectification and filtering Demodulate the FM signal using zero crossing pulses OOK Set up a carrier signal Set up a square wave data signal Generate an OOK signal Check its spectrum Recover the data signal using synchronous detection including filtering and squaring Recover the data signal using rectification filtering and squaring Repeat for a pseudo random bit stream FSK Set up two carrier signals Set up a square wave data signal Generate a FSK signal Check the spectrum of the FSK signal Demodulate the FSK signal using single sided detection filtering rectification filtering and squaring Demodulate the FSK signal using doublesided synchronous detection including filtering and squaring Repeat for a pseudo random bit stream 16 QAM Set up four different pseudo random bit streams Set up in phase and quadrature carriers Generate a 16 QAM signal to transmit the four data signals Using synchronous demodulation including filtering and
18. arrier signal produced at the receiver must be phase locked to X 1 whose carrier signal is produced at the transmitter For example if Y1 10 sin at 90 to the carrier then Vo 5 cosa t sinod 1 m cos Opt 25 1 M cos pt sin ot and there is no component near m we have just moved the received signal to have carrier frequency 20 rather than Check these conclusions in practice It is a matter of simple algebra to find similar conclusions for a suppressed carrier DSBSC signal however square law detection is useless Keep the AM signal circuit for the next section 15 Quadrature Amplitude M odulation This section requires the use of two chassis get a spare one or cooperate with another group of students Suppose that we generate an AM signal again using the method of section ii with Ac 3 Am 3 m lt 3 Vo 3 cos t 1 m cos m1t and add it to a second AM signal with the same carrier frequency but in quadrature with it namely v 3 sinod 14 M2 cos m t i e generate V o Vo tV From the combined signal V we may recover the first modulating signal alone using synchronous demodulation with an in phase carrier 10 cos t P 2 4 10 Elec321 Practical Notes while the second modulating signal alone may be also recovered fromV o using synchronous demodulation with the quadrature carrier 10sin t Demonstrate this technique you may like to use one or two nonsinusoidal modulati
19. at t 0 Comm Signal Sources Impulse to trigger the spectral analysis Connect three outputs of the analysis block to the plot block trigger blue and pink Run a simulation and change the axes of the various plots for an appropriate display Check up to 40 kHz and explain what you see or don t see e Recover the modulation signal from the AM using synchronous detection Multiply the AM signal by an in phase carrier and put the output through a low pass filter Comment on the parameters suggested below Use Number of Taps 2048 Cutoff Freq 4000 Hz Filter Type Lowpass Window Type Rectangular Plot all waveforms perhaps starting at 1 msec to avoid the filter transient You may like to establish a fine grid or add a negative dc voltage or use a high pass filter but this is not as easy as it looks to measure the amplitude of the demodulated signal Or you may save the data as a file and get the peak output values from the file Or you may use cross wires to read off x and or y values You may now like to save this version of the file then remove a few plots perhaps only leaving theAM signal P 9 10 15 ELEC321 Practical Notes e Attempt synchronous detection using a quadrature carrier As above except multiplying the AM signal by a quadrature carrier e Attempt synchronous detection using an off frequency carrier As above except using a carrier at 16250 Hz e Recover the modulation signal from the AM using rectificat
20. common to transmit n bits of data at a time by sending one of 2 distinguishable signals symbols Here wesend 2 bits at once using the signals 2 sin w t 7 4 2sin t 7 4 2 cos wt 7 4 2 cos t 7 4 These may be derived from data b1 bo by generating X cosot Y sinat where X 42if b 1 X 2if b 0 similarly for Y and bo The set of signals is often pictured as a constellation cf Schwartz Fig 4 13 or Lecture Notes Fig 24 8 b Since their amplitude is constant demodulation clearly needs a synchronous carrier coswt for X but sinoat for Y The modulator may be rather like Schwartz Fig 4 14 or Lecture N otes Fig 24 9 a the demodulator like Schwartz Fig 4 15 or Lecture N otes Fig 24 9 b Procedure You will use the Sequence Generator to produce a 2 bit stream of data so first study its characteristics above Use the bipolar ydlow outputs to get signals swinging between 2 and 2 volts In observing the data stream trigger the CRO off the SYNC of the Sequence Generator and usex10 MAGnification of the timebase A sequence length of 32 is appropriate Master Signals Sequence Generator Fig 1 QPSK Modulator P 6 8 15 Elec321 Practical Notes 60kHz Lowpass Filter Out Phase Shifter H 100 kHz cos or sin wt Master Signals Fig 2 QPSK Demodulator The set up is as shown in Figs 1 and 2 In the receiver you may like to use the Phase Shifter for fine adjustmen
21. d known to the reader so that only unusual methods need be reported fully The occasional heading to indicate a new section is appropriate Observe high standards in any graphs with measured points and axes clearly labelled label the graph itself to make clear what measurements it plots Write legibly in ink don t scrawl in pencil Because your report is handed in as you leave the session there is no opportunity to type the report if your writing is too terrible perhaps you could use a style nearer printing Write on A4 paper stapled no need for books or folders References Bruce M Cooper Writing Technical Reports Pam Peters Strategies for Student Writers C Turk amp J Kirkman Effective Writing Elec321 Practical Notes Some Practical Notes Practical Session 1 TIME AND FREQUENCY DOMAINS 1 Procedure The emphasis in this introductory session is not on taking lots of measurements and then comparing them with the proper theory Rather you will make relatively few measurements but are required to discuss their interpretation from several viewpoints The aim is to emphasize the way their behaviour in the frequency and time domains is related You will need to familiarise yourself with the Tektronix 2247A oscilloscope It is not difficult to use and in fact it makes some measurements such as dc or peak peak volts and frequency or even phase both more simply and more accurately than a more basic CRO just follow the menu but
22. dding two double sideband DSB signals derived from the same Suppressed carrier in phase quadrature The DSB signals will in general carry different messages If the bandwidth of each of the messages is B Hz then the bandwidth of each of the DSB signals will be 2 x B Hz Each DSB signal occupies the same place in the frequency spectrum so the bandwidth of the QAM signal is also 2 x B Hz Phase Division M ultiplexing And D emultiplexing Ideas p 35 The phase division multiplexer is a block which appears in many guises depending upon the nature of the signals involved In this example the two are different independent voice channels each converted to a DSB signal based on the same Suppressed carrier The carriers are ideally in quadrature at 90 phase difference P 6 8 5 Elec321 Practical Notes Each DSB signal has the same bandwidth and occupies the same band in the frequency spectrum at the same time The demodulator can separate the two channels by virtue of their phase differences Note especially that the carriers at the transmitter do not need to be in exact quadrature Their phases only need to be different for complete channel separation to be achieved The quadrature condition ensures best signal to noise performance but even a45 error from this is not exactly disastrous What is essential however is that once the transmitter phase has been set the receiver phase is adjusted not to maximise the wan
23. e that you can and do support this claim It is not enough to write Predicted value 22 7 Measured value 24 319 These values are not equal and it is no argument to say that they are nearly equal how close is close enough No more is it valid to claim that the two values are not equal so that the theory is wrong In all cases it is necessary to consider the uncertainties in your measurements and predictions and to show that the range of uncertainty of the measured value covers the predicted one Calculation of uncertainties need not always be a big deal if the predicted value depends on the values of a few 5 resistors and the predicted and measured values differ by 3 little more need be said Of course agreement of this kind is not the end of the story If more accurate measurements could be made it could well be that minor effects which were concealed by the uncertainties become apparent It is then necessary to add to your model of the circuit In designing your system we will often take some care to choose parameters which ensure that such second order effects are negligible while at other times we will allow them to make a significant difference The first alternative is most appropriate for introductory work the second for more advanced studies What to include Because of the nature of your work in ELEC321 your report need not be highly structured A report intended to convince a managing director that new equipment wa
24. ebands and the carrier as measured at the output of the Multiplier This is done by controlling the ratio of the amplitudes of the message and the DC at the output of the Adder When their amplitude values are equal at this point then the depth of modulation of the AM signal is 100 or m Note that e the depth of modulation m is not a function of the amplitude of the high frequency signal into the multiplier e theamplitude of the AM signal itself is directly proportional to the amplitude of the high frequency signal into the multiplier e thereis no need in this arrangement to make any phase adjustments e by lowering the DC value to zero the AM sgnal is reduced to a doublesideband suppressed carrier signal DSBSC Coherent D emodulation Ideas p 21 This is the standard demodulator for DSBSC signals see Fig 2 below But it will also demodulate other linearly modulated carriers It will demodulate SSB for example but only provided that there is not another sideband on the other side of the carrier Should this other sideband exist e g as the other sideband of an ISB signal or perhaps an adjacent unwanted signal then a true SSB demodulator is required Procedure Demodulated Carrier Phase Output 100 kHz Shifter y Fig 2 AM Demodulator P 6 8 2 Elec321 Practical Notes Set up an AM signal as in Fig 1 Demodulate as in Fig 2 first using the same carrier then trying phaseshifted versions You may
25. ectangular wave of 50 duty cycle at 500 Hz swinging between Oand 1V e Generate an OOK signal Multiply the two signals e Check its spectrum See earlier sections Try 32k for FFT size e Recover the data signal using synchronous detection including filtering and squaring Extract the modulation signal using a low pass filter with 512 taps at say 8 KHz discuss your choice of cutoff frequency Squaring may use a gt icon with appropriate comparison value a constant 1 block e Recover the data signal using rectification filtering and squaring Rectification may use Blocks Arithmetic abs e Repeat for a pseudo random bit stream Comm Signal sources PN sequence gets you the bit stream Use a shift register size of 10 bits Set the bit interval at 1 msec and the levels at Oand 1V P 9 10 20 ELEC321 Practical Notes 8 amp FSK Use Simulation Properties Frequency 1024000 Hz End at 0 032 sec e Setup two carrier signals Say 1V at 16 and 32 kHz e Set up a square wave data signal Say arectangular wave of 50 duty cycle at 500 Hz swinging between Oand 1V To make it easier to replace it later with a pseudo random data source send the output through a wire positioner top right solid arrow e Generate a FSK signal First get a complement to the data signal levels 0 1 various methods suggest themselves Multiply one carrier by the data signal and the other carrier by the complement of the data signa
26. ee gi ELEC321 Practical Notes Quantisation noise and companding Set up a signal Compress the original signal using u 255 Expand the compressed signal and plot its error Quantise the original signal to 8 bit accuracy Plot its error Determine the rms error Quantise the compressed signal Expand this signal and plot its error Determine the rms error Repeat for several other signal sizes Eye patterns Set up a Square wave Put it through a lowpass filter Avoiding the filter transient plot the eye pattern Add noise to the original signal and filter the result Plot the eye pattern of this filtered noisy signal Repeat for several degrees of noise and filtering Repeat all this for a pseudo random bit stream AM generation and detection Generate AM by multiplying a carrier by a suitable signal Generate AM by adding a DSBSC signal to a carrier Plot the spectrum of the AM signal Recover the modulation signal from the AM using synchronous detection Attempt synchronous detection using a quadrature carrier Attempt synchronous detection using an off frequency carrier Recover the modulation signal from the AM using rectification and filtering SSBSC generation and detection Generate a SSBSC signal using in phase and quadrature carrier and modulation Check the spectrum Makea small change to generate the other sideband Check the spectrum Recover the modulati on signal from the SSBSC using synchronous detection Attempt synchronous detection using a qu
27. equency of about 100 KHz to obtain a 100kH z FM signal FM Demodulation Ideas p 28 There are many methods for demodulating FM signals For example phase locked loop see below FM to AM conversion differentiation followed by envelope detection pulse counting detection see below slope detection using a single tuned circuit plus envelope detection ratio detector Foster Seeley discriminator Procedure Audio f Buffer Oscillator i Amplifier Fig 1 FM Generator Using a VCO an FM generator is simply set up as in Fig 1 With V jn 0 set the VCO frequency to 90 kHz Set the Audio Oscillator to give a few volts at a frequency of 1 kHz Increase the VCO gain to give a frequency deviation of 10 kHz i e fc 90 kH Z fm 1 kHz B 10 Check on the CRO that this is in fact an FM generator i e that the frequency deviation depends only on the amplitude of V jn not on its frequency Without a fixed carrier it is not easy to check that the phase deviation does differ P 6 8 9 Elec321 Practical Notes Phase Detector 60kHz LPF Feedback Fig 2 FM Receiver 1 PLL This FM signal may be demodulated using a phase locked loop as shown in Fig 2 Before wiring it as shown set up the VCO parameters to be much the same as those of the generator The gain needs to be fairly low else the feedback loop will oscillate Set the circuit up with the gain of the 60kH z LPF set to give a few volts of o
28. explain why ii Note that if either input amplitude is varied the amplitude of the output waveform varies but its two components remain of equal size iii Make S20 X1 5 cos jt 07 y1 100 f1 500 Hz P 2 4 3 Elec321 Practical Notes z2 5 cos w7t 42 f2 4kHz and observe the output out X 22 This waveform is the sum of two sine waves whose frequencies are in much the same ratio as those in the previous result so that if the timebase is half as fast the two displays should be similar Check this iv Notethat if either input amplitude is varied the behaviour is different from that observed if two inputs are multiplied Record and comment on the differences 5 Product of Two Sine Waves at the One Frequency If X1 A cos t 01 Y B cos wt 9 then their productis X1 Y1 AB cos wt 01 cos t 2 which using the same trigonometrical identity as before becomes X1Y1 A B 2 cos o1 02 AB 2 cos 2 t 01 02 so that the result is the sum of a dc component dependent on the phase difference the first term and a sinusoidal component at twice the input frequency If this signal is passed through a low pass filter we may reject the sinusoidal component and retain the dc component i Make X2 Y2 Z7 0 X1 10 cos wt 61 f 10 kHz Y1 10 cos wt 67 and observe the multiplier output OUT X 1Y1 10 N ote that the observed output consists of a dc component whose value varies
29. ge 2 08 kHz Generate a SSBSC signal as in Fig 4 then demodulate as in Fig 2 See also note below Phase Shift 902 7 Quadrature Phase Splitter Fig 4 SSBSC Modulator As with AM try first the correct carrier then a phaseshifted one then one at not quite the right frequency N otes on SSB Generation The Phase Shifter in Fig 4 could perhaps be saved by just using quadrature signals On the other hand it can be adjusted around 90 for best performance Each half of the Quadrature Phase Splitter produces a phase shift which is a function of frequency but one half produces at all audio frequencies close to 90 more phase shift than the other P 6 8 4 Elec321 Practical Notes Practical Sessions 6 8 2 Quadrature Amplitude M odulation Reading Lecture N otes 7 5 7 7 7 8 7 11 Message 2 08 kHz DSBSC Carrier 100 kHz Fig 1 DSBSC Modulator If a carrier signal is multiplied by a modulation signal then the result is a DSBSC double sideband suppressed carrier signal If a quadrature carrier is used the same band of frequencies results Surprisingly however two independent messages may be sent in the same bandwidth and recovered separately using synchronous detection D ouble Sideband Suppressed C arrier G eneration Ideas p 12 DSBSC V COSM mt cost This process can be modelled with a multiplier Quadrature Amplitude M odulation Ideas p 29 A QAM signal is made by a
30. ge extends beyond either power supply so without power there must be no signal N ote again that the signal voltage reading on the generator is the voltage that would exist with a 502 load so that with the light loads of these sessions the reading must be doubled 3 Initial Tests Ideally Vo Z X1 X2 x Y1 Y2 10 However each term of this expression is in practice subject to errors such as i zero errors offsets ii scale factor errors wrong gain and iii norrlinearity errors distortion Zero errors are normally temperature dependent while the other errors are moderately frequency dependent By working with large signals at moderate frequencies such errors may often be ignored Check this by making at least the following measurements Sketch a block diagram of the circuit in each case and throughout this session i Make Zo X2 Y 0 Y 410 0 X1 20Vpp sine waveat 500 Hz The output should be identical to X 1 Check this by comparing the two e g by subtracting them on the CRO Vary Y 1 alittle if necessary Now vary Y 1 from 10 through to 10 volts checking that the output varies in size Note the phase change around Y 1 O and see how small you can make the output by adjusting Y 1 Note particularly the frequency of this minimum output and comment ii Perform a similar set of measurements except with X 1 and Y signals swapped P 2 4 2 Elec321 Practical Notes TheX and Y inp
31. gered Spectral Output Magnitude Phase FFT Size 32k Output Freq Units kHz FFT Window Type Rectangular Power Spectrum Units dBm Hz Load 1 ohm Number of FFT Averages 1 ignore Unwrap Phase and Remove Linear Phase Set up a trigger to start the analysis using Comm Signal Sources Impulse and connect its output to the Trg input of the analysis block Add a plot block to display the spectrum right click the plot block and select External Trigger which will now bea red circle above the normal inputs Connect Trg Mag and freq from the analysis block to the trigger input the blue input and the pink magenta 4th arrowhead down input respectively of the plot block N ote that two of the wires are thick to denote that they are vectors not simple signals Change the plot properties selecting XY Plot using X axis value 4 Insert labels noting that the X axis is now trace 4 Make sure that the axis values are unbounded Run the simulation The spectrum display will not have very appropriate axes so you will need to change them with appropriate choices you can zero in on the exact values of the peaks of the response for comparison with theory Note however that there may be a bug in the software that sometimes ruins the plot if the vertical axis is changed appreciably so you may be unable to print with your preferred axes Sometimes the screen display is OK but the print not sometimes the reverse Even better perha
32. hat you check each waveform against the stated method of generating it It would be best to havea record in your report of what these methods are When you try the inverted codes explain your results don t just state what they are Try AC coupling using a series 47nF capacitor not the nonsense method given in the Ideas Explain these results also P 11 13 6 ELEC321 Practical Notes Practical Session 13 DELTA MODULATION Variation of step size and clock rate effect on overload noise and quantisation noise Study of various methods of modulation and demodulation Reading Lecture Notes 17 Schwartz 145 159 TIMS AMSI User Manual 12 21 TIMS AMSI Ideas for Experiments 19 29 It is unlikely that you will get through all of the tests suggested in the Ideas but do as much as you can A reasonable scheme is to go through sections 6 1 7 1 7 2 6 2 7 2 6 3 7 1 in that order testing demodulation for each modulator The modulators and demodulators given as Ideas do not correspond exactly to those in your printed notes so a few brief notes follow P 11 13 7 ELEC321 Practical Notes 6 1 Simple Delta M odulator Input 2 kHz TTL output Hard Limiter Bipolar Data Integrator This samples at well above the Nyquist rate Note that the circuit forms a feedback loop comparing the integral of the bipolar data output with the input in the adder which produces an error signal The loop attempts to make
33. he signal s Fourier components P 1 1 iv v Vi 1 ii iii 1 ii 1 ii Elec321 Practical Notes Up to how high a frequency does the output roughly resemble the input square wave in that it has recognizable rising and falling edges and is almost full size There is no unique answer here Discuss how this is related to the circuit s frequency response Your oscilloscope s vertical amplifier is said to be good up to 100 MHz i e frequency domain Estimate its rise time for a square wave input i e time domain Explain how a single measurement on a circuit using a square wave at a frequency considerably less than the circuit s 3dB frequency f can give an estimate of fy Don t just give a formula but explain the apparent paradox that the input frequency can be well within the pass band 4 High pass Circuit Time Response Set up a high pass circuit using C 47 nF R 220kQ Apply a square wave input at 100 Hz and record the output waveform using a x 10 probe to avoid loading effects Calculate the 3dB frequency of the circuit Calculate the effect of the circuit on the fundamental and on the first few harmonics of the input i e frequency domain Explain how these effects are linked to the shape of the output i e time domain Warning There is an apparent paradox here so give a careful explanation 5 Band pass Circuit Frequency Response Set up a tuned circuit co
34. he spectrum will be 1 T e g if you End at 0 008 sec the resolution will be 62 5 Hz The display will do a dot to dot picture so that a spectral line may appear widened look at the raw data as outlined below under Exact Data to check whether any widening of lines is real Remember that the spectrum is always given as a Fourier transform never a Fourier series Exact Data f you want to get exact values from a plot of a waveform or spectrum save the data from the plot in a file Block Properties Save Data to File The file will contain along list of x and y values but it is a simple task to delete the many entries of little interest and turn the vital data into a table which is easily incorporated in your report P 9 10 11 ELEC321 Practical Notes L Sampling and reconstruction Use Simulation Properties Frequency 4096000 Hz End at 0 001 sec e Setup a sampling waveform Comm Signal Sources Rectangular Pulses Pulse Frequency Hz 32000 High Leva 1 Low Leva 0 Duty Cycle Duty Cycle 25 e Setup an input message Sinusoid Frequency Hz 8000 Amplitude 1 Label Signal 8 kHz e Sample the message Multiply x the two waveforms Add and connect a Plot block to record the original blue and sampled orange signals e Filter the sampled signal to recover the message Add aFIR filter to the screen Comment on the parameters suggested below Use Number of Taps 2048 Cutoff Freq 1 15000 Hz Filter Type Lowpas
35. ich do not appear on a printout or manually incorporate them on the screen for example by adding a descriptive title at the end of the header and notes on details of the plots at the end of the footer but such comments are not permanently attached to the file or using a label Since CommSim files are stored in only a few kilobytes each feel free to save each version of any file separately this has the advantage that an identifying file name will be attached to each printout For each file name use the format 321g03p09sla g03 if that is in your user name p09 for Practical Session 9 sla for Section 1 Figure 1a this should identify each printout uniquely P 9 10 1 ELEC321 Practical Notes A report will be required for Session 10 being due about one week after its completion Make sure that your report is self contained with an introduction to the relevant theory and with frequent reference in detail and with associated calculations to how the results support the theory P 9 10 2 ELEC321 Practical Notes Practical Session 9 Comm SIM TUTORIAL CommSim is a package that simulates communication systems The purpose of this tutorial is to introduce you to CommSim getting you to near the point where you could devise and test your own implementation of a complex communication system CommsSim is installed for each PC in the ELEC321 laboratory Log on at aconvenient computer you will be given a user name and password at the sta
36. ill not be printed practical notes along the normal lines rather you will mostly work from Ideas for Experiments compiled by the TIMS manufacturer It is considered that this will normally be a good three hours work and you may well have to be selective Handouts provided for the session only in the laboratory include these instructions specifications for all modules used and often reading from sources other than Schwartz Some extra notes for each session are provided below Your report need not contain copies of large slabs of the printed instructions provided that you make it quite clear what you did by direct references to those notes As usual make sure that your report makes all possible qualitative and quantitative comparisons of experimental results with theory P 11 13 1 ELEC321 Practical Notes Practical Session 11 BiT ERROR RATES The effect of limited bandwidth and added noise on the transmission of data Eye patterns as visual indicators choosing a decision level Bit error rate as a function of SNR Reading Lecture Notes 19 26 F2 Schwartz 181 182 408 410 422 432 Extra reading may be found in the laboratory handout TIMS AMSI User Manual 6 11 22 24 TIMS AM SI Ideas for Experiments 5 13 reference only After examining waveforms and eye patterns for a variety of channel characteristics you study how the Decision Maker works This allows you to make measurements of the bit error rate as a func
37. ion Blocks Arithmetic abs and filtering as above P 9 10 16 ELEC321 Practical Notes 5 SSBSC generation and detection Use Simulation Properties Frequency 4096000 Hz End at 0 002 sec e Generate a SSBSC signal using in phase and quadrature carrier and modulation Use 16 kHz for the carrier and 1 kHz for the modulation You ll haveto watch the various phases Multiplication and addition are standard icons x J e Check the spectrum Increase the simulation time to 0 008 sec Add a spectrum analyser to the screen Comm Operators Spectrum Real Triggered 32 k Rectangular KHz dBm Hz 1 ohm Add a new plot block to the screen to display the spectrum External Trigger X Y Plot X Axis 4 Label the plot and the axes Add an Impulse at t 0 Comm Signal Sources Impulse to trigger the spectral analysis Connect three outputs of the analysis block to the plot block trigger blue and pink Run a simulation and change the axes of the various plots for an appropriate display Check up to 40kHz and explain what you see or don t see e Makea small change to generate the other sideband What is it e Check the spectrum e Recover the modulation signal from the SSBSC using synchronous detection Multiply the SSBSC signal by a carrier and put the autput through a low pass filter Comment on the parameters suggested below Use Number of Taps 2048 Cutoff Freq 4000 Hz Filter Type Lowpass Window Type Rectang
38. k that this amplitude varies as expected with the amplitude of X 7 Verify that when the output is large Using the frequency measurement function of the CRO Counter Timer this can be done very precisely 7 Electronic Fourier Analysis If a signal which contains a variety of Fourier components is multiplied by a sinusoidal signal at frequency the output of the multiplier contains Fourier components at the various sum and difference frequencies If the input frequency is approximately equal to one component at frequency n of the original signal the output component at frequency p is readily separated from all the others hence its amplitude determined Phase is not so simple By varying over an appropriate range it is possible to measure the amplitude of each of the Fourier components of the signal i Setup the signal of section 4 iii using one multiplier Use the other multiplier to multiply this by a 20Vpp sinusoidal input of variable frequency Remember the Important N ote of section 2 Observe the output of this second multiplier via the low pass filter of section 5 ii Verify the amplitude and frequency of the two Fourier components using the above method Note again that the signal generator frequency can be very precisely set to P 2 4 5 Elec321 Practical Notes ensure an output frequency of about 1 Hz The peak to peak voltage of this signal can be accurately measured by setting the
39. l To be more realistic you may liketo filter the data signals say low pass at 8 kHz before modulation to avoid excessive bandwidth e Check the spectrum of the FSK signal See earlier sections Try 32k for FFT size e Demodulate the FSK signal using singlesided detection filtering rectification filtering and squaring Use a low pass or high pass filter at Say 22 KHz with 1024 taps to get an OOK signal Rectification may use Blocks Arithmetic abs Extract the modulation signal with a low pass filter at say 8 kHz with 512 taps discuss your choice of cutoff frequency Squaring may use a gt icon with appropriate comparison value a constant 1 block e Demodulate the FSK signal using double sided synchronous detection including filtering and squaring Use one carrier at 16 kHz and one at 32 kHz to ge two outputs theoretically complementary Combine these two outputs to get a better signal to noise ratio e Repeat for a pseudo random bit stream Comm Signal sources PN sequence gets you the bit stream Use a shift register size of 10 bits Set the bit interval at 1 msec and the levels at Oand 1V P 9 10 21 ELEC321 Practical Notes 9 16QAM Use Simulation Properties Frequency 1024000 Hz End at 0 032 sec e Setup four different pseudo random bit streams Comm Signal sources PN sequence gets you a bit stream Use a shift register size of 10 bits Set each bit interval at 1 msec and use levels of 1 V
40. led for a little less than half the sampling period Try a variety of pulse widths and delays in the recovery circuit and record and explain what you see P 6 8 13 Elec321 Practical Notes Practical Sessions 6 8 5 Quadrature Phase Shift Keying Reading Schwartz 4 3 Lecture N otes 23 2 23 3 23 6 23 7 Pseudorandom Sequence G enerator Using a common external clock signal the Sequence Generator outputs two independent pseudorandom sequences X and Y A SYNC output is provided which is coincident with the start of the sequences The sequences may be stopped and restarted at any time via front panel controls Sequences X and Y are available as either standard TTL or analog level outputs Use Anexternal clock signal must be provided to operate the Sequence Generator This may be sinusoidal or TTL separate input sockets are used The sequences may be stopped at any time by either depressing the Reset button or applying a TTL HI signal to the Reset input To restart the sequences from the beginning release the Reset button or apply a TTL LO to the Reset input The length of the sequences may be selected by a PCB mounted DIP switch Four independent sequence pairs are available from lengths of 2 to 211 The sequences are selected as follows DIP Switch Sequence Code Length 2n fet oo P 6 8 14 Elec321 Practical Notes Theory Where bandwidth is limited but noise is not severe e g in the telephone system it is
41. llustrated with this multiplier apply at the normal radio frequencies but more specialised circuitry is normally used The multiplier has inputs X1 2 1 1 2 22 or X1 X2 Y1 2 22 which may take values in the range 10 V to 10 V The output is given by Vo Z2 X1 X2 x Y1 Y2 10 and must also have values in the range 10V to 10V to avoid distortion On the Multiplier A pplications chassis all multiplier inputs have pull down resistors of 22 kQ Except for critical measurements e g in section 3 unused inputs may therefore be regarded as grounded The chassis has two sinusoidal oscillators and phase shifters Each oscillator can be switched to one of three frequencies simultaneously the associated phase shifter s P 2 4 1 Elec321 Practical Notes response is altered so that 90 degrees is at about centre scale at the oscillator frequency with a range of about 1 130 degrees The other phase shifter is not so suitable Power supplies are derived from 15V regulators in the chassis so that external supplies of 18 20 V are required The chassis contains a dc supply variable from 10 V to 10 V but with a current rating of only 5 mA 2 Important Note Some sections require the use of an external signal source Keep this source turned down or off or disconnected until the multiplier is powered up and turn the source down or off or disconnect it before the multiplier is powered down The AD534 may be damaged if an input volta
42. ng signals 16 Single Sideband D etection If a single sideband is transmitted SSBSC the signal is of the form Square aw detection is clearly hopeless but synchronous demodulation can succeed if a phaselocked carrier is available SSBSC signals sometimes contain a small carrier content allowing this method to be implemented fairly easily The demonstration in practice is most simply done using a sinusoidal input as above directly rather than generating it as in section 10 If a single sideband and a carrier even a reduced one are transmitted a squaretaw detector could be used Verify this in theory and in practice if time permits This looks like a simple SSB detection method but there is a catch Can you find it P 2 4 11 Elec321 Practical Notes Practical Session 5 INTRODUCTION TO TIMS Telecommunications Instructional Modelling System 1 Procedure The aim is to become familiar with most of the dozen or so different modules that make up TIMS A systematic test procedure is suggested including some sections similar to those in sessions 2 4 You should use this to test the basic function of each module and if possible some of its limitations in turn recording your procedure and results in your report Particularly note for later reference any features of a module that are not obvious from the front panel or from the TIMS Quick Reference sheet You will use in the laboratory but may not borrow for h
43. nsisting of the parallel combination of L 10mH C 0 1 uF Apply a sinusoidal signal v to it through a 10kQ resistor and observe the output v across the tuned circuit Theory says that the maximum output is at a frequency given by L 1 C o and that the output drops by 3 dB at frequencies given approximately by MO x 1 1 2Q where Q R L Check these predictions by measurement In order to take account of losses in the tuned circuit the value R used in these calculations needs to be reduced to be effectively about 5700 Q 6 Band pass Circuit Time Response Apply a square wave at 50 Hz to the circuit and record the output The parameter Q quality factor has two alternative definitions a Itis the ratio of the centre frequency f to the 3dB bandwidth frequency domain b Itis the ratio of the total stored energy to the average energy lost per unit angle of the oscillation time domain Discuss how the output of the circuit is related to its properties in the frequency domain and in the time domain P 1 2 Elec321 Practical Notes Practical Sessions 2 4 FOURIER SERIES AND MODULATION 1 Procedure You will perform some of your practical work using the Multiplier Applications chassis illustrated below MULTIPLIER APPLICATIONS The multiplier Analog Devices type AD534 is usable to 1 MHz so has near ideal performance for the frequencies in the audio range at which you will use it The same principles as i
44. nto such situations later in ELEC321 Dual Analog Switch Sequence Generator Fig 3 QASK Modulator QASK In Out Carrier Phase y Variable 100 kHz Shifter DC Fig 4 QASK Demodulator Set up the QA SK modulator of Fig 3 Similarly set up half the demodulator as shown in Fig 4 using cost to get one data stream and sinwt to get the other data stream in turn Note and comment particularly on the need to choose a suitable REFerence level for the Comparator P 6 8 19 ELEC321 Practical Notes Practical Sessions 9 10 INTRODUCTION TO COMMSIM These sessions will be spent using the CommSim Communication Systems Simulation package which permits easy simulation of communication functions and systems Session 9 will be a tutorial one to get you familiar with CommSim You will work on your own at aPC doing some simple exercises with full instructions You will then do a slightly harder exercise with less detailed instructions it will simulate an earlier practical exercise and involves most of the skills needed for later exercises There will be no report required for Session 9 but Session 10 may carry more weight than a normal one It is thought that three hours will be adequate time to complete Session 9 you may even be able to get started on the work for Session 10 which will be allocated at Session 9 Please arrive on time for Session 9 which will start with a talk by Steven on various system details If you mis
45. odulator But what about the step size how is it determined P 11 13 8 ELEC321 Practical Notes A basic integrator produces an output Vout from an input Vin according to 1 Mout T Vin at wheret RC is the time constant of theR and C used in the circuit If the integrator has had an input of 2 V from the bipolar signal for time Ts the output will change between sample instants by 27 T and this is clearly the step size TheTIMS integrator has three possible values of R giving three step sizes to choose from It also has three possible clock frequencies note that increasing the time of integration also increases the step size In the first laboratory circuit there is an added complication the adder has adjustable gain from each input to the output In order to avoid overload when first it is tested you should probably set appreciably higher gain from the integrator input than from the signal input As a rough guide set the integrator gain control fully clockwise and the message gain control about half way The output should not spend appreciable time at a steady high or low voltage but should rapidly alternate most of the time the integrator output should not have long straight sections but should be quite jagged This adjustment may be refined later if necessary P 11 13 9 ELEC321 Practical Notes 6 2 Delta Sigma M odulator Input 2 kHz TTL output Hard Limiter Integrat
46. of the analysis block to the plot block trigger blue and pink Run a simulation and change the scales of the various plots for an appropriate display Check up to 50 kHz and explain what you see You may like to store the data values in a file so that the precise values are available You may use the following values of the Bessel function in predicting the spectrum Jo 2 0 223891 J1 2 0576725 J2 2 0 352834 J3 2 0128943 J 4 2 0 033996 J5 2 0007040 J6 2 0 001202 J7 2 0000175 e Demodulate the FM signal by differentiation rectification and filtering The circuit below will give a good approximation to differentiation for this purpose Get the delay from Comm Operators Delay Real and set the delay value to 2 SIM Steps X and are standard icons Signal The rectification is done by Blocks Arithmetic abs Usea FIR filter Comment on the parameters suggested below Use Number of Taps 4096 Cutoff Freq 1500 Hz Filter Type Lowpass Window Type Rectangular e Demodulate the FM signal using zero crossing pulses The circuit below will produce these pulses Use Comm Operators Delay Real Signal analogue ine Filter its output to recover the modulation signal Output P 9 10 19 ELEC321 Practical Notes 7 OOK Use Simulation Properties Frequency 1024000 Hz End at 0 032 sec e Set up acarrier signal Say a cosine wave of 1V at 16 kHz e Set up a square wave data signal Say ar
47. oise and filtering e Repeat all this for a pseudo random bit stream Instead of the square wave use Comm Signal Sources PN Sequence Set Shift Register Size 10 Bilevel 1 1 Timing Internal Bit Rate bps 10000000 10 M bps Change Plot Axis Interval to 0 2 MicroSeconds P 9 10 14 ELEC321 Practical Notes 4 AM generation and detection Use Simulation Properties Frequency 4096000 Hz End at 0 002 sec e Generate AM by multiplying a carrier by a suitable signal Set up a carrier at 16 KHz with amplitude 1 make it a cosine wave Set up a message signal at 1 KHz with amplitude 1 make it a cosine wave Add a dc constant voltage of 2 V to the message signal constant 1 summing junction D Multiply this sum by the carrier x plot all waveforms Modify to g amp 100 modulation with the same carrier component and plot these waveforms e Generate AM by adding aDSBSC signal to a carrier Usea multiplier to get the DSBSC signal and a summing junction to add a carrier Use cosine waves where possible Choose values to get 40 modulation with a peak voltage of 28 e Plot the spectrum of the AM signal Increase the simulation time to 0 008 sec Add a spectrum analyser to the screen Comm Operators Spectrum Real Triggered 32 k Rectangular KHz dBm Hz 1 ohm Add a new plot block to the screen to display the spectrum External Trigger X Y Plot X Axis 4 Label the plot and the axes Add an Impulse
48. ome use a copy of the TIMS User Manual First read pages 1 3 of the manual Switch on TIMS power switch at rear left of unit 2 Variable DC p 23 Just check its operation on the CRO 3 Voltage Controlled Oscillator pp 24 25 Control it with the Variable DC and check its two frequency ranges using the CRO 4 Frequency Counter p 8 Use it to again check the two frequency ranges of the VCO 5 Tuneable Lowpass Filter p 20 Check its operation for several TUNEings and on both frequency ranges using the output of the VCO as a test signal LO range 6 Master Signals pp 10 11 1 Check each of these briefly Using the 2kHz message to trigger the CRO check that the other four signals are locked in phase to it ii Convert the 8 3kHz sample clock into a sine wave using the tuneable LPF and check the frequency ratios of the master signals using Lissajous figures on the CRO 7 Audio Oscillator p 5 1 Briefly check its operation ii Apply the two analogue outputs to the X Y inputs of the CRO and check the display over the frequency range P 5 1 Elec321 Practical Notes 8 Buffer Amplifiers p 6 Briefly check their operation using a signal from the VCO Leave them set for a gain of 0 5 9 Adder p 4 Add setting g G 1 a 2Vpp sine wave at 1 kHz derived from the audio oscillator and a 2Vpp sine wave at 9 kHz derived from the VCO Compare this waveform with that obtained in section 4 of sessions 2
49. omplete suppression can be achieved The price of an error from phase quadrature is a reduced signal to noise ratio because of a reduced signal Procedure Phase Jf Shift 90 Dual Fig 2 QAM Generator Set up the combined message as in Fig 2 Usethe 2kHz Master signal for one message and the Audio Oscillator for the other Use the 100kHz Master signal for the carrier At first just use the cos and sin outputs instead of the Phase Shifter as noted above an exact 90 is not needed Modulated Lowpass Signal Filter Demodulated Carrier Phase A 100 kHz Shifter Output Fig 3 Demodulator P 6 8 7 Elec321 Practical Notes Demodulate as in Fig 3 Check that by careful phase adjustment one signal may be received with little sign of the other Is the adjustment correct when the carrier used is exactly in phase with the original carrier Check that the modulator need not use exactly 90 phase shift between the carriers P 6 8 8 Elec321 Practical Notes Practical Sessions 6 8 3 Wideband FM Reading Schwartz 4 9 4 11 Lecture Notes 10 1 10 5 10 9 Wideband FM Generation Ideas p 27 The VCO can be used to generate a wideband FM signal The TTL output from the VCO will be rich in harmonics of the carrier frequency and each of these will be frequency modulated It is nvenient to extract the fundamental with a lowpass filter eg the 100kH z lowpass Channel Filter if the VCO is set to a fr
50. or Bipolar Data Here the integrator is moved from its previous position the error is integrated rather than the output H owever the error is an analogue voltage and is not digitised N ote that what is compared with the input in the adder is the bipolar output so that we could expect the output to represent the input directly not its rate of change For a given dc input voltage the output should alternate rapidly between HIGH and LOW with the duty cycle adjusting so that the average voltage is equal to the input voltage The adder output swings rapidly between two equal and opposite voltages so that the integrator output ramps above and below the dc input voltage the average error is zero and the output alternates rapidly between HIGH and LOW The demodulator need only average the bipolar data and the frequency response reaches right down to dc Note that both an RC circuit and an integrator can be regarded as low pass filters for the demodulator However an integrator has a 1 f frequency response which an RC circuit only has beyond the cutoff frequency P 11 13 10
51. ort will clearly illustrate how the TIMS units worked how results agree with theory how you understand the drcuit behaviour and so on Procedure 1 Setup a random bit sequence docked at 2 KHz Audio Sequence Oscillator Generator Trigger the CRO ch 3 5V with x10 probe off the SYNC output of the Sequence Generator for most of the time except for eye patterns Check that some pulses of the output are only 05 msec wide and note that Xa and Xd are the inverse of each other as well as having different levels 2 S amp up a sequence which simulates the result of using areal channa i e limited frequency response and added noise Tuneable Noise LPF Generator Real IN OUT Output B signal Set g with G 0 so that the output swings 2 V Now set G with 20 cB of noise so that the noise swings about 2 V about the signal P 11 13 3 from Audio Osc 3 Refinethese settings using the True RMS Meter Disconnect thenoise from the Adder Adjust g to givean indicated Adder output of 200VRMS Disconnect the bit stream from the Adder and connect the noise 20 dB Adjust G to givean indicated Adder output of 141VRMS Retain these G and g settings With both Adder inputs connected measure the RMS output voltage does it agree with a value predicted from the separate signal voltages Some students get dose to the correct answer by adding the two voltages and dividing the sum by the square root of 2 but
52. predicted by theory Comment N ote the LPF inverts Now replace the 2kKHz message in Fig 1 by a variable frequency one from the Audio Oscillator Check that the output is still the same shape as the input for frequencies of a few kHz or less Then try the full range of audio frequencies record some representative results and comment Dual Analog Switch Tuneable Lowpass Filter Twin Pulse Generator Fig 2 Variable Sampling Set up as in Fig 2 which allows the width and phase of the sampling pulse to be varied Record the effect of these variations and check for a few cases that the output shape and size are as predicted by theory P 6 8 12 Elec321 Practical Notes Time D ivision Multiplexing TDM Ideas p 32 If samples of two or more bandlimited messages are interlaced in time the resulting signal is known as a time division multiplexed signal Using commutation techniques the samples from a particular channel can be isolated at the receiver and then reconstructed as illustrated previously for the single message case The mode of Fig 3 below is a two channel time division multiplexer By reducing the sample widths more channels can be accommodated Dual Analog Switch Twin Pulse Generator Fig 3 Time Division Multiplexing Dual Analog Switch Pulse Generator Fig 4 Time Division Demultiplexing Set up a time division multiplexing system as in Figs 3 and 4 with each message samp
53. ps is to right click the spectral plot then Save Data to File This file will bea simple list of x y values it is very long but it is easy to delete large slabs of useless data and produce a compact table of the vital data For better printing from Notepad if thatis what dat files open to use Edit Set Font Another ploy is to read off x and y values using cross wires right click the plot block and click Read Coordinates when you can record quite precise values particularly if you Zoom in on a relevant section of waveform by altering the axis limits You may like to save your diagram at this stage as what follows may make it too crowded and you may like to remove some plots Another proof of the proposition is to put the product signal through a filter to select one or other of the predicted Fourier components and check that the output is as predicted Add afilter to the diagram using Comm Filters FIR Finite Impulse Response Choose Number of Taps 8192 or 8191 Cutoff Freq 1 1500 or 7000 Hz for Filter Type Lowpass or Highpass respectively Window Type Rectangular OK Connect the product signal to the input run a simulation and plot the output Check the result first selecting one component and then the other or use two different filters for simultaneous plots Note that if you finish early after thoroughly checking all of the above you may start on the work for Practical Session 10 You should probably start with
54. rt of the session Launch CommSim Start Programs Commsim2001 Commsim2001 left click Use the full screen There may bea paneon the left labelled Diagram 1 which is not much use left click drag its right boundary left to conceal it if necessary At the top is a menu bar which lets you do almost anything Check out the dropdown windows File Theusual Edit Note Repaint Screen Preferences In the latter tick Snap to Grid Simulate Under Simulation Properties for the moment choose a Frequency of 4096000 Hz to End at 0 001 seconds both for Defaults and Range Blocks Do a quick survey of what is available View Tick Block Labels Connector Labels Status Bar Tool Bar and Presentation Mode not Display Mode for the moment Under Colors choosea dark green for Wires We want to see them on screen and from a black white printer Comm Do a quick survey of the comms blocks that are available Help Take a quick look It is not always very helpful to my mind Below the Menu bar are icons shortcuts for popular functions They are grouped into separate toolbars Look them over Main The usual things to deal with files and editing the display Hold the mouse over the icons in turn when a tooltip will give you a shortform identification of the function and the status bar at the bottom of the screen will give you more detail Sim Control Starts with a green arrowhead CommSim runs a simulation on sampled data and these
55. s Window Type Rectangular Connect the sampled signal to the input and plot the output Perhaps start the timeaxis a little late to avoid the filter transient Perhaps also plot a constant input equal to the predicted recovered amplitude e Find the spectrum of the sampled message Increase the simulation time to 0 008 sec Add a spectrum analyser to the screen Comm Operators Spectrum Real Triggered 32 k Rectangular KHz dBm Hz 1 ohm Add a new plot block to the screen to display the spectrum External Trigger X Y Plot X Axis 4 Label the plot and the axes Add an Impulse at t 0 Comm Signal Sources Impulse to trigger the spectral analysis Connect three outputs of the analysis block to the plot block trigger blue and pink Run a simulation and change the scales of the various plots for an appropriate display Check up to 320 KHz and explain what you see or don t see These values have been carefully chosen to avoid certain spurious effects To see a sample of what we avoided try an input at 8002 Hz or asampling frequency of 32002 Hz or 32250 Hz instead e Compare with the spectrum of the sampling signal the pulses Back with an 8kHz signal and 32kH z sampling set up a second spectrum analyser and plot to display this on the same basis as the sampled signal e Repeat all this with a signal near half the sampling frequency Change the input frequency to 14 kHz and record the altered plots e Repeat with
56. s it you may have trouble even logging on Session 10 will have you using CommSim to perform a variety of communication systems functions and get hard copy of the resulting waveforms and spectra Each student will perform working on their own three exercises chosen from a pool of nine The selection of exercises has been done to try to give you each work of comparable difficulty and diversity Which set you get will be decided by a blind draw at Session 9 you will therefore have time to revise the theory before starting While you are welcome to perform exercises of your own devising and such attempts will be highly regarded our experience is that it is not easy to choose all parameters appropriately at the first attempt and the whole lot often has to be repeated Accordingly unless you are particularly confident we suggest that you basically follow the course suggested in the handouts available at Session 10 which must be returned at the end of that session This has the advantage that if you run into trouble you can compare your results with our records Improvements or additions to the suggested exercises will be highly regarded See Footnote Make sure that every printout you make includes your name and the date somewhere perhaps in the header as detailed for Session 9 Make sure also that every waveform is fully labelled Make each screen as useful as possible before printing it Keep your own record of all parameters wh
57. s needed for a production line would have a very different structure However too many students go to the opposite extreme they may give tables of values without making it clear whether they are predicted or measured ones perhaps not even revealing to what measurements they apply It is a waste of effort to copy slabs of prose from the practical notes to your report but you should include enough detail to make the report intelligible without constant reference back to the notes Identify what measurements are being made in plain English and not just by section number explain concisely how the measurements were obtained referring to the practical notes if appropriate but giving full details if you had to depart from the methods in the notes Explain how you analysed these results in a similar style don t just have stacks of raw equations Present the results of your measurements clearly and in such a way as to make the comparison with the predicted values easy to follow How to write it We do not expect your report to be a major literary feat but it should be in good basic English Write complete sentences not disconnected snippets Try to avoid spelling mistakes a report written to the standard of a ten year old does not inspire confidence In published papers it is necessary to spell out the exact experimental procedure to strangers so a high degree of detail and structure is appropriate However for ELEC321 the equipment is quite basic an
58. servations 9 Amplitude Modulation AM Make x2 X1 22 5 cosa ct fc 4kHz y1 10 cos Ot fm 50Hz P 2 4 6 Elec321 Practical Notes The output is OUT 5 cos ct 5 cos ct cos mt 5 cos ct 1 cos Opt The latter relation shows that the amplitude of the carrier is again modulated but never passes through zero note the difference of the zero crossings from section 8 A component at the carrier frequency is now present as well as the two sidebands of section 8 i Vary the amplitude of they input to observe degrees of modulation of less than 100 Sketch and comment ii AnAM waveform may alternatively be generated by making X1 10 cos wt X2 0 Y1 Am COS Wmt Y2 Ac Z2 0 What now determines the amplitudes of the carrier and sidebands Check this method in particular making Ac lt Am but Ac Am lt 10 why to observe the effect of overmodulation But note that overmodulation gives a different effect in anormal AM transmitter 10 Phase M odulation PM Make X2 Y2 0 X1 5 cos t fc 4kHz Y 5 COS Opt fm 50 Hz Z2 5 sinod i e we use quadrature carriers The output is OUT 5 sin t 25 cosa ct cos pt The second term can be regarded as being in phase with cos t but with a waxing and waning amplitude at frequency Opm P 2 4 7 Elec321 Practical Notes This phasor is at 90 to the carrier 5 sin ct so it advances and retards its phase at ra
59. squaring recover each of the four data signals from the 16 QAM signal P 9 10 9 ELEC321 Practical Notes Added N otes You are welcome to vary the parameter values from those suggested in the detailed notes However we would suggest that you consider the following guidelines Sampling Frequency Make this a reasonably large multiple of the carrier frequency particularly if you are interested in details of carrier phase If you intend to calculate spectra ensure that the waveform repeats itself after 4096 8192 16384 or 32768 sample points as the Fast Fourier Transform will always assume that the waveform is periodic outside the analysis interval You can t do this for a random bit stream but should analyse at least 8192 points to make the spectral lines as near continuous as possible Duration of Simulation For clear waveforms at a single frequency regard 1024 sample points as a minimum While a small number gives faster processing choose a value nearer 8192 particularly if you have a wide range of frequencies nice to have plenty of points per carrier period and intend to use filters which waste hundreds of points in a turn on transient Signal Frequencies Choose 1 kHz for the modulation frequency or bit rate For arandom bit stream the number of points between bit changes is n x repetition period x sampling frequency Make it an integer which is a multiple of the number of points in a carrier cycle Probably choose 16 kH z for
60. t and for selecting X or Y as the final output Wesave on use of Multipliers by getting only one output at once You may square up the output pulse using a Comparator Utilities with O V an appropriate REF erence level Watch out for an output which may be an inverted version of the original input You may like to replace the 60kH z LPF by a Tuneable LPF which can scarcely be made wide enough to get reasonably shaped output pulses P 6 8 16 Elec321 Practical Notes Practical Sessions 6 8 6 Amplitude Shift Keying Quadrature ASK Reading Schwartz 4 2 Lecture N otes 22 1 22 2 22 5 22 5 100kKH z Channel Filters Three switch selectable 100KH z channels are provided comprising two different filters and onestraight through connection Use Only one channel may be selected and used at a time Note that each of the three channels may be AC or DC coupled by front panel toggle switches Channel Characteristics Before using any of these three channels in experiments each channel should be characterised by actual measurement of amplitude and phase responses As a minimum the cut off and stop band frequencies should be measured using the VCO and true rms meter modules or an oscilloscope Basic Specifications Input coupling AC or DC Channels 1 to 3 Channel responses Channa 1 straight through Channel 2 bandpass filter as per specs but lowpass in fact Channel 3 lowpass filter as per specs but bandpass in fact
61. te m since the amount of phase shift is independent of m we have phase modulation rather than frequency modulation With only the two sidebands from the second term the amount of phase modulation can only besmall before the amplitude modulation becomes unacceptable Vary the amplitude of y1 to observe various degrees of modulation trigger the CRO off Z2 to make the modulation easily visible Calculate values of the modulation index peak phase deviation in radians and compare with expected values 11 Phase M ultiplication This uses frequency doubling Make X 1 Y 1 the phasemodulated signal output of section 10 Note the increase in the amount of phase modulation the increase in amplitude modulation needs to be countered Calculate the index of modulation is it doubled 12 Single Sideband M odulation SSBSC Make a suppressed carrier modulated wave by multiplying 10 cos t by 10 cos pt V1 10 cos t cos Omt 5 COS c Om t 5 cs c m Make a second modulated wave using quadrature inputs V2 10 sin ct sin om 5 cCOS c Mm t 5 COS M m Xt If these two modulated waves v1 and v are added using aZ 2 input the result is Vo V1 V2 10 cos Wc Mm t so that both the carrier and one of the sidebands are missing This method is not popular in practice eg it needs a quadrature generator usable over the whole audio band Moresimply Vo 10 cos ct cos mt sin ct sin mt 10 cOS Mp t
62. ted channel but to null the unwanted channel H ow carrier frequency synchronisation is achieved and phase tracking maintained is of course of great importance in practice This experiment serves to illustrate the principles of the multiplexing process Question If 90 is not essential then why not use a smaller angle and fit in an extra channel or two D ouble Sideband Suppressed C arrier D emodulation Ideas p 13 DSBSC V x x t x cos t Here the message is x t A simple form of message is x t A coso mt A demodulator will recover the message x t from the DSBSC signal Demodulation may be performed by multiplying the DSBSC signal by a carrier of the same frequency and phase and filtering the message from the result thus x t cost x V ostot 6 Y x t cosp YA x t cos 2m t 6 Note that the phase is set to maximise the amplitude of the recovered message The term at twice the carrier frequency is removed with a lowpass filter P 6 8 6 Elec321 Practical Notes Quadrature Amplitude D emodulation Ideas p 30 If the DSB signals are derived from different messages these messages can be independently recovered by synchronous demodulation with appropriate phases N ote that if at the transmitter there is a phase error from quadrature between the two DSB signals then this demodulator will be able to separate the two messages if its Phase Shifter is re adjusted to match that of the transmitter c
63. the carrier frequency to ensure that the spectrum does not fold around zero frequency and that carrier cycles and modulation cycles may be viewed on the one timebase These values also allow easy comparison with the results of others See also Sampling Frequency and make it fairly easy to filter modulation frequencies from carrier frequencies Signal Delay For easiest comparison with earlier practical sessions and the theory ensure an initial phase of 90 You can t do this with a random bit stream and 0 is probably more appropriate Duty Cycle Itis best to make each pulse last for an integer number of sample points Filters f you have to filter one frequency from another choose a cutoff frequency at the geometric mean so that the filter will pass one well and stop the other well To get a sharp filter response you ll need lots of calculations so the calculation time will be long We suggest that you make the Number of Taps about equal to the number of sample points in one period of the lowest frequency of interest a filter to renove dc from a demodulated output is the worst case If in doubt click on the Block Properties View Response option to see if the filter is satisfactory P 9 10 10 ELEC321 Practical Notes Spectral Analysis For the FFT Size choose 4k 8k 16k or 32k making sure as noted earlier that the waveform is periodic after this number of points Note that if the simulation runs for timeT the resolution of t
64. the lowpass filter allows a suitable set of samples to be taken of a single sine wave The sampling theorem requires that for undistorted reconstruction fs gt 2fm For the case fs 2fm the reconstruction filter would need to have a brick wall response For a practical filter with a finite transition band slope it would need to be correspondingly wider Reconstruction The circuit of Fig 1 below recovers the original sine wave from the set of samples above using the lowpass filter The above could be used as a simple qualitative demonstration of sampling and reconstruction It can be made quantitative by examining more closely the relationships between the system parameters including fs fm pulse width filter cutoff f and input and output amplitudes Sampling and reconstruction as illustrated above is an example of pulse amplitude modulation PAM Bandlimited Channel Pulse Shaping The above is a simple introduction to sampling and message recovery The experiment can be extended to cover pulse shaping channel band limiting and so on P 6 8 11 Elec321 Practical Notes Procedure Dual Analog Switch Tuneable Lowpass Filter Sample Clock JUL 83k Fig 1 Basic Sampling First set up the Tuneable LPF to cut off at about 4 kHz near top of NORMal range Gain 2 Set up the basic sampling circuit of Fig 1 Record the sampled waveform Check that the output shape and sizeis as
65. the topic whose theory you understand best and prepare for the other exercises in the intervening week by studying the theory behind them P 9 10 6 ELEC321 Practical Notes Practical Session 10 EXERCISES USING CoMMSIM See the earlier notes about this session particularly the final warning about comparing the precise results obtained with the theory this is what your report will be judged on The exercises to be performed have been split into three groups and each student will be allocated one from each group Group A Waveforms 1 Sampling and reconstruction 2 Quantisation noise and companding 3 Eye patterns Group B Analogue modulation 4 AM generation and detection 5 SSBSC generation and detection 6 FM generation and detection Group C Digital modulation 7 OOK 8 FSK 9 16QAM A brief description of the content of each exercise is given below and detailed suggestions will be provided in the laboratory L Sampling and reconstruction e Set up a sampling waveform e Set up an input message e Sample the message e Filter the sampled signal to recover the message e Find the spectrum of the sampled message Compare with the spectrum of the sampling signal the pulses e Repeat all this with a signal near half the sampling frequency e Repeat with a signal chosen to demonstrate aliasing P 9 10 7 e gt gt o gt gt o Pf e gt o o ww e gt o gt gt o ooN eeee e
66. the two inputs to the adder equal and opposite If the input is zero the output will alternate rapidly with equal times for a HIGH and a LOW value so that the output of the integrator is zero equal to the input If the input now suddenly changes to a positive value the bipolar data output will jump to aconstant state which opposes the change at the adder and will stay there until the integrator output reaches the new input value the output will then revert to a 50 duty cycle and the integrator will hold the new input voltage Note that any dc input will give the same output a 50 duty cycle If the input is aramp of voltage the output will take on a duty cycle corresponding to that average voltage which applied to the integrator tracks the ramp the faster the ramp the higher the average voltage What is sent is therefore the rate of change of the input after integration in the Delta Modulator it tracks the input voltage The demodulation process will need to perform integration on the transmitted data stream but the result will have an arbitrary dc level Note again that what is compared with the input in the adder is the integral of the bipolar output which should therefore be the differential of the input Overload noise can occur if the output is a constant 2 V or 2 V the resulting slope of the ramp from the integrator is the maximum slope of the input before overload occurs So much for the principle of this data m
67. this is nonsense For example what if one of the signals were zero Turn the noise down to OdB Now try to square up the bit stream using the Decision Maker Note that we used the analogue bit stream for two reasons e Itisaninverted version of the digital bit stream but this compensates for inversion in the Adder e When set to NRZ L the Decision Maker has a threshold at OV central on the waveform However the bit stream out of the Decision Maker is bipolar so it needs to be set to TTL levels for later use To CRO ch 4 5V Decision Utilities Maker Comparator Corrupted Decided IN1 OUTI IN bit stream bit stream 2kHz clock BCLK REF Observe IN 1 OUT1 Z MOD on the CRO while still triggering off the SYNC of the Sequence generator ch 3 The short pulses of ZMOD indicate the times at which the output leva is changed depending on whether the input is then positive or negative The decision should not be made while the input is changing so adjust the Decision Point until Z MOD pulses avoid such transitions but coincide with the centre of the shortest input pulses P 11 13 4 ELEC321 Practical Notes Check that the output is now a good copy of the input notehowever that the output has to be a ddayed version of the input You may like to try other filters set the Tuneable LPF to its widest setting and put this signal also through the Baseband Channel Filters N B 2 amp 4areswapped
68. tion of SNR and compare this with the theory These measurements must take account of the delays inversions and level shifts needed to ensure that the Decision Maker works correctly and that the final output is fairly compared with the original data N otes on how to make these measurements are given below and need to be followed to the letter if the results are to be valid P 11 13 2 ELEC321 Practical Notes Bit Error Rate as function of SNR etc Set up these units from left to right 1 Audio Oscillator Set Af to give 2 KHz 2 Sequence Generator Set switches for minimum sequence length both up 3 Tuneable LPF Initially set Tune clockwise Gain top centre Wide 4 Baseband Channe Filters Initially set to 1 5 NoiseGenerator Initially set to OCB 6 Adder Initially s G and g to top centre 7 Wideband TrueRMS Meter Initially setto AC 10V 8 Decision Maker Set to NRZ L INT 9 Utilities 10 Error Counting Utilities Check the PCB switches they should bein the default positions as follows ji NORM AdiveLevd Trig HI swi Gate LO J Count Mult xl SW2 11 Twin Pulse Generator Initially set Width to min Delay to max 12 Twin Pulse Generator Asfor 11 Note that the Procedure that follows tells you how to get sensible results It does not teal you eveything you should do and observe and record Think of useful and informative things to do and record at each stage so that your rep
69. tons Use the normal timebase A rather than the delayed timebase B for your measurements AUTO SETUP is a magic button 1 ii iii iv 1 ii iii 2 Low pass Circuit Frequency Response Set up a low pass circuit using R 22 KQ C 47 nF Calculate its 3dB frequency fo Take enough measurements to allow you to check this value and to sketch the amplitude and phase response for a decade or so either side of fo Up to what frequency is the amplitude response constant to within 10 Up to what frequency is the phase response constant to within 10 of a radian Note how different are the results of these apparently similar criteria 3 Low pass Circuit Time Response Apply a 50Hz square wave to the circuit and record the output waveform In the time domain we may solve the differential equation involved namely C dvy dt vj V9 R i If the input v swings from V to V the output v may be shown to be given by check for t 0 and t Vo V 2 Vexp t T where t RC is called the time constant The output takes a couple of time constants to rise or fall This is related to the time needed to charge or discharge C through R Discuss the frequency response in similar terms to these E g for a given input voltage what is the maximum rate of change of output voltage Discuss the observed output waveform in terms of the effect of the circuit s frequency response on t
70. two sine waves and display the result as an orange trace 2nd plot input up Display the 1kHz sine wavein blue 2nd plot input down You may need to shuffle the blocks around somewhat to get a good clear schematic with no wires or labels or blocks obscured Try to make as many wires as possible have no corners Run the simulation The two trace colours suggested are easily distinguished on the screen and on a black white print where one is near black and the other dotted dark grey N ote When you edit a diagram the screen may be not fully updated You can fix this using Edit Repaint Screen but this is used often enough that you may like to add a special icon for this function Go through View Toolbar User OK then Edit Toolbar and for Button 0 choose Function Edit gt Repaint Screen OK Try selecting a set of blocks by left clicking to their top left and dragging a dotted box over the selected part of the diagram Move the selected section a little to check At this stage you will probably have to add the printer to your configuration Go through Start Settings Printers click Add Printer double click NEXT click Network Printer tick NEXT click Find Now click E64 219 laser double click FINISH click Add your name and the date and time to the diagram so that your printout can be identified go to File Page Setup then after F put a few spaces then D date amp time then a few spaces then your name in full
71. ular Plot all waveforms perhaps starting at 1 msec to avoid the filter transient You may like to establish a fine grid or add a negative dc voltage or use a high pass filter but this is not as easy as it looks to measure the amplitude of the demodulated signal Or you may save the data as a file and get the peak output values from the file e Attempt synchronous detection using a quadrature carrier Asabove except multiplying the SSBSC signal by a quadrature carrier e Attempt synchronous detection using an off frequency carrier As above except using a carrier at 16250 Hz P 9 10 17 ELEC321 Practical Notes 6 FM generation and detection Use Simulation Properties Frequency 4096000 Hz End at 0 002 sec e Generate AM by adding aDSBSC signal to a carrier Set up a carrier at 16 kHz with amplitude 2 make it a cosine wave Set up a message signal at 1 kHz make it a cosine wave Usea multiplier x to get the DSBSC signal and a summing junction 2 to add a carrier Use cosine waves where possible Choose values to get 40 modulation with a peak voltage of 28 V e Makea small change to produce narrowband FM What is it e Check the amplitude modulation and phase deviation of this signal For example compare it directly with the original carrier e Set up a wideband FM signal Get an FM modulator Comm Modulators Real FM Re Suitable parameters are Translation Frequency Hz 16000 the carrier frequency
72. use the signal generator for fm if you wish Now try using an output from the VCO as a local carrier first adjust the VCO to close to 100 kHz This should be hopeless sin wt extracted carrier Feedback path Fig 3 Phase Locked Loop You may now like to try extracting a good carrier from the AM signal using a phase locked loop as in Fig 3 This setup will recover a carrier from the complete AM signal but needs careful adjustment First reduce the modulation to zero Set both Tuneable LPF controls mid way Turn the VCO gain right down and adjust f until very close tof Then turn up the VCO gain just a little until the two signals lock in frequency Adjust thefo control to get the two roughly in quadrature 0 V out of the LPF Perhaps adjust the VCO gain to the centre of the usable range Now turn up the modulation and check that it still locks Check that this extracted carrier with perhaps some phase shift may be successfully used instead of the original carrier in the demodulator of Fig 2 Single Sideband G eneration Phasing M ethod Ideas p 18 See Fig 4 below The output may be changed from one sideband to the other depending upon e which local carrier leads the other e whether the adder is additive or subtractive Amplitude balancing controls are available in the Adder P 6 8 3 Elec321 Practical Notes The output is based on a carrier of frequency O Carrier 100 kHz Messa
73. utput Check that the output amplitude and frequency vary with that of the original modulation signal Vary the parameters of the feedback loop of Fig 2 and record and comment on the results of those variations record more than just the output waveform if the circuit misbehaves 10 us Tuneable Lowpass Filter Twin Pulse TTL Generator Q 47 nF Fig 3 FM Receiver 2 pulse count Now alter the FM generator to have parameters fc 80 kHz fm 1 kHz 10 Demodulate the signal using a pulsecounting method i e produce a standard pulse for each period of the FM input and filter this pulse train The basic set up is as in Fig 3 note that because the pulse train has a large dc component the ac output will be rather small unless that dc is removed You may do this by adding a dc offset or by capacitive coupling as shown in Fig 3 Check that the pulse width is appreciably less than a period of fc 8 fm otherwise reduce fc somewhat but check that f B fm is not too small Record the various waveforms and compare your results with an appropriate theory P 6 8 10 Elec321 Practical Notes Practical Sessions 6 8 4 Sampling and Reconstruction Reading Schwartz 3 2 3 3 Lecture N otes 12 1 12 2 12 6 12 8 Sampling The sampling theorem shows that a band limited signal may be completely reconstituted from a sequence of suitably spaced and shaped samples The circuit of Fig 1 below without
74. uts affect the output using quite different mechanisms iii Repeat i and ii not so thoroughly but with the sine wave at 25 kHz Look particularly for phase shifts and feedthrough output that can t be reduced with a dc offset Comment on what these measurements reveal about the imperfections of the multiplier Refer back to these measurements if one of your later circuits does not work as well as it might 4 Product of Two Sine Waves at Different Frequencies X1 A cos jt Y1 B cos 7t 07 then their product is X1 Y A B cos jt 1 cos apt 02 which using a standard trigonometrical identity becomes X1Y1 A B 2 cos 1 2 t 01 62 cos 1 02 gt 1 2 so that if 1 and are not equal the result is the sum of two sinusoidal oscillations at two new frequencies 1 and 1 07 and with equal amplitudes A B 2 This surprising result is very important in the study of communication systems with a wide variety of interpretations applications i Make X2 Y2 Z7 0 X1 10 cos t 01 f1 5 kHz Y1 10 cos w7t 02 f2 4kHz and observe the multiplier output OUT X 1Yj1 10 Check that it appears to be a slow 1KHz sine wave with a fast 9kHz sine wave superimposed Check the precise f and f values using the CRO Investigate various trigger sources for the most appropriate display triggering off the very highest peaks of the output may give the best display but
75. with the phase shift between X1 and Yj and a sinusoidal component of constant amplitude ii Patch up a simple R C low pass filter consisting of R 220kQ C O047uF Calculate its 3dB frequency Check its function on the CRO using a x10 probe to avoid loading the filter unduly Measure the dc output of the multiplier as in part i using this filter and check and plot the dependence of this voltage on the phase difference o1 02 P 2 4 4 Elec321 Practical Notes 6 Product of Two Sine Waves at Nearby Frequencies Remember that if sine waves at frequencies and are multiplied the output is the sum of two sine waves at frequencies 1 and 2 If then itis relatively easy with a low pass filter to pass only the difference frequency i Make X2 Y7 Z7 0 X1 10 cos jt 01 f1 4kHz Y 10 cos wgt 02 whereY 1 is derived from a signal generator to allow to be varied Read again the Important N ote of section 2 Pass the output through the low pass filter used in section 5 and observe the output on the CRO Vary f about 4 kHz carefully adjusting it to obtain the largest possible output amplitude Make this output only a few Hz by precise selection of the frequency of the signal from the generator N ote that with an input frequency of the order of 1Hz the CRO amplifier must be dc coupled for accurate results Verify that the output amplitude is 5 volts as expected Chec

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