American University of Sharjah
College of Engineering
Department of Electrical Engineering
ELE361L – Communication System Lab
Lab 8 – Digital Modulation Systems
Names
IDs
Abdelrahman Akmal
B00084488
Khalifa Al Shamsi
B00078654
Nour Hassania
G00085100
Due Date: 25/04/2023
Spring 2023
Instructor: Mr. Ibrahim Abu Seif
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Abstract:
This lab was focused on understanding and performing Amplitude Shift Keying (ASK) modulation
using an analog switch, then using a standard envelope detector to demodulate the signal.
Moreover, students modulate a VCO with Frequency Shift Keying (FSK), then use filtering and an
envelope detector to demodulate the signal. Lastly, students developed an understanding of how to
demodulate PSK modulation using coherent and non-coherent methods. But in the actual
application of the lab, students focused on the use of non-coherent methods, which was the use of a
bandpass filter and an envelope detector. Lastly, students garnered a deeper understanding of
Differential Phase Shift Keying (DPSK), this was through observing its output and learning its
demodulation models.
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Procedure
1- ASK Signal Generation
In this procedure section you will generate an ASK signal using the analog switch module on the Telecoms101 trainer.
 Connect the system represented by the block diagram in figure 8. The Sequence Generator module is used
to model a digital signal and its SYNC output is used to trigger the scope to provide a stable display. The
Dual Analog Switch module is used to generate the ASK signal with a 100 KHz carrier.
Figure 8 ASK Signal Generation
 Sketch the digital message and the ASK signal below each other.
Q1) What is the relationship between the digital signal and the presence of the carrier at the modulator output?
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When the digital signal is at logic 0 does not have the carrier signal present, while the digital signal at logic 1
has the carrier is present.
Q2) What form of ASK modulation is this? Can the multiplier replace the switch for ASK generation? Would
a bipolar message be suitable for proper ASK generation using a multiplier?
The sine carrier in this type of modulation has two amplitude values, which are determined by the data stream.
The basic form is where the carrier is keyed. When the data bit is 1, the modulator sends the carrier, while
when the data bit is 0, it removes. This is called on-off ASK.
 Adjust the vertical position control of channel one to overlay the digital message with the ASK envelope and
compare them.
2- ASK Signal Detection
An ASK signal is really just like AM (with a digital message instead of speech or music), it can be demodulated
using any of the AM demodulation schemes.
In this procedure section you will demodulate the ASK signal using an envelope detector.
 Connect the ASK signal to the rectifier and the tunable low pass filter as shown in figure 9. You can locate
the rectifier on the Utilities module.
 Turn the filter cut-off frequency and gain controls fully clockwise.
Figure 9 ASK Signal Generation and Detection
 Display the rectifier’s output signal on channel two of the oscilloscope. Sketch the rectifier output and
compare with the ASK signal.
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 Display the recovered digital signal at the filter output on channel two and keep channel one connected to
the original message. Sketch the two signals and compare them.
Q3) Is the recovered signal an exact copy of the original message (ignoring phase shift) or a band limited
version? What is the cause of this band limiting distortion?
This is because the cut off frequency of the low pass filter is lower than some of the higher frequency
components of the message signal. Additionally the transients of the filter and propagation delay cause the
recovered message to have ripples and phase shifts.
 Connect the filter’s output to the REF input of the comparator on the Utilities module and connect the
variable DC voltage on the Variable DCV module to the second input of the comparator.
 Move the oscilloscope channel two to the comparator’s output and vary the DC voltage until the recovered
digital signal is a perfect copy of the original message.
 Sketch both signals as seen on the oscilloscope.
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Q4) Is the comparator output a perfect copy of the original message? Did the comparator eliminate the band
limiting distortion of the LPF output?
The comparator is used to compare its input to a reference voltage. Therefore, if the input is above a certain
threshold, the output is shown as a logic 1. If the input is below that threshold, the output is shown as a logic
0. Therefore, the voltage comparator is an excellent method of cleaning a recovered signal.
3. FSK Signal Generation
In this procedure section, you will generate an FSK signal using the VCO module on the trainer.
 Connect the setup shown in figure 10 below.
 Set the VCO Gain control to about half its travel, the Frequency adjust control to about quarter of its travel,
and the Range control to the LO position.
 Set the Sequence generator module’s dip-switches to 00.
 Set the scope’s time base control to the 0.5 ms/div position.
 Observe and sketch the digital message from the sequence generator and the FSK signal out of the VCO as
shown the scope.
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 Measure the FSK signal frequency that corresponds with logic-0s in the digital data and the frequency that
corresponds with logic-1s.
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High frequency (logic-1) = 7.14 kHz
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Low frequency (logic-0) = 2.38 kHz
Q5) Calculate the FSK signal bandwidth using carson’s rule?
7.14 − 2.38
∆𝑓 =
= 2.38
2
BW = 2(∆𝑓 + 𝑓𝑚) = 2(2.38 + 2) = 8.76 𝑘𝐻𝑧
Figure 10: FSK Signal Generation
4. FSK Signal Detection
 Turn the VCO frequency adjust control to about the position of the number 2 on a clock face.
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 Set the tunable LPF cut-off frequency control fully clockwise, and gain control fully clockwise.
 Modify the set-up as shown in figure 11.The envelope detector comprises the Rectifier and the Baseband
LPF in the channel module.
 Monitor the Tuneable LPF output on channel 2 and turn the LPF’s cut-off frequency control slowly from
fully clockwise until the higher frequency component of the FSK signal becomes equal to zero.
 Sketch the digital signal and the filter output as shown on the scope.
Q6) Describe the filtered FSK signal and explain the function of the Tuneable LPF in the detection process?
The signal looks like an ASK signal. This is because its filter operates like a bandpass. This means that the
FSK is converting to ASK as the higher frequency components of the signal are attenuated, while the lower
frequency components are not effected as much. This causes the distorted signal to appear at the output.
Figure 11: FSK Generation and Envelope Detection
 Move channel 2 to the output of the rectifier, then the baseband LPF and sketch each signal along with the
original message.
 Add the voltage comparator to the set-up as in part 2. Move channel 2 to the comparator’s output and adjust
the VDC to restore a clean version of the original message.
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Q7) Why the recovered digital message is inverted?
This is because the logic sequence was inverted when converting FSK to ASK.
 Sketch the recovered message and the original message as shown on the scope.
5. PSK Signal Generation
In this procedure section you will generate a binary PSK signal using a multiplier.
 Connect the set up shown in figure 12 which can be represented by the block diagram in figure 13. The
sequence generator is used to model a digital message signal and the multiplier is used to generate the PSK
signal by implementing its mathematical model.
 Observe both the digital signal and the PSK signal on the scope. Sketch the two waveforms and compare
them.
Q8) What happens to the PSK signal on the data stream binary transitions? Is the BPSK signal similar to a
DSBSC signal with a square wave message?
The PSK signal is phase shifted by 180 degrees.
 Use the oscilloscope channel one vertical position control to overlay the digital signal with the PSK signal
envelopes and compare them.
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Figure 12: PSK Signal Generation Setup
Figure 13: PSK Signal Generation Block Diagram
6- PSK Signal Detection
Since the PSK signal is simply a DSBSC signal, it can be demodulated using any of the DSBSC demodulation
schemes.
 Apply product detection for the PSK signal as shown in figure 14.
 Monitor and sketch the product detector’s multiplier output (before the LPF) on channel 2.
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Q9) Explain the multiplier’s output signal and clarify its spectral components?
The output of the multiplier is the same as the product detector multiplier output of the DSB-SC. The output
of the multiplier adds 2fc of high frequency components to the message.
 Monitor and sketch the LPF and comparator output signals. Adjust the comparator VDC until the restored
digital signal is a copy of the original digital message ignoring the phase shift.
Figure 14: PSK Signal Generation and Detection
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7. Differential Phase Shift Keying (DPSK) and Carrier Acquisition
•
•
Batch the block diagram for DPSK generation using a sequence generator, a line code encoder and a
multiplier. Use the NRZ-M output to phase modulate the 100 KHz sinewave carrier. As shown in
figure 15.
Use 8.3 KHz Master clock for the line code encoder and supply the bit clock (B. Clock) to the
sequence generator as shown in figure 15. This will set the Bit clock to 2.083 KHz (approximately 2
KHz).
Figure 15: DPSK Signal Generation
•
•
Observe and sketch the DPSK signal together with the NRZ-M signal on the scope. Verify that the
DPSK signal phase shifts at every transition in the NRZ-M signal.
Batch the PLL carrier acquisition block diagram shown in figure 16 to regenerate an exact copy of
the transmitter carrier.
Figure 16: PLL Carrier Acquisition for DPSK
•
Display the regenerated carrier at the VCO output together with the original carrier on the scope.
Adjust the VCO center frequency and gain to achieve lock. Use a phase shifter to eliminate the phase
shift between the regenerated carrier and the original carrier. Successful carrier acquisition requires
that the regenerated carrier is at the same frequency and phase as the original carrier at the
transmitter. Take a screen shot for the in phase regenerated carrier and the transmitter carrier.
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•
Using the regenerated carrier, apply product detection for the DPSK signal. Take screenshots of the
output of each receiver module together with the original NRZ-M digital data message.
Simulation Assignment
ASK Generation:
LPF and VAR DC:
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FSK Generation VCO/FSK module:
100 kHz Channel filters:
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Comparator output:
Rectifier output:
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LPF:
PSK Generation:
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Multiplier Output:
Rectifier:
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LPF:
Comparator
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