Instructor's Manual
to accompany
Lab Sheets
Instructor's Manual
to accompany
Lab Sheets
rev 1.1
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Sydney AUSTRALIA
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Preface
This Instructor`s Manual is to support TIMS Lab Sheets.
Comments and criticisms are welcome - tim@tims.com.au
Tim Hooper
Sydney, 2001
TIMS LAB SHEETS
INSTRUCTOR`S MANUAL
VER 1.2
Please read the notes (if any) for an experiment before running it.
Most experiments are concerned with the transmission of a message – these can take
two general forms, depending on the context.
analog messages
In analog experiments requiring a message this comes typically from an AUDIO
OSCILLATOR. 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 possible
loss 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 simply
measured. A more complex message is a two-tone. This is also easily represented
analytically, and reasonably easy to measure. Following testing, however, the
ultimate message will probably be speech.
Speech can be obtained from a SPEECH MODULE. The two available speech
channels should be recorded using bandlimited speech. The subject matter should
be sufficiently different as to simplify channel identification.
Although a SPEECH MODULE is seldom specified in the module inventory, it is
suggested that one be added to the inventory where appropriate. Its inclusion can be
of benefit, especially as regards signal visualisation in the time domain.
A SPEECH MODULE can also be a source of periodic messages by recording the
output of, say, a VCO. Using both of the speech channels a two-tone test signal can
be 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 messages
The most common form of digital message in TIMS is the pseudo random binary
sequence (PRBS), provided by a SEQUENCE GENERATOR module.
Do not forget the very useful DC signal. Both a TTL LO and HI are provided by the
VARIABLE DC module.
oscilloscope displays
There is nothing more annoying – especially when giving a demonstration – than an
unstable oscilloscope display.
Wherever possible choose an appropriate
synchronizing signal, and use it to trigger the oscilloscope by connecting it to the
TIMS Lab Sheets Instructor`s Manual
ver 1.2
-1
ext. synch. or ext. trig input.
NOTE: this is a draft version of the Instructor’s Manual. Feedback is sought
regarding the kind of information required by Instructor’s in the next update
(comments to tim@tims.com.au)
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TIMS Lab Sheets Instructor`s Manual
Introduction to TIMS
Modelling equations
DSBSC – generation
mini 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 from
MASTER SIGNALS. However a variable frequency message is to be preferred in
order to observe more inter-relationships. For example it is often stated that the
carrier 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.
cases.
This preferable in most
The ADDER enables a pilot carrier to be added to the DSBSC. This simplifies
carrier acquisition by a demodulator. It is possible to calculate the amount of
carrier present from observations, in the time domain, of the relative amplitudes of
alternate peaks of the DSB envelope.
To ensure that both waveforms are stationary on the screen (Figure 2) the message
must be a sub-multiple of the carrier frequency. A pair of such frequencies is easily
obtained by using a 100 kHz carrier and a 2 kHz message (actually 2.083 kHz) from
MASTER SIGNALS. However, the frequency ratio of 48:1 is rather high. Instead
use an AUDIO OSCILLATOR tuned close to 8.333 kHz, and then synchronized to
the 8.333 kHz TTL from MASTER SIGNALS. This gives a frequency ratio of
24:1, which matches the waveforms of Figure 2.
BEWARE: the block diagram of Figure 1 ensures no phase difference between the
DSBSC carrier and the pilot carrier. The modules of Figure 3 probably achieve this
too. But what if there is a phase shift between the two (as there often is in a lessthan-ideal situation) ? The display will change dramatically as this phase difference
is increased - until the signal is reminiscent of that of Armstrong. Ambitious
students should be given a PHASE SHIFTER so as to investigate further.
Product demodulation
mini version: no AUDIO OSCILLATOR (use 2 kHz message), and only one
PHASE SHIFTER.
For most displays use the message as the synchronising signal, connected to the ext.
synch. input of the oscilloscope.
In the list of Lab Sheet experiments this one (entitled Product demodulation) comes
before those detailing the generation of AM and SSB. That is unfortunate, since
TIMS Lab Sheets Instructor`s Manual ver 1.2
3
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
message
(µ)
g
G
carrier
(100kHz)
(ω)
AM
out
carrier
typically ω >> µ
adjust phase
The ADDER is used to add pilot, or full, carrier, as required. The PHASE
SHIFTER will set the appropriate phase for AM (not set by measuring phase, but by
observing the envelope of the AM itself).
An exact model of this block diagram may not quite give AM ! This is because
there will be an inevitable, but small, phase shift between the DSBSC and the
carrier, at the adding point. The PHASE SHIFTER may not have enough range to
add further shift to rotate this back to the in-phase condition. Thus it is best to use
one of the quadrature outputs of the 100 kHz from MASTER SIGNALS to the
multiplier, and the other to the PHASE SHIFTER. This inserts a close to 900 phase
difference between the DSBSC and carrier, which is easily increased to 1800 by the
PHASE SHIFTER (whereas an increase of 1800 may be difficult).
Demodulation AM and DSB(SC) using a non-synchronous carrier is not a practical
proposition, which can be demonstrated. Small frequency errors (say less than 1 Hz)
produce a slowly varying output amplitude (which includes periods of zero
amplitude). 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 go
un-noticed with speech (and even music). This also is often mis-reported in text
books.
When the output is required to be the exact frequency of the message then no
frequency error can be tolerated at the demodulator.
It us interesting to show that, whereas a phase other than zero at the transmitter will
effect the shape of the envelope of AM, this has no effect upon the message
waveform recovered by a product demodulator.
Not so, of course, from an
envelope detector.
AM - amplitude modulation – I
For most displays use the message as the synchronising signal, connected to the ext.
synch. input of the oscilloscope.
AM - amplitude modulation – II
mini version: no AUDIO OSCILLATOR – use 2 KHz MESSAGE.
For most displays use the message as the synchronising signal, connected to the ext.
synch. input of the oscilloscope.
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TIMS Lab Sheets Instructor`s Manual
The aim is to get the carrier in phase with the DSBSC (actually with the resultant
phasor of the DSBSC, which is where the carrier would be if it were present). In
Armstrongs system (see separate Lab Sheet) the aim is for the phase difference to be
900. 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 the
phase is being reduced from 900 towards 00 (in fact 900, 700, 450, with 00 not
shown). The kiss will happen at 00.
You might gain further insight by looking at the Lab Sheet entitled Armstrong`s
phase modulator.
Envelope detection
mini version: no AUDIO OSCILLATOR or 60 kHz LPF
For most displays use the message as the synchronising signal, connected to the ext.
synch. input of the oscilloscope.
To recover the message from the envelope of a signal the shape of the envelope must
be a ‘copy’ of the shape of the message. The obvious choice is an amplitudemodulated signal with m < 1). For the case m > 1 the envelope is not a copy of the
message.
The fact that the envelope is no longer a copy of the message means that the
message can no longer be recovered using an envelope detector. But the envelope
that results (when m > 1) is exactly what analysis predicts. So there is, in the strict
sense, 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 without
distortion. Text books often suggest/infer/imply that the envelope cannot be
recovered when what is meant is that the message cannot be recovered. It is the
approximation 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 (or
its requirements not being met).
The well-designed envelope detector uses an ‘ideal rectifier’ and a separate LPF of
adequate bandwidth. The ubiquitous diode detector, a diode-plus-RC-filter, is an
approximation to this. Unless the carrier frequency (ω) is very much higher than
the highest message frequency (µ) it fails miserably.
TIMS Lab Sheets Instructor`s Manual ver 1.2
5
Mini version of this experiment omits the 60 kHz LPF and the AUDIO
OSCILLATOR (for the message); use instead the 2 kHz fixed-frequency message
from MASTER SIGNALS. Omitting the TUNEABLE LPF from the inventory
would 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 significant
components beyond 12 kHz (the upper band width of the TUNEABLE LPF). Thus
the filter output is a fair, but obviously imperfect, copy of the envelope. Swapping
to 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 lowest
possible message frequency (near 250 Hz), and now the 12 kHz bandwidth of the
TUNEABLE LPF is more than adequate to recover all significant components of the
DSBSC envelope. Reduce the bandwidth and show that the recovered envelope
shape is badly distorted.
SSB generation
mini version: no AUDIO OSCILLATOR
While confirming the individual DSBSC, use the message as the synchronising
signal, connected to the ext. synch. input of the oscilloscope. Sweep speed to show
a few message periods. Use it also when minimizing the envelope of the ADDER
output (the SSB), since it makes the envelope stationary on the screen. To observe
the SSB itself (derived from a single tone), rather than its envelope, increase the
sweep speed to show a few periods of the 100 kHz SSB, and use the SSB itself as
the synchronizing signal.
SSB demodulation
ISB - independent sideband
Armstrong`s phase modulator
mini version: no AUDIO OSCILLATOR or UTILITIES.
For most displays use the message as the synchronising signal, connected to the ext.
synch. input of the oscilloscope.
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 to
recover the envelope. By listening to the recovered envelope it is easy to determine
when the 900 point is reached. The fundamental of the message can be hard to null
out at that condition. This is a very sensitive test (and eliminates the need for an
oscilloscope). Even in the presence of the higher harmonics this null is obvious –
an example of psycho-acoustics. This phenomenon is not mentioned in the Lab
Sheet, but see the equivalent experiment in the Student Text (Volume A2).
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TIMS Lab Sheets Instructor`s Manual
FM - generation by VCO
mini version: no AUDIO OSCILLATOR.
Synchronize the oscilloscope to the FM signal itself when displaying Figure 1(a).
FM - demodulation by PLL
FM - demodulation by ZX counting
mini version: no AUDIO OSCILLATOR.
Sampling
mini version: no SPEECH.
PAM and TDM
FDM - frequency division multiplex
Phase division multiplex – generate
mini version: no SPEECH.
Phase division multiplex – demod
mini 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 version
of this experiment, since it would allow simultaneous recovery of two messages.
But that is perhaps an unnecessary luxury. A single demodulator patched between
channels should be sufficient.
If an AUDIO OSCILLATOR is not available for one of the messages perhaps a DC
signal could be substituted (with DC coupling of the relevant MULTIPLIER at the
TIMS Lab Sheets Instructor`s Manual ver 1.2
7
generator), and use of the oscilloscope to detect an output.
PWM - pulse width modulation
mini version: AUDIO OSCILLATOR or TUNEABLE LPF.
Carrier acquisition – PLL
mini version: no AUDIO OSCILLATOR.
The WAVE ANALYSER
Complex analog messages
PCM – encoding
If available a WIDEBAND TRUE RMS METER could be added to the module
inventory, but is not essential (see experiment text).
PCM – decoding
ASK – generation
ASK – demodulation
mini version: AUDIO OSCILLATOR or DECISION MAKER.
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TIMS Lab Sheets Instructor`s Manual
BPSK – modulation
mini version:
no LINE-CODE ENCODER,
CHANNEL FILTERS.
TUNEABLE LPF,
100 kHz
TUNEABLE LPF,
100 kHz
BPSK – demodulation
mini version:
no LINE-CODE ENCODER,
CHANNEL FILTERS.
QPSK – generation
mini version: 100 khz channel filters
QPSK – demodulation
mini version: inventory reduced by 1 x MULTIPLIER , 1 x PHASE SHIFTER, 1 x
TUNEABLE LPF.
FSK – generation
mini version: no AUDIO OSCILLATOR
FSK - envelope demodulation
No mini version. AUDIO OSCILLATOR essential
Signal constellations
DSSS - spread spectrum
Eye patterns
TIMS Lab Sheets Instructor`s Manual ver 1.2
9
PRBS messages
mini version: no TUNEABLE LPF.
This is an elementary introduction to the SEQUENCE GENERATOR module. If
two such generators are available (recommended) then alignment of the two can be
demonstrated.
A Lab Sheet entitled BER INSTRUMENTATION is an important extension of this
experiment. It illustrates a method of alignment when there is delay between the
two SEQUENCE GENERATOR modules.
Although a TUNEABLE LPF is suggested as an optional extra module, it is not
essential.
Detection with the DECISION MAKER
The noisy channel
This sheet does not describe an experiment. It serves as a reference to the NOISY
CHANNEL model used in later experiments.
BER instrumentation
This sheet does not describe an experiment. It serves as a reference to the
INSTRUMENTATION model used in later experiments.
BER measurement – introduction
Line coding & decoding
Delta modulation
Before plugging in the DELTA MODULATION UTILITIES set the on-board
switch SW2A ON, and SW2B OFF. Set the ADDER and the two BUFFER
AMPLIFIERS each to unity gain (use a sine wave). Do NOT ever change the
ADDER gains.
Patch up Figure 3 and check you have the waveforms of Figure 2 (for a completely
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TIMS Lab Sheets Instructor`s Manual
stationary display it is necessary to fine-trim the oscilloscope triggering).
Adjust one of the buffer gains to get the ‘best’ approximation between the two
waveforms on the screen.
1.
2.
3.
Failing the above check that, with the input removed from the B input of the
ADDER:
the HARD LIMITER output is ±2.5V inverted square in synch with the message
the SAMPLER output is ±5V (non-inverted) square in synch with the message
the BUFFER output is the same as the SAMPLER output
Delta-sigma modulation
Adaptive delta modulation
Delta demodulation
mini version: no SPEECH.
Bit clock regeneration
It is stated in the notes that a non-linear operation on the data stream will introduce a
spectral line at the bit clock rate (and harmonics) which was otherwise absent. This
is only partially true. A squarer is suggested. A perfect bi-polar signal, when
squared, produces a DC component only – with no harmonics. But if first processed
in some way, these ideal conditions are lost and a component may appear. By
‘processing’ is meant any operation which produces the desired effect ! Lowpass
filtering is effective. Other methods include introducing some delay (or difference)
into one of the arms of the squarer, or switching the squarer (the MULTIPLIER) into
the AC coupled mode. Whilst these methods are effective, they are not necessarily
optimum. They are mentioned here to make you aware that an effective method
should be both investigated and understood theoretically.
QAM – generation
QAM – demodulation
TIMS Lab Sheets Instructor`s Manual ver 1.2
11
BPSK
Broadcasting (two mini versions)
The full experiment includes modules for both AM and FM transmissions. Mini
versions 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 receiver
selectivity is provided by the BPF in the 100 kHz Rx ANTENNA UTILITIES
module. This is in excess of ±10 kHz centred around 100HZ. Better selectivity
would be obtained by using a product demodulator, and the LPF in the
HEADPHONE AMPLIFIER module. This arrangement would allow two or more
transmitted signals to be transmitted (and recovered independently) in the 90 to
110 kHz region. This arrangement is not described in the Lab Sheet, and modules
for it are not included in the normal inventory.
Fibre optic transmission
multi-channel digital fibre link
mini 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 not
mandatory, but are compatible.
Since the FDM itself has a bandwidth up to about 15 kHz, the sampling rate of the
delta modulator must be increased from the ‘normal’ 100 kHz rate to something
much 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 front
panel). 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.
2.
3.
the ADDER in the two QUADRATURE UTILITIES modules to unity
the ADDER of the DELTA MODULATOR model to unity (and leave them so set permanently)
each of the BUFFER AMPLIFIERS to unity
PCM-TDM ‘T1’ implementation
mini version: no FIBRE modules.
Cannot use speech for messages, or even an AUDIO OSCILLATOR, as sampling
rate is far too slow. Yes, the (standard, non-storage) oscilloscope does flicker when
looking at the internal periodic message.
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TIMS Lab Sheets Instructor`s Manual
DPSK & BER
This is a relatively complex experiment and must be carefully set up.
In order to conserve modules it is possible to omit the bandlimiting channel. Band
limiting (particularly of the wideband noise) is provided at the demodulator output
by 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 extra
modules leaves no room for the WIDEBAND TRUE RMS METER.
However,
this is used only once (for setting up the SNR at the detector – the DECISION
MAKER - input). At that time the modules for error counting are not required, so
one of these may be removed (to accommodate the meter) while the reference SNR
is set up.
You may refer to use a separate MULTIPLIER module at the receiver, rather than
the second multiplier of the QUADRATURE UTILITIES (although the two
multipliers of the QUADRATURE UTILITIES module are entirely independent,
some students may feel uncomfortable having to ‘loop back’ into the transmitter for
part of the receiver).
It is unfortunately necessary to make a one-time adjustment of the ADDER in the
QUADRATURE UTILITIES module. This is to set the signal level while adjusting
the SNR. An extender board would help here.
Students will be required to view both snap shots of the message sequence, and eye
patterns. 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 should
supply your own. Some suggestions follow (taken from the related experiment in
the Student Text).
1.
2.
initially set the SEQUENCE GENERATORs to a short sequence (toggles of SW2 both UP).
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 noise
into 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 lower
MULTIPLIER of the QUADRATURE UTILITIES. Steal the 100 kHZ carrier from the Tx (same
phase). Because the channel has no delay, there is no real need for a PHASE CHANGER in the path
of 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 DECISION
MAKER 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 the
centre of an eye (a long sequence is the ideal, but a short sequence is adequate here). It is assumed
that the Z-modulation options on the DECISION MAKER have been checked to suit the
oscilloscope.
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 ADDER
representing the ‘wideband channel’. Its gain for the noise path has been set to max already.
TIMS Lab Sheets Instructor`s Manual ver 1.2
13
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 the
DECISION 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 measure
the rms voltage level at the same point.
c)
remove noise, add signal, and using the ADDER in the QUADRATURE UTILITIES (this is
the ‘wideband channel’), set its level, measured at the input to the DECISION MAKER, to the
same rms level as was the noise.
When noise is now included, the SNR = 0 dB. This is a reference SNR. All SNR
above 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 some
useful gain, but also enables a fine adjustment of the DC level to the input threshold
of the DECISION MAKER. Set it to minimize BER (if necessary seek help for this
adjustment).
Now set up the instrumentation. Align the received and reference sequences with no
noise, and confirm no errors. Add noise, and confirm the error rate worsens as the
SNR is reduced.
The fine adjustment of DC to the detector is now made by setting BER to a few
counts/sec, then adjusting to reduce this. As it slows to say 1 count/secs then
decrease SNR and loop until no further improvement is possible.
T1 bit clock regeneration
mini version: no FIBRE, DECISION MAKER, TUNEABLE LPF, and provision
for only one channel (second PCM ENCODER and PCM DECODER missing).
Using a straight-through connection – that is, with no channel filter – setting up is
straightforward, as detailed in the sheet.
The addition of an optical link – using FIBRE OPTIC TX and a FIBRE OPTIC RX
modules would add realism – would require no other changes to the model.
For further realism a channel filter (TUNEABLE LPF, or a BASEBAND
CHANNEL FILTERS module) could be added. Either of these options would
require a DECISION MAKER at the receiver to ‘clean up’ the bandlimited signal
waveform. Remember to select the appropriate line code with the on-board switch
SW1 of the DECISION MAKER each time the line code is changed.
Reconstruction of a periodic message from the PCM DECODER is not possible
unless 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 12slot TIMS 301 can still be used by omitting the fibre optic link when including the
band limited channel.
Remember that the (standard, non-storage) oscilloscope does flicker when looking at
the internal periodic message.
DPSK and carrier acquisition
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TIMS Lab Sheets Instructor`s Manual
Intro to DSP: analog & digital
implementations compared
A typical two-tone test signal in this area has tones of equal amplitude, and
frequencies close with respect to their mean frequency. Moving the frequency
location while keeping the frequency difference fairly constant – useful when
watching for envelope distortion – is clumsy. Using a DSBSC as a two-tone test
signal 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, since
the multiplier required to generate the DSBSC would introduce more distortion than
the system under test (although this is not necessarily unacceptable if envelope
distortion is being examined). But it is probably legitimate in a less demanding
situation, such as the present one.
TCM – trellis coding
Please install the TCM Viterbi decoder EPROM in the TIMS320 DSP-HS module.
1.
2.
3.
To save time you could set the switches of the INTEGRATE & DUMP module:
SW1 to I&H1
SW2 to I&D2.
SW3 – upper toggle LEFT, lower toggle RIGHT
Note the SNR is measured at the output of the INTEGRATE & HOLD sub-system (I&H
1). The bandwidth of the channel – the TUNEABLE LPF – plays no part as it is
tuned to be wide band. Filtering is in effect performed by the integrator. The
TUNEABLE LPF symbolises the channel, and its variable gain performs a useful
function in setting levels.
It is critical to set the no noise peak of the four-level signal into the Viterbi decoder
to ±1.5 volt (the other two will be ±0.5 volt).
matched filter detection
The divide-by-two to generate a bit clock of 1.042 kHz is for compatibility with the
TCM experiment. If performed as a stand-alone experiment there is no need for
this division.
Ensure the z-mod jack J1 of the DECISION MAKER is set to suit your laboratory
oscilloscopes. To save time you could also set the switch SW1 to NRZ-L and SW2 to
INT.
This experiment is complete in itself, but is also used as a system to compare with
TCM (see the Lab Sheet entitled TCM – trellis coding). The latter uses a bit clock
derived from the 8.333 kHz sample (as do many other experiments). The TCM
encoding 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 the
8.333 kHz sample clock. The LINE-CODE ENCODER contains a divide-by-four
sub-system; the extra divide-by-two may be obtained by using a DIGITAL
UTILITIES module (total division-by-sixteen). But it could alternatively be derived
from the divide-by-sixteen within the CONVOLU’L ENCODER (if that experiment
is also to be performed).
TIMS Lab Sheets Instructor`s Manual ver 1.2
15
Note the SNR is measured at the output of the INTEGRATE & HOLD sub-system (I&H
1). The bandwidth of the channel – the TUNEABLE LPF – plays no part as it is
tuned to be wide band. Filtering is in effect performed by the integrator. The
TUNEABLE LPF symbolises the channel, and its variable gain performs a useful
function in setting levels.
TCM – coding gain
Use the previous two Lab Sheets – TCM – trellis coding and matched filter detection
for 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 is
2.5 dB. See the relevant TIMS Interactive for more info
CDMA – introduction
This is a qualitative experiment. It provides experience in setting up a single direct
sequence spread spectrum – DSSS – transmission.
Some observations are
suggested, but many more are possible.
The mini version inventory omits the VCO, which limits the possible range of PN
clock frequencies.
CDMA – processing gain
This is a quantitative experiment, which measures the processing gain.
Other measurements are possible, but the addition of a second channel is left to the
next experiment.
The mini version inventory omits the VCO, so cannot measure bandwidth of noise
generator.
CDMA – 2 channel
Co-channel interference is introduced.
Mini version omits modules for BER measurement.
CDMA – multichannel
This 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 a
time.
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TIMS Lab Sheets Instructor`s Manual
unknown signals – 1
TIMS is an ideal environment for stretching the imagination of communications
students. One avenue is to use TRUNKS to send unknown unknown signals to all
students.
The terms of reference are that each of the unknown signals was made from a
message composed of one or more baseband signals (eg, message bandwidth less
than say 3 kHz), which was then multiplied with (translated in frequency with) a
100 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 each
signal.
The signals are intended to deceive. They are all ‘trick’ signals, in that they are not
what they appear to be when viewed on an oscilloscope.
Initial oscilloscope viewing is simplified by using the signal envelope to
synchronize the display (use the DIODE + LPF in the utilities module for envelope
extraction).
A reliable method of analysis is to first multiply (demodulate) the unknown signal
with a 100 kHz phase-adjustable sinewave, and then baseband filter the product. If
two audio sinewaves are present the TUNEABLE LPF should be sufficient to isolate
one of them.
The frequency of the other can be deduced by measuring the
frequency of the envelope of this two-tone signal.
Signal #1
Figure 1 could be defined as the output of an SSB transmitter, with a single
sinewave (say 2kHz) and a DC component as the message (although a typical SSB
transmitter would reject any DC component in the message). Alternatively it could
be 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 a
DSBSC derived from a single tone, as it appears to be from a first viewing on an
oscilloscope. 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 an
upper sideband) derived from a general message, but a simplified expression for the
specific signal just defined.
There is no need to make an SSB generator to generate it. It can be synthesised, as
the equation suggests, by adding two equal-amplitude sinewaves of 100 kHz and
102 kHz.
It can be analysed by multiplying it with a 100 kHz sinewave, and noting (after
baseband 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 lower
sideband (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 which
the ambiguity can be resolved.
Signal #2
A two-tone SSB. Similar to #1, except that there is now a two-component AC
output (and no DC), requiring determination of the frequency of each. The higher
frequency component can be removed with the TUNEABLE LPF, and the difference
frequency derived from the envelope frequency.
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Synthesise from a DSBSC using (say) a 1 kHz message, and a VCO as carrier (say
102 kHz). The spectral components will be 101 kHz and 103 kHz. Demodulator
output will be no DC, and 1 kHz and 3 kHz sinewaves (easily separated with the
TUNEABLE LPF).
Alternatively add the outputs of two VCO modules (101 kHz and 103 kHz).
Signal #3
A DSBSC and carrier of amplitude ratio 1.2:1, added in phase quadrature. This is
Armstrong’s signal before amplitude limiting – sometimes called ‘quadrature
modulation’.
The purists could argue that such a transmitter is not generally
available, 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 phase
difference of 900. That the latter (2 kHz) amplitude can be controlled by phase
adjustment shows that there must be both an upper and lower sideband component.
CDMA at carrier freq: example 1
This experiment is best performed by students who have completed some of the
earlier 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 to
be able to estimate the bandwidth of their DSSS signals in order to ensure they can
pass via the channel BPF (if one is supplied). The ratio of spreading sequence to
message bandwidth will not be large (there is no need to be constrained to the
frequencies suggested – use the DIGITAL UTILITIES module to suit.
The model illustrated uses no channel filter; band limiting (essential to constrain the
noise 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 is
included, then the WIDEBAND TRUE RMS METER can be omitted. To make
SNR measurements the ERROR COUNTING UTILITIES can be removed
temporarily.
Since this is a relatively complex experiment, care is needed when patching up. A
suggested sequence is:
1.
2.
3.
4.
5.
6.
7.
8.
9.
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set sequences in MULTIPLE SEQUENCES SOURCE to 0 and 1.
set sequence in CDMA DECODER to 0
set message SEQUENCE GENERATOR to a short sequence (both toggles UP)
patch all clock leads, and confirm frequencies
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).
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.
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 this
to 2 volt peak, the analog reference level).
Connect this ADDER output to the stand-alone ADDER, setting this output to 1 volt peak (in
anticipation of NOISE being added later).
Connect the ADDER output to the LOWER multiplier of the receiver QUADRATURE UTILITIES.
TIMS Lab Sheets Instructor`s Manual
10. Connect the output to the LOWER multiplier of the receiver QUADRATURE UTILITIES to the
CARRIER LPF of the CDMA DECODER. Adjust to maximum amplitude using the PHASE
CHANGER. If it is thought necessary the internal ADDER of the QUADRATURE UTILITIES can
be 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 be
identical, 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 be
identical, although may not be aligned. If not perfectly aligned, the X-OR operation needs to be
gated by a pulse thinner than that of the clock supplied by the DIGITAL DIVIDE, and adjustable
over 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’, realign, and confirm the second message recovery.
The system is now almost set up for the experiment proper can begin. There is no
detail given in the Lab Sheet as to what to examine, but you might consider some
BER/SNR measurements. For example: with one channel, add noise, and set up for
a moderate BER. Measure SNR at DATA LPF output. Add the second channel,
note new BER/SNR, etc etc.
Degrees of freedom include:
1.
2.
3.
the number of channels added (each pair requires a SEQUENCE GENERATOR and a MULTIPLE
SEQUENCES SOURCE module. Summing can be implemented via a BUFFER AMPLIFIER as
described in the Lab Sheet entitled CDMA – multichannel).
data rates and PN sequence clock rates (both their absolute values, and their ratio)
system bandwidth
CDMA at carrier freq: example 2
N/A
pulse position modulation
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speech
Headphones are required for this experiment. They are not listed in the ‘module’
inventory.
voice modems
The four lab sheets to follow aim at introducing, in graded steps, a voiceband
modem. Further sheets on this topic are forthcoming.
binary data via voiceband
For this and related experiments the eye pattern is adequate for estimating maximum
data rates. BER instrumentation has been included in the model, but can be omitted
if necessary. It is used no so much to quantify BER but as to indicate the onset of
errors. Note that, if implemented, the X-OR gate in the ERROR COUNTING
UTILITIES must be clocked with a thin pulse, the timing of which may need
adjustment. See the Advanced Modules Users Manual. A suitable pulse, of
adjustable position, has been implemented with the DELAY within the INTEGRATE
& DUMP module.
multilevel data via voiceband
data rates and voiceband modems –
transmission
A pair of BESSEL filters from BASEBAND FILTERS modules will be used as the
receiver filters, since these have the ‘best’ characteristic of the three (superior to
that of a TUNEABLE LPF). Having a fixed bandwidth (about 3 kHz), all other
parameters of the demodulator must be designed around it. This results in the
transmission channel being approximately twice as wide (say 6 kHz). This is not
representative of a typical voice channel bandwidth, but the consequent frequency
scaling by a factor of two should not lessen the realism of the model.
data rates and voiceband modems –
demodulation
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contents
Introduction to TIMS.....................................................................................................................3
Modelling equations......................................................................................................................3
DSBSC – generation .....................................................................................................................3
Product demodulation...................................................................................................................3
AM - amplitude modulation – I .....................................................................................................4
AM - amplitude modulation – II ....................................................................................................4
Envelope detection ........................................................................................................................5
SSB generation..............................................................................................................................6
SSB demodulation .........................................................................................................................6
ISB - independent sideband...........................................................................................................6
Armstrong`s phase modulator .......................................................................................................6
FM - generation by VCO...............................................................................................................7
FM - demodulation by PLL ...........................................................................................................7
FM - demodulation by ZX counting...............................................................................................7
Sampling .......................................................................................................................................7
PAM and TDM..............................................................................................................................7
FDM - frequency division multiplex..............................................................................................7
Phase division multiplex – generate..............................................................................................7
Phase division multiplex – demod .................................................................................................7
PWM - pulse width modulation .....................................................................................................8
Carrier acquisition – PLL .............................................................................................................8
The WAVE ANALYSER .................................................................................................................8
Complex analog messages ............................................................................................................8
PCM – encoding ...........................................................................................................................8
PCM – decoding ...........................................................................................................................8
ASK – generation ..........................................................................................................................8
ASK – demodulation......................................................................................................................8
BPSK – modulation.......................................................................................................................9
BPSK – demodulation ...................................................................................................................9
QPSK – generation .......................................................................................................................9
QPSK – demodulation...................................................................................................................9
FSK – generation ..........................................................................................................................9
FSK - envelope demodulation .......................................................................................................9
Signal constellations .....................................................................................................................9
DSSS - spread spectrum ................................................................................................................9
Eye patterns ..................................................................................................................................9
PRBS messages ...........................................................................................................................10
Detection with the DECISION MAKER ......................................................................................10
The noisy channel........................................................................................................................10
BER instrumentation ...................................................................................................................10
BER measurement – introduction................................................................................................10
Line coding & decoding ..............................................................................................................10
Delta modulation ........................................................................................................................10
Delta-sigma modulation..............................................................................................................11
Adaptive delta modulation ..........................................................................................................11
Delta demodulation.....................................................................................................................11
Bit clock regeneration .................................................................................................................11
QAM – generation.......................................................................................................................11
QAM – demodulation ..................................................................................................................11
BPSK...........................................................................................................................................12
Broadcasting (two mini versions)................................................................................................12
Fibre optic transmission .............................................................................................................12
multi-channel digital fibre link....................................................................................................12
PCM-TDM ‘T1’ implementation .................................................................................................12
DPSK & BER ..............................................................................................................................13
T1 bit clock regeneration ............................................................................................................14
DPSK and carrier acquisition.....................................................................................................14
Intro to DSP: analog & digital implementations compared ........................................................15
TCM – trellis coding ...................................................................................................................15
matched filter detection...............................................................................................................15
TCM – coding gain .....................................................................................................................16
CDMA – introduction..................................................................................................................16
CDMA – processing gain ............................................................................................................16
CDMA – 2 channel......................................................................................................................16
CDMA – multichannel.................................................................................................................16
unknown signals – 1 ....................................................................................................................17
CDMA at carrier freq: example 1 ..............................................................................................18
CDMA at carrier freq: example 2 ..............................................................................................19
pulse position modulation ...........................................................................................................19
speech .........................................................................................................................................20
binary data via voiceband ...........................................................................................................20
multilevel data via voiceband......................................................................................................20
data rates and voiceband modems – transmission ......................................................................20
data rates and voiceband modems – demodulation .....................................................................20
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