Department of Electrical and Computer Engineering
332:427
Communications Systems Design
Fall 2008
LABORATORY EXERCISE #9: DIGITAL MODULATION AND DEMODULATION
This laboratory exercise is essentially split into two parts: the first covering Amplitude Shift
Keying (ASK) and the second Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift
Keying (QPSK).
The purpose of the first part of this laboratory exercise is to investigate two digital modulation
techniques: ASK and FSK. System considerations including signal generation, modulated signal
spectral occupancy and data detection/ recovery will be explored.
1. Amplitude Shift Keying (ASK)
a) Using the AM synthesizing feature of the HP function generator, produce an ASK waveform
of alternating ones and zeros. Set the frequency of the carrier signal to 100 kHz. Use a 12
kHz square pulse signal from the Tektronix function generator as the modulating waveform.
Adjust the amplitude of this signal so that the synthesized ASK waveform has a modulation
index of nearly unity. How does the observed waveform compare with standard AM (DSBLC)? Next, connect the ASK signal to the spectrum analyzer and plot the resulting spectrum
(be sure to AC couple the signal). Comment on how the resulting spectrum relates to what
you would expect from theory.
b) Using the TIMS experiment sheet L-026 ASK Generation, connect the Sequence Generator,
Adder and Multiplier modules to generate an ASK waveform consisting of a 100 kHz
sinusoid modulated by a baseband 8.333 kHz PRBS. In order to achieve the 8.333 kHz bit
rate, use the 8.333 kHz digital sampling clock, also found on the TIMS hardware chassis. Be
sure to connect the 8.333 kHz signal to the digital (i.e., red) clock input port on the Sequence
Generator module. Connect the resulting waveform to the oscilloscope. (Use the SYNC
output of the Sequence Generator module as an external trigger to the oscilloscope.) Adjust
the gain of the Adder module to achieve a modulation index as close to unity, as possible.
Determine the number of carrier periods for each information bit. Next, connect the ASK
signal to the spectrum analyzer (be sure to AC couple the signal). Plot the resulting spectrum
and compare with the theoretical spectrum for ASK. Estimate the bandwidth of the main lobe
of the resulting spectrum. Is there power at the carrier frequency in the main lobe? Next,
estimate the frequency offset between the main lobe and the first side lobe. By approximately
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how many dB is the amplitude of the first sidelobe suppressed with respect to the amplitude
of the main lobe?
c) Now, implement pulse shaping of the Sequence Generator output by inserting the LPF
module between the Sequence Generator and Adder modules as shown in the experiment
sheet. Briefly, view the filtered baseband signal on the oscilloscope to get a sense of the
filter's effect on the pulse shape. Next, observe the oscilloscope and spectrum analyzer traces
of the resulting ASK waveform, as done in part (b). In each case, adjust the tuning knob of
the LPF module to adjust the bandwidth of the LPF. Comment on the effect of the baseband
filter on the ASK waveform shape and bandwidth.
d) In this part, non-coherent detection of the ASK waveform generated in part (c) will be
performed. Connect the part (c) ASK waveform to the non-coherent detection scheme
depicted in the TIMS experiment sheet L-027 ASK Demodulation. Be sure that the DC Ref
port of the Utilities module is connected to an appropriate DC source (run the Variable DC
output through the Buffer Amplifier module, found on the TIMS chassis, to create the
appropriate voltage reference). Contrast, via the oscilloscope, the Comparator output with
that of the unfiltered baseband PRBS, adjusting the DC Ref input to the Comparator for
optimum performance. Verify that non-coherent recovery of the original bit stream is being
performed. Next, reduce the bandwidth of the LPF stage. What is the minimum signal
bandwidth required for successful non-coherent data recovery?
e) Now, coherent data recovery will be performed. First change the clock input of the Sequence
Generator from the 8.333 kHz digital sampling signal to a 2 kHz digital signal. Tune the TTL
output of the Audio Oscillator module to realize this 2 kHz clock signal. Connect the
modules needed for ASK demodulation in accordance with the coherent data recovery
diagram of the TIMS experiment sheet L-027 ASK Demodulation. (The reason the 2 kHz
clock source is used here is because the Decision Maker module used for coherent data
recovery is optimized to operate on an input waveform of around 2 kHz..) Verify that ASK
demodulation and subsequent data recovery has been successfully performed by contrasting
the input data sequence with the recovered output on the same oscilloscope screen. Next,
view of the eye diagram for the demodulator output (i.e., the input signal to the Decision
Waveform). By adjusting the bandwidth of the LPF used in the ASK generation, estimate the
bandwidth required for successful coherent data recovery based on when the "eye" of the
demodulator output closes. Follow the clock and trigger guidelines of the supplemental notes
on the Decision Maker module operation to view a stable version of the recovered data
sequence and the eye diagram of the Decision Maker input. Discuss the pros and cons of
using coherent data recovery versus non-coherent data recovery, as done in part (d).
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The purpose of the second part of this experiment is to demonstrate the processes of modulation
and demodulation involved with Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift
Keying (QPSK). BPSK and QPSK are closely related in the sense that QPSK is effectively the
sum of two BPSK waveforms in quadrature. The basic principle is to form two carriers in
quadrature and modulate one with the even-numbered data bits and the other with the oddnumbered data bits. The result is then linearly combined to form the QPSK signal.
2. Binary Phase Shift Keying (BPSK)
Using the Telecommunications Instructional Modeling System (TIMS), the following Binary
Phase Shift Keying (BPSK) exercises are to be performed.
a) Connect the Sequence Generator and Multiplier modules as shown in Figure 3 of the TIMS
experiment sheet L-028 BPSK - Modulation. View the resulting BPSK waveform on the
oscilloscope. Based on the oscilloscope image, how is the information content of the data
stream conveyed by the BPSK waveform? Estimate the number of carrier periods per period.
b) Now, study the frequency spectrum of the BPSK waveform (be sure to AC couple the
spectrum analyzer input). Note the relative amplitudes of the sidelobes and estimate the
sidelobe separation in kHz. In your lab report, compare the observed spectrum with that
predicted by theory.
c) To implement a bandlimited BPSK waveform, interpose a Tunable LPF module between the
Sequence Generator and Multiplier modules. Again, study the frequency spectrum of the
resulting BPSK waveform. Compare with the frequency spectrum generated in part (b).
d) To demodulate the BPSK waveform, connect the Phase Shifter, Multiplier and Tunable LPF
modules as shown in Figure 2 of the TIMS experiment sheet L-029 BPSK - Demodulation.
As for the ASK experiment of Lab Exercise #9, the Decision Maker module will be used for
coherent data recovery. Using the divide-by-N utility of the Bit Clock Regeneration module,
reduce the bit clock input to the Sequence Generator module from 8.333 kHz to 2.0833 kHz.
Adjust the Phase Shifter and Tunable LPF gain as necessary. Verify that the Sequence
Generator module is set to the short (32 bit) PRBS. Now, connect the demodulated waveform
to the Decision Maker input. Verify by inspection, with the oscilloscope, that the original
data stream has been recovered. Discuss the process of synchronous demodulation and data
recovery, in your lab report.
e) To analyze the effects of a noise channel, connect the BPSK waveform and output of the
Noise Generator module to an Adder module. Initially set the gain (g) of the noise input to
zero. Adjust the gain (G) of the BPSK waveform in order to change the signal amplitude
input to the Decision Maker module as necessary. Estimate the RMS voltage of the BPSK
waveform by using the Adder module output as input to the Wideband True RMS Meter.
Now, gradually increase the gain (g) of the noise input. By viewing the output of the
Decision Maker module, determine the point at which the noisy input begins to produce
errors in the output bit stream of the Decision Maker module. Now, zero the gain (G) of the
BPSK waveform inputted to the aforementioned Adder module. Using the Adder module
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output as input to the Wideband True RMS Meter module, estimate the RMS voltage of the
noisy input.
f) Return now to the mode of non-bandlimited BPSK scenario by bypassing the Tunable LPF
module used in the modulation stage. Repeat part (e) to assess the performance of nonbandlimited BPSK in the presence of additive channel noise. Compare qualitatively with the
results of part (e). Discuss the advantages and disadvantages of bandlimited BPSK vs. the
non-bandlimited case.
3. Quadrature Phase Shift Keying (QPSK)
Using MATLAB and the associated Communications Toolbox, design and simulate a QPSK
system in the presence of additive white Gaussian noise.
a) Design and simulate a baseband QPSK system. Use a block of 1000 channel symbols within
a 1.0 second time interval. For the MATLAB Communications Toolbox, use the standard
built-in functions (e.g., randint, dmodce, butter, cheby1, cheby2, randn, ddemodce, biterr,
etc.) to develop the QPSK system.
b) Design and simulate a passband QPSK system. Use a block of 1000 channel symbols within
a 1.0 second time interval and a carrier frequency of 10 kHz. Use the appropriate built-in
functions (e.g., dmod and ddemod) of the MATLAB Communications Toolbox for passband
modulation and demodulation.
For both Parts (a) and (b), use the MATLAB built-in functions, plot, abs, fft and fftshift to obtain
both time and frequency domain plots of the signals that are output from the various modules of
the respective QPSK systems.
c) Perform a Monte Carlo simulation for a digital communication system that employs QPSK
signals operating at baseband. That is, obtain via simulation methods the bit error probability
performance of the digital communication system.
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