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Instructor's ManualInstructor's ManualInstructor's ManualInstructor's Manual

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Lab SheetsLab SheetsLab SheetsLab Sheets

Instructor's ManualInstructor's ManualInstructor's ManualInstructor's Manual

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Lab SheetsLab SheetsLab SheetsLab Sheets

rev 1.1Emona Instruments Pty Ltd(ABN 79 069 417 563)86 Parramatta RoadCamperdown NSW 2050Sydney AUSTRALIA

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PrefacePrefacePrefacePreface

This Instructor`s Manual is to support TIMS Lab Sheets.

Comments and criticisms are welcome - [email protected]

Tim HooperSydney, 2001

TIMS Lab Sheets Instructor`s Manual ver 1.2 - 1

TIMS LAB SHEETSTIMS LAB SHEETSTIMS LAB SHEETSTIMS LAB SHEETSINSTRUCTOR`S MANUALINSTRUCTOR`S MANUALINSTRUCTOR`S MANUALINSTRUCTOR`S MANUAL

VER 1.2VER 1.2VER 1.2VER 1.2Please read the notes (if any) for an experiment before running it.

Most experiments are concerned with the transmission of a message – these can taketwo general forms, depending on the context.

analog messagesanalog messagesanalog messagesanalog messages

In analog experiments requiring a message this comes typically from an AUDIOOSCILLATOR. In order to minimize the module count this can often be omitted,and the 2 kHz MESSAGE provided by the MASTER SIGNALS module used instead.But this restricts the number of observations which can be made, with the possibleloss of some insights into the systems behaviour.

In the interests of good teaching this economy is not recommended.

The sinewave message is easy to represent analytically, and its properties simplymeasured. A more complex message is a two-tone. This is also easily representedanalytically, and reasonably easy to measure. Following testing, however, theultimate message will probably be speech.

Speech can be obtained from a SPEECH MODULE. The two available speechchannels should be recorded using bandlimited speech. The subject matter shouldbe sufficiently different as to simplify channel identification.

Although a SPEECH MODULE is seldom specified in the module inventory, it issuggested that one be added to the inventory where appropriate. Its inclusion can beof benefit, especially as regards signal visualisation in the time domain.

A SPEECH MODULE can also be a source of periodic messages by recording theoutput of, say, a VCO. Using both of the speech channels a two-tone test signal canbe created. The VCO is then free to be used in another part of the experiment.

Do not forget a DC message - often useful in an analog system.

digital messagesdigital messagesdigital messagesdigital messages

The most common form of digital message in TIMS is the pseudo random binarysequence (PRBS), provided by a SEQUENCE GENERATOR module.

Do not forget the very useful DC signal. Both a TTL LO and HI are provided by theVARIABLE DC module.

oscilloscope displaysoscilloscope displaysoscilloscope displaysoscilloscope displays

There is nothing more annoying – especially when giving a demonstration – than anunstable oscilloscope display. Wherever possible choose an appropriatesynchronizing signal, and use it to trigger the oscilloscope by connecting it to the

2 TIMS Lab Sheets Instructor`s Manual

ext. synch. or ext. trig input.

NOTE: this is a draft version of the Instructor’s Manual. Feedback is soughtregarding the kind of information required by Instructor’s in the next update(comments to [email protected])

TIMS Lab Sheets Instructor`s Manual ver 1.2 3

Introduction to TIMSIntroduction to TIMSIntroduction to TIMSIntroduction to TIMS

Modelling equationsModelling equationsModelling equationsModelling equations

DSBSC – generationDSBSC – generationDSBSC – generationDSBSC – generationmini version: no AUDIO OSCILLATOR or ADDER introduces some restriction!

For most displays use the message as the synchronising signal, connected to the ext.synch. input of the oscilloscope.

If an AUDIO OSCILLATOR is not available then use the single tone message fromMASTER SIGNALS. However a variable frequency message is to be preferred inorder to observe more inter-relationships. For example it is often stated that thecarrier frequency (ω) should be much greater than the message frequency (µ).Ambitious students can try ‘violating’ this inequality by using the 2 kHz message(as the message), and an AUDIO OSCILLATOR as the carrier.

Remember to trigger the oscilloscope to the message. This preferable in mostcases.

The ADDER enables a pilot carrier to be added to the DSBSC. This simplifiescarrier acquisition by a demodulator. It is possible to calculate the amount ofcarrier present from observations, in the time domain, of the relative amplitudes ofalternate peaks of the DSB envelope.

To ensure that both waveforms are stationary on the screen (Figure 2) the messagemust be a sub-multiple of the carrier frequency. A pair of such frequencies is easilyobtained by using a 100 kHz carrier and a 2 kHz message (actually 2.083 kHz) fromMASTER SIGNALS. However, the frequency ratio of 48:1 is rather high. Insteaduse an AUDIO OSCILLATOR tuned close to 8.333 kHz, and then synchronized tothe 8.333 kHz TTL from MASTER SIGNALS. This gives a frequency ratio of24:1, which matches the waveforms of Figure 2.

BEWARE: the block diagram of Figure 1 ensures no phase difference between theDSBSC carrier and the pilot carrier. The modules of Figure 3 probably achieve thistoo. But what if there is a phase shift between the two (as there often is in a less-than-ideal situation) ? The display will change dramatically as this phase differenceis increased - until the signal is reminiscent of that of Armstrong. Ambitiousstudents should be given a PHASE SHIFTER so as to investigate further.

Product demodulation Product demodulation Product demodulation Product demodulationmini version: no AUDIO OSCILLATOR (use 2 kHz message), and only onePHASE SHIFTER.


In the list of Lab Sheet experiments this one (entitled Product demodulation) comesbefore those detailing the generation of AM and SSB. That is unfortunate, since


both AM and SSB signals are required for this experiment.

Remember to set the on-board switches of both PHASE SHIFTER modules to HI.

A suitable source of signal is shown modelled in the Figure below.

DSBSC

carrier

AM out

adjust phase

g

G

message

carrier (100kHz)

( µµµµ )

( ωωωω ) typically ωωωω >> µµµµ

The ADDER is used to add pilot, or full, carrier, as required. The PHASESHIFTER will set the appropriate phase for AM (not set by measuring phase, but byobserving the envelope of the AM itself).

An exact model of this block diagram may not quite give AM ! This is becausethere will be an inevitable, but small, phase shift between the DSBSC and thecarrier, at the adding point. The PHASE SHIFTER may not have enough range toadd further shift to rotate this back to the in-phase condition. Thus it is best to useone of the quadrature outputs of the 100 kHz from MASTER SIGNALS to themultiplier, and the other to the PHASE SHIFTER. This inserts a close to 900 phasedifference between the DSBSC and carrier, which is easily increased to 1800 by thePHASE SHIFTER (whereas an increase of 1800 may be difficult).

Demodulation AM and DSB(SC) using a non-synchronous carrier is not a practicalproposition, which can be demonstrated. Small frequency errors (say less than 1 Hz)produce a slowly varying output amplitude (which includes periods of zeroamplitude). Larger frequency errors result in other undesirable effects (try it !).Text books often mis-report the effect. SSB is an exception; errors up to 10 Hz goun-noticed with speech (and even music). This also is often mis-reported in textbooks.

When the output is required to be the exact frequency of the message then nofrequency error can be tolerated at the demodulator.

It us interesting to show that, whereas a phase other than zero at the transmitter willeffect the shape of the envelope of AM, this has no effect upon the messagewaveform recovered by a product demodulator. Not so, of course, from anenvelope detector.

AM - amplitude modulation – IAM - amplitude modulation – IAM - amplitude modulation – IAM - amplitude modulation – IFor most displays use the message as the synchronising signal, connected to the ext.synch. input of the oscilloscope.

AM - amplitude modulation – AM - amplitude modulation – AM - amplitude modulation – AM - amplitude modulation – IIIIIIIImini version: no AUDIO OSCILLATOR – use 2 KHz MESSAGE.



The aim is to get the carrier in phase with the DSBSC (actually with the resultantphasor of the DSBSC, which is where the carrier would be if it were present). InArmstrongs system (see separate Lab Sheet) the aim is for the phase difference to be900. In either case it is the envelope which is used to make the adjustment.

Figure 3: The AM signal at various stages of phase adjustment.Message at the top, then the model output waveform (going from top to bottom thephase is being reduced from 900 towards 00 (in fact 900, 700, 450, with 00 notshown). The kiss will happen at 00.

You might gain further insight by looking at the Lab Sheet entitled Armstrong`sphase modulator.

Envelope detectionEnvelope detectionEnvelope detectionEnvelope detectionmini version: no AUDIO OSCILLATOR or 60 kHz LPF


To recover the message from the envelope of a signal the shape of the envelope mustbe a ‘copy’ of the shape of the message. The obvious choice is an amplitude-modulated signal with m < 1). For the case m > 1 the envelope is not a copy of themessage.

The fact that the envelope is no longer a copy of the message means that themessage can no longer be recovered using an envelope detector. But the envelopethat results (when m > 1) is exactly what analysis predicts. So there is, in the strictsense, no envelope distortion introduced by the equipment or the generation method.

Semantics ? Perhaps, but this phenomenon is often mis-reported in text books.

Note also that a well-designed envelope detector can recover this envelope withoutdistortion. Text books often suggest/infer/imply that the envelope cannot berecovered when what is meant is that the message cannot be recovered. It is theapproximation to the ideal envelope detector – the so-called ‘diode detector’ -which may fail in many cases – but that is the fault of the approximation process (orits requirements not being met).

The well-designed envelope detector uses an ‘ideal rectifier’ and a separate LPF ofadequate bandwidth. The ubiquitous diode detector, a diode-plus-RC-filter, is anapproximation to this. Unless the carrier frequency (ω) is very much higher thanthe highest message frequency (µ) it fails miserably.


Mini version of this experiment omits the 60 kHz LPF and the AUDIOOSCILLATOR (for the message); use instead the 2 kHz fixed-frequency messagefrom MASTER SIGNALS. Omitting the TUNEABLE LPF from the inventorywould limit the number of observations to be made about the envelope bandwidth.

Note that with a 2 kHz message, the envelope of a DSBSC has significantcomponents beyond 12 kHz (the upper band width of the TUNEABLE LPF). Thusthe filter output is a fair, but obviously imperfect, copy of the envelope. Swappingto the 60 kHz LPF confirms this, since the filter output is now much improved.

If you have an AUDIO OSCILLATOR, but not a 60 kHz LPF, then use the lowestpossible message frequency (near 250 Hz), and now the 12 kHz bandwidth of theTUNEABLE LPF is more than adequate to recover all significant components of theDSBSC envelope. Reduce the bandwidth and show that the recovered envelopeshape is badly distorted.

SSB generationSSB generationSSB generationSSB generationmini version: no AUDIO OSCILLATOR

While confirming the individual DSBSC, use the message as the synchronisingsignal, connected to the ext. synch. input of the oscilloscope. Sweep speed to showa few message periods. Use it also when minimizing the envelope of the ADDERoutput (the SSB), since it makes the envelope stationary on the screen. To observethe SSB itself (derived from a single tone), rather than its envelope, increase thesweep speed to show a few periods of the 100 kHz SSB, and use the SSB itself asthe synchronizing signal.

SSB demodulationSSB demodulationSSB demodulationSSB demodulation

ISB - independent sidebandISB - independent sidebandISB - independent sidebandISB - independent sideband

Armstrong`s phase modulatorArmstrong`s phase modulatorArmstrong`s phase modulatorArmstrong`s phase modulatormini version: no AUDIO OSCILLATOR or UTILITIES.


Don`t forget to set the PHASE CHANGER to the HI range (on-board SW1).

Figure 3 contains al the info required to set the phase to 900.

If you have a set of headphones it is instructive to use an envelope detector torecover the envelope. By listening to the recovered envelope it is easy to determinewhen the 900 point is reached. The fundamental of the message can be hard to nullout at that condition. This is a very sensitive test (and eliminates the need for anoscilloscope). Even in the presence of the higher harmonics this null is obvious –an example of psycho-acoustics. This phenomenon is not mentioned in the LabSheet, but see the equivalent experiment in the Student Text (Volume A2).


FM - generation by VCOFM - generation by VCOFM - generation by VCOFM - generation by VCOmini version: no AUDIO OSCILLATOR.

Synchronize the oscilloscope to the FM signal itself when displaying Figure 1(a).

FM - demodulation by FM - demodulation by FM - demodulation by FM - demodulation by PLLPLLPLLPLL

FM - demodulation by FM - demodulation by FM - demodulation by FM - demodulation by ZX countingZX countingZX countingZX countingmini version: no AUDIO OSCILLATOR.

SamplingSamplingSamplingSamplingmini version: no SPEECH.

PAM and PAM and PAM and PAM and TDMTDMTDMTDM

FDM - frequency division multiplexFDM - frequency division multiplexFDM - frequency division multiplexFDM - frequency division multiplex

Phase division multiplex – generatePhase division multiplex – generatePhase division multiplex – generatePhase division multiplex – generatemini version: no SPEECH.

Phase division multiplex – Phase division multiplex – Phase division multiplex – Phase division multiplex – demoddemoddemoddemodmini version: no SPEECH or AUDIO OSCILLATOR.

A total of four MULTIPLIER modules and a second HEADPHONE AMPLIFIER(or the equivalent such as a TUNEABLE LPF) would give the most realistic versionof this experiment, since it would allow simultaneous recovery of two messages.But that is perhaps an unnecessary luxury. A single demodulator patched betweenchannels should be sufficient.

If an AUDIO OSCILLATOR is not available for one of the messages perhaps a DCsignal could be substituted (with DC coupling of the relevant MULTIPLIER at the


generator), and use of the oscilloscope to detect an output.

PWM - pulse width modulationPWM - pulse width modulationPWM - pulse width modulationPWM - pulse width modulationmini version: AUDIO OSCILLATOR or TUNEABLE LPF.

Carrier acquisition – Carrier acquisition – Carrier acquisition – Carrier acquisition – PLLPLLPLLPLLmini version: no AUDIO OSCILLATOR.

The WAVE ANALYSERThe WAVE ANALYSERThe WAVE ANALYSERThe WAVE ANALYSER

Complex analog messagesComplex analog messagesComplex analog messagesComplex analog messages

PCM – encodingPCM – encodingPCM – encodingPCM – encodingIf available a WIDEBAND TRUE RMS METER could be added to the moduleinventory, but is not essential (see experiment text).

PCM – decodingPCM – decodingPCM – decodingPCM – decoding

ASK – generationASK – generationASK – generationASK – generation

ASK – demodulationASK – demodulationASK – demodulationASK – demodulationmini version: AUDIO OSCILLATOR or DECISION MAKER.


BPSK – modulationBPSK – modulationBPSK – modulationBPSK – modulationmini version: no LINE-CODE ENCODER, TUNEABLE LPF, 100 kHzCHANNEL FILTERS.

BPSK – demodulationBPSK – demodulationBPSK – demodulationBPSK – demodulationmini version: no LINE-CODE ENCODER, TUNEABLE LPF, 100 kHzCHANNEL FILTERS.

QPSK – generationQPSK – generationQPSK – generationQPSK – generationmini version: 100 khz channel filters

QPSK – demodulationQPSK – demodulationQPSK – demodulationQPSK – demodulationmini version: inventory reduced by 1 x MULTIPLIER , 1 x PHASE SHIFTER, 1 xTUNEABLE LPF.

FSK – generationFSK – generationFSK – generationFSK – generationmini version: no AUDIO OSCILLATOR

FSK - envelope demodulationFSK - envelope demodulationFSK - envelope demodulationFSK - envelope demodulationNo mini version. AUDIO OSCILLATOR essential

Signal constellationsSignal constellationsSignal constellationsSignal constellations

DSSS - spread spectrumDSSS - spread spectrumDSSS - spread spectrumDSSS - spread spectrum

Eye patternsEye patternsEye patternsEye patterns


PRBS messagesPRBS messagesPRBS messagesPRBS messagesmini version: no TUNEABLE LPF.

This is an elementary introduction to the SEQUENCE GENERATOR module. Iftwo such generators are available (recommended) then alignment of the two can bedemonstrated.

A Lab Sheet entitled BER INSTRUMENTATION is an important extension of thisexperiment. It illustrates a method of alignment when there is delay between thetwo SEQUENCE GENERATOR modules.

Although a TUNEABLE LPF is suggested as an optional extra module, it is notessential.

Detection with the DECISION MAKERDetection with the DECISION MAKERDetection with the DECISION MAKERDetection with the DECISION MAKER

The noisy channelThe noisy channelThe noisy channelThe noisy channelThis sheet does not describe an experiment. It serves as a reference to the NOISYCHANNEL model used in later experiments.

BER instrumentationBER instrumentationBER instrumentationBER instrumentationThis sheet does not describe an experiment. It serves as a reference to theINSTRUMENTATION model used in later experiments.

BER measurement – introductionBER measurement – introductionBER measurement – introductionBER measurement – introduction

Line coding & decodingLine coding & decodingLine coding & decodingLine coding & decoding

Delta modulationDelta modulationDelta modulationDelta modulationBefore plugging in the DELTA MODULATION UTILITIES set the on-boardswitch SW2A ON, and SW2B OFF. Set the ADDER and the two BUFFERAMPLIFIERS each to unity gain (use a sine wave). Do NOT ever change theADDER gains.

Patch up Figure 3 and check you have the waveforms of Figure 2 (for a completely


stationary display it is necessary to fine-trim the oscilloscope triggering).

Adjust one of the buffer gains to get the ‘best’ approximation between the twowaveforms on the screen.

Failing the above check that, with the input removed from the B input of theADDER:

1. the HARD LIMITER output is ±2.5V inverted square in synch with the message2. the SAMPLER output is ±5V (non-inverted) square in synch with the message3. the BUFFER output is the same as the SAMPLER output

Delta-sigma modulationDelta-sigma modulationDelta-sigma modulationDelta-sigma modulation

Adaptive delta modulationAdaptive delta modulationAdaptive delta modulationAdaptive delta modulation

Delta demodulationDelta demodulationDelta demodulationDelta demodulationmini version: no SPEECH.

Bit clock regenerationBit clock regenerationBit clock regenerationBit clock regenerationIt is stated in the notes that a non-linear operation on the data stream will introduce aspectral line at the bit clock rate (and harmonics) which was otherwise absent. Thisis only partially true. A squarer is suggested. A perfect bi-polar signal, whensquared, produces a DC component only – with no harmonics. But if first processedin some way, these ideal conditions are lost and a component may appear. By‘processing’ is meant any operation which produces the desired effect ! Lowpassfiltering is effective. Other methods include introducing some delay (or difference)into one of the arms of the squarer, or switching the squarer (the MULTIPLIER) intothe AC coupled mode. Whilst these methods are effective, they are not necessarilyoptimum. They are mentioned here to make you aware that an effective methodshould be both investigated and understood theoretically.

QAM – generationQAM – generationQAM – generationQAM – generation

QAM – demodulationQAM – demodulationQAM – demodulationQAM – demodulation


BPSKBPSKBPSKBPSK

Broadcasting (two mini versions)Broadcasting (two mini versions)Broadcasting (two mini versions)Broadcasting (two mini versions)The full experiment includes modules for both AM and FM transmissions. Miniversions cater for one or the other of these only.

Other forms of modulation are of course possible, but are not discussed.

For the AM case only envelope detection is described. This means receiverselectivity is provided by the BPF in the 100 kHz Rx ANTENNA UTILITIESmodule. This is in excess of ±10 kHz centred around 100HZ. Better selectivitywould be obtained by using a product demodulator, and the LPF in theHEADPHONE AMPLIFIER module. This arrangement would allow two or moretransmitted signals to be transmitted (and recovered independently) in the 90 to110 kHz region. This arrangement is not described in the Lab Sheet, and modulesfor it are not included in the normal inventory.

Fibre optic transmissionFibre optic transmissionFibre optic transmissionFibre optic transmission

multi-channel digital fibre linkmulti-channel digital fibre linkmulti-channel digital fibre linkmulti-channel digital fibre linkmini version: no FIBRE modules (!) and no SPEECH.

Students should be encouraged to consider the choice of FDM carrier frequencies,messages, and the delta modulator sampling frequency. Those suggested are notmandatory, but are compatible.

Since the FDM itself has a bandwidth up to about 15 kHz, the sampling rate of thedelta modulator must be increased from the ‘normal’ 100 kHz rate to somethingmuch higher. Fortunately the delta modules can work at a rate up to about 1 MHz,and such a TTL signal is available as the clock of a TUNEABLE LPF (at the frontpanel). This ranges from a few hundred kilo Hertz to a few mega Hertz.

The model needs to be set up in a systematic manner. For example:

1. the ADDER in the two QUADRATURE UTILITIES modules to unity2. the ADDER of the DELTA MODULATOR model to unity (and leave them so set permanently)3. each of the BUFFER AMPLIFIERS to unity

PCM-TDM ‘T1’ implementationPCM-TDM ‘T1’ implementationPCM-TDM ‘T1’ implementationPCM-TDM ‘T1’ implementationmini version: no FIBRE modules.

Cannot use speech for messages, or even an AUDIO OSCILLATOR, as samplingrate is far too slow. Yes, the (standard, non-storage) oscilloscope does flicker whenlooking at the internal periodic message.


DPSK & DPSK & DPSK & DPSK & BERBERBERBERThis is a relatively complex experiment and must be carefully set up.

In order to conserve modules it is possible to omit the bandlimiting channel. Bandlimiting (particularly of the wideband noise) is provided at the demodulator outputby the TUNEABLE LPF.

Use of a channel filter requires in addition a PHASE SHIFTER for the stolen carrier,to account for the delay introduced by the channel. The presence of these two extramodules leaves no room for the WIDEBAND TRUE RMS METER. However,this is used only once (for setting up the SNR at the detector – the DECISIONMAKER - input). At that time the modules for error counting are not required, soone of these may be removed (to accommodate the meter) while the reference SNRis set up.

You may refer to use a separate MULTIPLIER module at the receiver, rather thanthe second multiplier of the QUADRATURE UTILITIES (although the twomultipliers of the QUADRATURE UTILITIES module are entirely independent,some students may feel uncomfortable having to ‘loop back’ into the transmitter forpart of the receiver).

It is unfortunately necessary to make a one-time adjustment of the ADDER in theQUADRATURE UTILITIES module. This is to set the signal level while adjustingthe SNR. An extender board would help here.

Students will be required to view both snap shots of the message sequence, and eyepatterns. Oscilloscope synchronising signal is the start-of-sequence, and bit clock,respectively.

Since there is no room on a single Lab Sheet to give detailed instructions you shouldsupply your own. Some suggestions follow (taken from the related experiment inthe Student Text).

1. initially set the SEQUENCE GENERATORs to a short sequence (toggles of SW2 both UP).2. set the on-board switch SW1 of the DECISION MAKER to accept differential encoding (NRZ-M),

and SW2 to ‘INT’ (manual decision point adjustment).3. on the QUADRATURE UTILITIES board rotate the ADDER gain control A about 25% clockwise

(will require re-setting later), and B fully clockwise.4. patch up and check the transmitter, using (say) the upper MULTIPLIER of the QUADRATURE

UTILITIES. The 100 kHz carrier comes from MASTER SIGNALS. Use the NRZ-M line code.5. connect the Tx output to the A input of the ADDER in the QUADRATURE UTILITIES. This

ADDER acts as a wideband channel to the Rx. Noise will be added later (initially omit the noiseinto ADDER input B).

6. the output of the channel (ADDER) is the input to the Rx, so connect it to the A input of the lowerMULTIPLIER of the QUADRATURE UTILITIES. Steal the 100 kHZ carrier from the Tx (samephase). Because the channel has no delay, there is no real need for a PHASE CHANGER in the pathof the stolen carrier.

7. patch in the Rx demodulator TUNEABLE LPF (set to max bandwidth), and the receiver ADDER.The ‘Rx ADDER’ serves to set signal level (and eventually SNR), and DC offset, at the DECISIONMAKER input.

8. observe the signal at the input to the DECISION MAKER (assumes channel attenuation not zero).Set up the oscilloscope for an eye pattern (ext synch to bit clock). Set the decision instant to thecentre of an eye (a long sequence is the ideal, but a short sequence is adequate here). It is assumedthat the Z-modulation options on the DECISION MAKER have been checked to suit theoscilloscope.

9. confirm the received sequence is a copy (delayed by the LPF) of the sent message.10. set the SNR and peak level at the input to the detector – the DECISION MAKER. To do this:

a) remove the signal and insert maximum noise (attenuator at +22 dB) into the ADDERrepresenting the ‘wideband channel’. Its gain for the noise path has been set to max already.


b) ideally levels at all module interfaces will not exceed the TIMS ANALOG REFERENCE LEVEL(2V peak). Using the gains of the TUNEABLE LPF and the ADDER at the input to theDECISION MAKER, set the noise level to half the desired peak level, using the oscilloscope.This is not a precise adjustment – setting a little higher is probably acceptable. Then measurethe rms voltage level at the same point.

c) remove noise, add signal, and using the ADDER in the QUADRATURE UTILITIES (this isthe ‘wideband channel’), set its level, measured at the input to the DECISION MAKER, to thesame rms level as was the noise.

When noise is now included, the SNR = 0 dB. This is a reference SNR. All SNRabove this are set by using the calibrated attenuator of the NOISE GENERATOR.But what of the peak level at the detector input ? Confirm it is about 2V peak.

There is an ADDER at the input to the DECISION MAKER. This provides someuseful gain, but also enables a fine adjustment of the DC level to the input thresholdof the DECISION MAKER. Set it to minimize BER (if necessary seek help for thisadjustment).

Now set up the instrumentation. Align the received and reference sequences with nonoise, and confirm no errors. Add noise, and confirm the error rate worsens as theSNR is reduced.

The fine adjustment of DC to the detector is now made by setting BER to a fewcounts/sec, then adjusting to reduce this. As it slows to say 1 count/secs thendecrease SNR and loop until no further improvement is possible.

T1 bit clock regenerationT1 bit clock regenerationT1 bit clock regenerationT1 bit clock regenerationmini version: no FIBRE, DECISION MAKER, TUNEABLE LPF, and provisionfor only one channel (second PCM ENCODER and PCM DECODER missing).

Using a straight-through connection – that is, with no channel filter – setting up isstraightforward, as detailed in the sheet.

The addition of an optical link – using FIBRE OPTIC TX and a FIBRE OPTIC RXmodules would add realism – would require no other changes to the model.

For further realism a channel filter (TUNEABLE LPF, or a BASEBANDCHANNEL FILTERS module) could be added. Either of these options wouldrequire a DECISION MAKER at the receiver to ‘clean up’ the bandlimited signalwaveform. Remember to select the appropriate line code with the on-board switchSW1 of the DECISION MAKER each time the line code is changed.

Reconstruction of a periodic message from the PCM DECODER is not possibleunless a Version 2 or higher is available (which has a suitable LPF).

Note that the inventory for the full experiment includes 14 modules. However, a 12-slot TIMS 301 can still be used by omitting the fibre optic link when including theband limited channel.

Remember that the (standard, non-storage) oscilloscope does flicker when looking atthe internal periodic message.

DPSK and carrier acquisitionDPSK and carrier acquisitionDPSK and carrier acquisitionDPSK and carrier acquisition


Intro to Intro to Intro to Intro to DSP: analog & digitalDSP: analog & digitalDSP: analog & digitalDSP: analog & digitalimplementations comparedimplementations comparedimplementations comparedimplementations compared

A typical two-tone test signal in this area has tones of equal amplitude, andfrequencies close with respect to their mean frequency. Moving the frequencylocation while keeping the frequency difference fairly constant – useful whenwatching for envelope distortion – is clumsy. Using a DSBSC as a two-tone testsignal has the advantage of its being tuneable with a single control – the ‘carrier’source. The DSBSC test signal is probably not acceptable in a hi-fi situation, sincethe multiplier required to generate the DSBSC would introduce more distortion thanthe system under test (although this is not necessarily unacceptable if envelopedistortion is being examined). But it is probably legitimate in a less demandingsituation, such as the present one.

TCM – trellis codingTCM – trellis codingTCM – trellis codingTCM – trellis codingPlease install the TCM Viterbi decoder EPROM in the TIMS320 DSP-HS module.

To save time you could set the switches of the INTEGRATE & DUMP module:1. SW1 to I&H12. SW2 to I&D2.3. SW3 – upper toggle LEFT, lower toggle RIGHT

Note the SNR is measured at the output of the INTEGRATE & HOLD sub-system (I&H1). The bandwidth of the channel – the TUNEABLE LPF – plays no part as it istuned to be wide band. Filtering is in effect performed by the integrator. TheTUNEABLE LPF symbolises the channel, and its variable gain performs a usefulfunction in setting levels.

It is critical to set the no noise peak of the four-level signal into the Viterbi decoderto ±1.5 volt (the other two will be ±0.5 volt).

matched filter detectionmatched filter detectionmatched filter detectionmatched filter detectionThe divide-by-two to generate a bit clock of 1.042 kHz is for compatibility with theTCM experiment. If performed as a stand-alone experiment there is no need forthis division.

Ensure the z-mod jack J1 of the DECISION MAKER is set to suit your laboratoryoscilloscopes. To save time you could also set the switch SW1 to NRZ-L and SW2 toINT.

This experiment is complete in itself, but is also used as a system to compare withTCM (see the Lab Sheet entitled TCM – trellis coding). The latter uses a bit clockderived from the 8.333 kHz sample (as do many other experiments). The TCMencoding process results in a bit clock of 1.042 kHz.

So this experiment uses a bit clock of 1.042 kHz also. This is derived from the8.333 kHz sample clock. The LINE-CODE ENCODER contains a divide-by-foursub-system; the extra divide-by-two may be obtained by using a DIGITALUTILITIES module (total division-by-sixteen). But it could alternatively be derivedfrom the divide-by-sixteen within the CONVOLU’L ENCODER (if that experimentis also to be performed).


Note the SNR is measured at the output of the INTEGRATE & HOLD sub-system (I&H1). The bandwidth of the channel – the TUNEABLE LPF – plays no part as it istuned to be wide band. Filtering is in effect performed by the integrator. TheTUNEABLE LPF symbolises the channel, and its variable gain performs a usefulfunction in setting levels.

TCM – coding gainTCM – coding gainTCM – coding gainTCM – coding gainUse the previous two Lab Sheets – TCM – trellis coding and matched filter detectionfor setting up details.

Aim for a bit error rate of a few hundred errors in 105 clock periods in each system.The SNR will be between 5 and 10 dB for the TCM case. Theoretical coding gain is2.5 dB. See the relevant TIMS Interactive for more info

CDMA – introductionCDMA – introductionCDMA – introductionCDMA – introductionThis is a qualitative experiment. It provides experience in setting up a single directsequence spread spectrum – DSSS – transmission. Some observations aresuggested, but many more are possible.

The mini version inventory omits the VCO, which limits the possible range of PNclock frequencies.

CDMA – processing gainCDMA – processing gainCDMA – processing gainCDMA – processing gainThis is a quantitative experiment, which measures the processing gain.

Other measurements are possible, but the addition of a second channel is left to thenext experiment.

The mini version inventory omits the VCO, so cannot measure bandwidth of noisegenerator.

CDMA – 2 channelCDMA – 2 channelCDMA – 2 channelCDMA – 2 channelCo-channel interference is introduced.

Mini version omits modules for BER measurement.

CDMA – CDMA – CDMA – CDMA – multichannelmultichannelmultichannelmultichannelThis is a variation on the previous experiment, in that analog messages are possible.

The mini version sends four channels, but can receive only a single channel at atime.


unknown signals – 1unknown signals – 1unknown signals – 1unknown signals – 1TIMS is an ideal environment for stretching the imagination of communicationsstudents. One avenue is to use TRUNKS to send unknown unknown signals to allstudents.

The terms of reference are that each of the unknown signals was made from amessage composed of one or more baseband signals (eg, message bandwidth lessthan say 3 kHz), which was then multiplied with (translated in frequency with) a100 kHz sine wave to produce a bandpass signal in the 100 kHz region.

The object of the experiment is to determine the analytical description of eachsignal.

The signals are intended to deceive. They are all ‘trick’ signals, in that they are notwhat they appear to be when viewed on an oscilloscope.

Initial oscilloscope viewing is simplified by using the signal envelope tosynchronize the display (use the DIODE + LPF in the utilities module for envelopeextraction).

A reliable method of analysis is to first multiply (demodulate) the unknown signalwith a 100 kHz phase-adjustable sinewave, and then baseband filter the product. Iftwo audio sinewaves are present the TUNEABLE LPF should be sufficient to isolateone of them. The frequency of the other can be deduced by measuring thefrequency of the envelope of this two-tone signal.

Signal #1Signal #1Signal #1Signal #1

Figure 1 could be defined as the output of an SSB transmitter, with a singlesinewave (say 2kHz) and a DC component as the message (although a typical SSBtransmitter would reject any DC component in the message). Alternatively it couldbe defined as the output from an overmodulated compatible single sideband (CSSB)transmitter (message amplitude too large). What ever you might call it, it is not aDSBSC derived from a single tone, as it appears to be from a first viewing on anoscilloscope. Its analytical expression is:

y(t) = E.cosω1t + Ecosω2t

where ω1/(2.π) = 100 kHz and ω2/(2.π) = 102 kHz

Note that the equation is not an analytical expression for an SSB signal (here anupper sideband) derived from a general message, but a simplified expression for thespecific signal just defined.

There is no need to make an SSB generator to generate it. It can be synthesised, asthe equation suggests, by adding two equal-amplitude sinewaves of 100 kHz and102 kHz.

It can be analysed by multiplying it with a 100 kHz sinewave, and noting (afterbaseband filtering) that the output is a DC component (amplitude is phase sensitive)and a 2 kHz sinewave (amplitude is phase insensitive).

However, there is an ambiguity. Is the 2 kHz output derived from an upper or lowersideband (of 100 kHz) ? If a VCO has been provided, multiplication with 102 kHz(DC output) and with 98 kHz (no DC output) will give different results, from whichthe ambiguity can be resolved.

Signal #2Signal #2Signal #2Signal #2

A two-tone SSB. Similar to #1, except that there is now a two-component ACoutput (and no DC), requiring determination of the frequency of each. The higherfrequency component can be removed with the TUNEABLE LPF, and the differencefrequency derived from the envelope frequency.


Synthesise from a DSBSC using (say) a 1 kHz message, and a VCO as carrier (say102 kHz). The spectral components will be 101 kHz and 103 kHz. Demodulatoroutput will be no DC, and 1 kHz and 3 kHz sinewaves (easily separated with theTUNEABLE LPF).

Alternatively add the outputs of two VCO modules (101 kHz and 103 kHz).Signal #3Signal #3Signal #3Signal #3

A DSBSC and carrier of amplitude ratio 1.2:1, added in phase quadrature. This isArmstrong’s signal before amplitude limiting – sometimes called ‘quadraturemodulation’. The purists could argue that such a transmitter is not generallyavailable, it more likely being the exciter of an FM transmitter; but so be it.

Analytically:

y(t) = E.cosµt.cosωt + E.sinωt

where µ is a message frequency (say at 2 kHz) and ω/(2.π) = 100 kHz

At the demodulator output the DC and AC amplitudes maximise at carrier phasedifference of 900. That the latter (2 kHz) amplitude can be controlled by phaseadjustment shows that there must be both an upper and lower sideband component.

CDMA at carrier freq: example 1CDMA at carrier freq: example 1CDMA at carrier freq: example 1CDMA at carrier freq: example 1This experiment is best performed by students who have completed some of theearlier CDMA Lab Sheets (thus assuming familiarity with the CDMA modules,sequence alignment, BER measurement).

Due to space constraints considerable detail has been omitted. Students will need tobe able to estimate the bandwidth of their DSSS signals in order to ensure they canpass via the channel BPF (if one is supplied). The ratio of spreading sequence tomessage bandwidth will not be large (there is no need to be constrained to thefrequencies suggested – use the DIGITAL UTILITIES module to suit.

The model illustrated uses no channel filter; band limiting (essential to constrain thenoise bandwidth) is performed after the CDMA is translated back to baseband.

A full 12-slot rack is required. If the 100 kHz CHANNEL FILTERS module isincluded, then the WIDEBAND TRUE RMS METER can be omitted. To makeSNR measurements the ERROR COUNTING UTILITIES can be removedtemporarily.

Since this is a relatively complex experiment, care is needed when patching up. Asuggested sequence is:

1. set sequences in MULTIPLE SEQUENCES SOURCE to 0 and 1.2. set sequence in CDMA DECODER to 03. set message SEQUENCE GENERATOR to a short sequence (both toggles UP)4. patch all clock leads, and confirm frequencies5. patch 100 kHz to the frequency translater multipliers; sin to the transmitter, and cos to the receiver

via a PHASE CHANGER (whose on-board switch is set to HI).6. initially set up one channel only. Connect the X TTL message to the UPPER X-OR gate, and its

product to the UPPER multiplier of the generator QUADRATURE UTILITIES.7. Connect the upper QUADRATURE UTILITIES multiplier output to the A input of its internal

ADDER. Set the ADDER output to 1 volt peak (in anticipation of a second channel increasing thisto 2 volt peak, the analog reference level).

8. Connect this ADDER output to the stand-alone ADDER, setting this output to 1 volt peak (inanticipation of NOISE being added later).

9. Connect the ADDER output to the LOWER multiplier of the receiver QUADRATURE UTILITIES.


10. Connect the output to the LOWER multiplier of the receiver QUADRATURE UTILITIES to theCARRIER LPF of the CDMA DECODER. Adjust to maximum amplitude using the PHASECHANGER. If it is thought necessary the internal ADDER of the QUADRATURE UTILITIES canbe used to increase the input level to this LPF.

11. Connect the CARRIER LPF output to the UPPER multiplier of the QUADRATURE UTILITIES,and its output to the DATA LPF input; thence to the COMPARATOR input.

12. Synchronize the oscilloscope to the message start-of-sequence SYNCH output.13. Obtain a stable display of the source message on one channel.14. Display the COMPARATOR output on a second channel. If dis-similar (almost inevitable) align the

two spreading sequence generators (SYNCH of Tx to RS of Rx). The two traces should now beidentical, albeit not (quite) aligned (change phase of demodulator by 1800 if of opposite polarity).Check patching until demodulation/decoding is successful.

Patch up BER instrumentation.

15. Observe A and B inputs of the X-OR GATE of the ERROR COUNTING module. These should beidentical, although may not be aligned. If not perfectly aligned, the X-OR operation needs to begated by a pulse thinner than that of the clock supplied by the DIGITAL DIVIDE, and adjustableover a clock period. Such a clock is provided by the DELAY sub-system of the CDMA DECODER.

16. To suit conditions, set the on-board jumper of the CDMA DECODER to the DATA position.17. Adjust the delay for zero output (TTL LO) from the X-OR output (no pulses). This is the no-error

condition, and is possible since no noise is present.18. Add the second DSSS (as per patching diagram; PN sequence 1 already set)) and confirm the

original message is still be recovered. Change PN sequence at the CDMA DECODER to ‘1’, re-align, and confirm the second message recovery.

The system is now almost set up for the experiment proper can begin. There is nodetail given in the Lab Sheet as to what to examine, but you might consider someBER/SNR measurements. For example: with one channel, add noise, and set up fora moderate BER. Measure SNR at DATA LPF output. Add the second channel,note new BER/SNR, etc etc.

Degrees of freedom include:

1. the number of channels added (each pair requires a SEQUENCE GENERATOR and a MULTIPLESEQUENCES SOURCE module. Summing can be implemented via a BUFFER AMPLIFIER asdescribed in the Lab Sheet entitled CDMA – multichannel).

2. data rates and PN sequence clock rates (both their absolute values, and their ratio)3. system bandwidth

CDMA at carrier freq: example 2CDMA at carrier freq: example 2CDMA at carrier freq: example 2CDMA at carrier freq: example 2

N/A

pulse position modulationpulse position modulationpulse position modulationpulse position modulation


speechspeechspeechspeechHeadphones are required for this experiment. They are not listed in the ‘module’inventory.

voice modemsThe four lab sheets to follow aim at introducing, in graded steps, a voicebandmodem. Further sheets on this topic are forthcoming.

binary data via binary data via binary data via binary data via voicebandvoicebandvoicebandvoicebandFor this and related experiments the eye pattern is adequate for estimating maximumdata rates. BER instrumentation has been included in the model, but can be omittedif necessary. It is used no so much to quantify BER but as to indicate the onset oferrors. Note that, if implemented, the X-OR gate in the ERROR COUNTINGUTILITIES must be clocked with a thin pulse, the timing of which may needadjustment. See the Advanced Modules Users Manual. A suitable pulse, ofadjustable position, has been implemented with the DELAY within the INTEGRATE& DUMP module.

multilevel data via multilevel data via multilevel data via multilevel data via voicebandvoicebandvoicebandvoiceband

data rates and data rates and data rates and data rates and voiceband modems –voiceband modems –voiceband modems –voiceband modems –transmissiontransmissiontransmissiontransmission

A pair of BESSEL filters from BASEBAND FILTERS modules will be used as thereceiver filters, since these have the ‘best’ characteristic of the three (superior tothat of a TUNEABLE LPF). Having a fixed bandwidth (about 3 kHz), all otherparameters of the demodulator must be designed around it. This results in thetransmission channel being approximately twice as wide (say 6 kHz). This is notrepresentative of a typical voice channel bandwidth, but the consequent frequencyscaling by a factor of two should not lessen the realism of the model.

data rates and data rates and data rates and data rates and voiceband modems –voiceband modems –voiceband modems –voiceband modems –demodulationdemodulationdemodulationdemodulation


contentsIntroduction to TIMS.....................................................................................................................3Modelling equations......................................................................................................................3DSBSC – generation .....................................................................................................................3Product demodulation...................................................................................................................3AM - amplitude modulation – I .....................................................................................................4AM - amplitude modulation – II ....................................................................................................4Envelope detection ........................................................................................................................5SSB generation..............................................................................................................................6SSB demodulation .........................................................................................................................6ISB - independent sideband...........................................................................................................6Armstrong`s phase modulator .......................................................................................................6FM - generation by VCO...............................................................................................................7FM - demodulation by PLL ...........................................................................................................7FM - demodulation by ZX counting...............................................................................................7Sampling .......................................................................................................................................7PAM and TDM..............................................................................................................................7FDM - frequency division multiplex..............................................................................................7Phase division multiplex – generate..............................................................................................7Phase division multiplex – demod .................................................................................................7PWM - pulse width modulation .....................................................................................................8Carrier acquisition – PLL.............................................................................................................8The WAVE ANALYSER .................................................................................................................8Complex analog messages ............................................................................................................8PCM – encoding ...........................................................................................................................8PCM – decoding ...........................................................................................................................8ASK – generation ..........................................................................................................................8ASK – demodulation......................................................................................................................8BPSK – modulation.......................................................................................................................9BPSK – demodulation ...................................................................................................................9QPSK – generation .......................................................................................................................9QPSK – demodulation...................................................................................................................9FSK – generation ..........................................................................................................................9FSK - envelope demodulation .......................................................................................................9Signal constellations .....................................................................................................................9DSSS - spread spectrum ................................................................................................................9Eye patterns ..................................................................................................................................9PRBS messages ...........................................................................................................................10Detection with the DECISION MAKER ......................................................................................10The noisy channel........................................................................................................................10BER instrumentation ...................................................................................................................10BER measurement – introduction................................................................................................10Line coding & decoding ..............................................................................................................10Delta modulation ........................................................................................................................10Delta-sigma modulation..............................................................................................................11Adaptive delta modulation ..........................................................................................................11Delta demodulation.....................................................................................................................11Bit clock regeneration .................................................................................................................11QAM – generation.......................................................................................................................11QAM – demodulation ..................................................................................................................11BPSK...........................................................................................................................................12Broadcasting (two mini versions)................................................................................................12Fibre optic transmission .............................................................................................................12multi-channel digital fibre link....................................................................................................12PCM-TDM ‘T1’ implementation .................................................................................................12DPSK & BER ..............................................................................................................................13T1 bit clock regeneration ............................................................................................................14DPSK and carrier acquisition.....................................................................................................14Intro to DSP: analog & digital implementations compared ........................................................15TCM – trellis coding ...................................................................................................................15matched filter detection...............................................................................................................15TCM – coding gain .....................................................................................................................16CDMA – introduction..................................................................................................................16CDMA – processing gain ............................................................................................................16CDMA – 2 channel......................................................................................................................16CDMA – multichannel.................................................................................................................16unknown signals – 1 ....................................................................................................................17CDMA at carrier freq: example 1 ..............................................................................................18CDMA at carrier freq: example 2 ..............................................................................................19pulse position modulation ...........................................................................................................19speech .........................................................................................................................................20binary data via voiceband ...........................................................................................................20multilevel data via voiceband......................................................................................................20data rates and voiceband modems – transmission ......................................................................20data rates and voiceband modems – demodulation .....................................................................20

Instructor's Manual to accompany - boya.xmu.edu.cnboya.xmu.edu.cn/comm/tims/LS-instructor.pdf ·...

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Transcript of Instructor's Manual to accompany - boya.xmu.edu.cnboya.xmu.edu.cn/comm/tims/LS-instructor.pdf ·...