Analog & Digital Communications Lab
Habib University
LAB EXPERIMENT 11
PULSE AMPLITUDE MODULATION (PAM) & DEMODULATION
Objectives
• Understanding the principles of pulse amplitude modulation and demodulation using
MATLAB Simulink.
• Generating a waveform from an analog signal which looks like pulses and contains
the information present in the analog waveform by modulation.
• Applying sampling technique to produce a PAM signal.
• Performing the demodulation by recovering the original analog waveform from the
PAM signal through a low-pass filter.
Theory
Pulse amplitude modulation (PAM) and demodulation is the process of converting an
analog signal to a pulse-type signal in which the amplitude of the pulse denotes the analog
information and then recovering the original signal from the pulse amplitude modulated
signal. Two types of sampling are used to produce a PAM signal which are natural
sampling and instantaneous sampling that produces a flat-top pulse. If 𝜔(𝑡) is an analog
waveform bandlimited to 𝐵 hertz, the PAM signal that uses natural sampling (gating) is
𝜔𝑠(𝑡) = 𝜔(𝑡)𝑠(𝑡)
(1)
where
∞
𝑡 − 𝑘𝑇𝑠
𝑠(𝑡) = ∑ Π (
)
𝑟
(2)
𝑘=−∞
is a rectangular wave switching waveform and 𝑓𝑠 = 1⁄𝑇𝑠 ≥ 2𝐵. The duty cycle (ratio) of
𝑠(𝑡) is 𝑑 = 𝑟⁄𝑇𝑠 where 𝑟 is the pulse width and 𝑇𝑠 is the sampling time.
0
t
(a) Baseband Analog Waveform
0
t
Ts
(b) Switching Waveform with Duty Cycle d = /Ts = 1/3
𝜔
t
0
Ts
(c) Resulting PAM Signal (natural sampling, d = /Ts = 1/3)
Figure 1 PAM signal with natural sampling
Analog multiplier
( four-quadrant multiplier )
𝜔
PAM (natural sampling)
Low-pass filter
cos(n 𝜔 s t )
Oscillator
𝜔 o = n 𝜔s
– fco
fco
where
f
B < fco< fs – B
Figure 2 Demodulation of a PAM signal (naturally sampled).
Procedure
1. Switch on the computer and click on the MATLAB icon.
2. Go to start at the bottom of the command window, then select “Simulink” then go to
library browser and drag it into creating file. (or) Once you open the MATLAB, click
on the Simulink icon . Go to file and select new and then select model. You will get a
new window.
3. Arrange the functional blocks as shown in Simulink model.
4. Assign required parameters to each functional block.
5. Observe the outputs on scope.
6. Change the frequency of pulse generator to 100 Hz. Observe and comments on the
changes in PAM waveform. Attach the waveform of Scope 2.
7. Vary the amplitude and frequency of modulating (analog) signal. Observe and
comments on the changes in output. Also, attach its waveform.
Simulink Model
Parameters
Output: (Attach the output of Scope 1,2 and 3 and comments on the result as well)
AT SCOPE 1
AT SCOPE 2
AT SCOPE 3
NOW CHANGING FREQUENCY OF PULSE GENERATOR TO 100 Hz
AT SCOPE 02
AT SCOPE 03
NOW CHANGING FREQUENCY OF PULSE GENERATOR TO 1 Hz
AT SCOPE 2
AT SCOPE 3
OBSERVATION ON GRAPHs VARIATION IN FREQUENCY
When the pulse generator frequency shifts to 100 Hz, the PAM waveform
exhibits longer time period, consequently shorter wavelength. The Quantization
levels also observed to be reduced. The signal quality of Modulated Signal also
got compromised. However, when we used frequency of 1 Hz, the modulated
signal took relatively smoother shape. Similarly, when frequency used is 100
Hz the demodulated signal no longer retains the original signal and losses its
information. On the other hand, in the frequency of 1 Hz, the demodulated
signal closely resembles with the analogue signal at the input, suggesting that
original signal can be recovered.
VARYING THE AMPLITUDE
AMPLITUDE TO 10
AT SCOPE 2
AT SCOPE 3
VARYING THE AMPLITUDE TO 0.1
AT SCOPE 2
AT SCOPE 3
OBSERVATION ON GRAPHs VARIATION WITH AMPLITUDE
The changing of Amplitude of Analogue signal has direct impact upon the amplitudes of
modulated and demodulated signals.
When we have used the Amplitude of 0.1, then the modulated signal has the amplitude of 0.1
whereas the demodulated one has 0.06.
Similarly, when the Amplitude of 10 is used, the amplitude of modulated signals appears to be 10
and that of de-Modulated signal is around 6.
In essence, higher the amplitude of Analogue Signal the greater the amplitude of deModulated Signal.
Tasks:
1.
Which is better, natural sampling or flat-topped sampling and why?
Natural Sampling and the flat-topped sampling are the sampling techniques, in which the
original signal got preserved in the Natural Sampling while the signal got modified or
reduced in the flat-topped sampling.
However Natural Sampling technique is good and used, in general. This technique
samples the signal at Nyquist rate while avoiding the aliasing problem. Hence through
this way the original signal got preserved. But the scenarios in which anti-aliasing
sampling is difficult such that high components of frequencies are residing near the
Nyquist rate we prefer flat-topped sampling. Hence the choice between both the
sampling techniques will be decided, depending upon specific application.
2.
Why PAM is not preferable in digital transmission?
Pulse Amplitude Modulation (PAM) is not preferable in digital transmission, as it is
highly sensitive and prone to noise signals. Moreover, the Low Bandwidth efficiency
between signal levels, more power requirement, and complex design of receiver make it
less preferrable.
3.
Which multiplexing technique is used to transmit both digital and analog signals?
To transmit both digital and analog signals, we used Time Division Multiplexing (TDM)
technique.
4.
What is the process of sampling an analog signal at a high rate?
While sampling analog signal at a high rate, we assign the discrete levels of uniform
spacing for continuous signal to be represented in Discrete domain. For the sampling we
usually keep it slight greater than Nyquist Rate (which is twice of fs, the sampling
frequency). This technique avoids overlapping of band limited signals and hence avoid
aliasing. Through this we have translated the analogue signal into digital signal with L
uniform levels.
Dhanani School of Science and Engineering, Habib University
Name & ID: Munim ul Haq mh07731
Date: 19 Feb 2024
EE-322L Analog and Digital Communications Lab
Spring 2024
Habib University
Dhanani School of Science & Engineering
Lab#11 Pulse Amplitude Modulation & Demodulation
Lab #11 Marks distribution:
In-Lab
Tasks
Task 1
LR1=05
LR3=05
LR4=05
LR5=15
AR4=20
05
05
05
05
10
20
LR5=15
AR4=20
Task 2
Total
Marks
50
Lab #11 Marks Obtained:
LR1=05
In-Lab
Tasks
LR3=05
LR4=05
Task 1
Task 2
Total
Marks
1
Analog and Digital Communication Lab (EE-322L)
Dhanani School of Science and Engineering, Habib University
Lab Evaluation Assessment Rubric
#
Assessment Elements
(0<Level 1<=4)
(4< Level 2<=6)
(6< Level 3<=8)
(8< Level 4<=10)
Components are wired
and didn’t show neat and
clean connections and
minimal efforts shown.
Components are wired
with untidy connection
and didn’t show neat and
clean connections
Few but not all
Components are wired
with neat and clean
connections
Complete components are
wired with neat and clean
tight connections. Given
task completed in due
time.
Unable to identify the
fault/minimal effort
shown
Able to identify the fault
but unable to remove it.
Able to identify the fault
but partially removes it.
Able to identify the fault
and able to make
necessary steps and
actions to correct it.
Data Collection
Measurements are
incomplete, inaccurate
and imprecise.
Observations are
incomplete or not
included. Symbols, units
and significant figures are
not included
Measurements are
somewhat inaccurate and
very imprecise.
Observations are
incomplete or recorded in
a confusing way. Major
errors using symbols, units
and significant digits
Measurements are mostly
accurate.
Observations are
generally complete. Minor
errors using symbols,
units and significant digits
Measurements are both
accurate and precise.
Observations are very
thorough. Includes
appropriate symbols,
units and significant digits
and completed in due
time.
LR5
Results & Plots
Figures, graphs, tables
contain errors or are
poorly constructed, have
missing titles, captions,
units missing or incorrect,
etc.
Most figures, graphs,
tables OK, some still
missing some important
or required features
All figures, graphs, tables
are correctly drawn, but
some have minor
problems or could still be
improved
All figures, graphs, tables
are correctly drawn and
contain titles/captions.
LR6
Clean-up
All equipment/PC is not
powered off.
All items left at station
and not cleaned.
Many equipment/PC is
not powered off.
Many items left at station.
Some equipment/PC is
not powered off.
Some items left at
station.
All equipment/PC is
powered off.
Station left neat and
clean.
LR8
Contribution
No participation towards
the group tasks
Slight participation
towards the group tasks
Substantial participation
towards the group tasks
Outstanding participation
towards the group tasks
Couldn’t provide good
summary of in-lab tasks.
Some major tasks were
completed but not
explained well.
Submission on time.
Some major plots and
figures provided.
Good summary of In-lab
tasks. All major tasks
completed except few
minor ones. The work is
supported by some
decent explanations,
Submission on time, All
necessary plots, and
figures. provided
Outstanding Summary of
In-Lab tasks, All task
completed and explained
well, submitted on time,
good presentation of
plots and figure with
proper label, titles and
captions.
Late submission after 4
days and within a week.
Late submission after the Timely submission of the
lab timing and after 2
report and in lab time.
days of the due date.
LR1
Neat and Clean Circuit
layout
LR3
Troubleshooting
LR4
LR9
Report
No summary provided.
The number/ amount of
tasks completed below
the level of satisfaction
and/or submitted late.
AR4
*Report Submission
Late submission after 1
week.
*Report: Report will not be accepted after due date
2
Analog and Digital Communication Lab (EE-322L)