Electrical
Communications Systems
Course No. ECE.09.433
Laboratory
Project 2
AM and DSB-SC Modulation and Demodulation
Objectives
This project has 2 parts. In Part 1, you will investigate the performance of
the envelope detector. In Part 2, you will model AM and DSB-SC communications
systems using the Matlab Simulink Communications
Systems Toolbox.
In both parts, you will test the system with single-tone AM (w and w/o added
Gaussian noise) and multi-tone AM signals (w/o added noise).
Equipment
and Software
- You will be synthesizing waveforms in Matlab and creating their corresponding electrical
signals using the Arbitrary Waveform Generator. In order to do this, you
will need the Keysight IO Libraries Suite. Instructions
have been created by Yang Qi with sample Matlab code.
- You will be capturing signals from the
Oscilloscope/Spectrum Analyzer and will be analyzing them in Matlab.
- You will use electronic components to breadboard an
envelope detector circuit.
- You will use Matlab Simulink
Communications Systems Toolbox to model AM and DSB-SC communications
systems.
Project Requirements
Part 1: Envelope Detector
- Design the envelope detector circuit described in the Lab
2: Pre-Lab Lecture. The design equation is fco
= 1/(2pR1C1) where fco
is the cut-off frequency of the low-pass filter R1C1
. Cc is a coupling capacitor, chosen such that Cc >>C1.
(As part of the theory section in your report, you are expected to derive
this design equation).
Single-Tone AM Detection: No added Gaussian Noise
- Using the HP Arb. Function Generator, generate an AM
wave with carrier frequency in the AM band and modulation frequency in the
audio range (single tone).
- Vary the modulation
depth from 10% to 120%.
- For each modulation depth,
observe and digitally capture the input and output waveforms.
- Perform a spectral
analysis of the input and output waveforms. Observe the spectral
components due to ripple and harmonic distortion.
- Listen to the output
signal using the PC speakers. Compare with the pure tone of the same
frequency.
- Set the modulation index to an optimal value (max value
for minimum distortion) and vary the modulating signal frequency to
determine the 3-dB bandwidth of the detector (value at which the output
signal falls to 0.707 of its maximum value).
- Experiment with varying the carrier frequency and/or
cut-off frequency of the LPF.
Multi-Tone AM Detection: No added Gaussian Noise
- Obtain a multi-tone baseband signal by playing legally
downloaded music on your laptop out of the headphone jack.
- Feed this baseband signal through a variable gain
inverting OpAmp circuit and then into the
external AM modulating source input at the back of the HP 33120A Function
Generator.
- Choose a suitable carrier frequency (in the AM
broadcast band) and generate an AM signal by choosing external modulating
source in the HP 33120A menu.
- Observe the modulated signal on the oscilloscope. Vary
the modulation depth by varying the gain of the operational amplifier.
- Apply the multi-tone AM signal to the input of the
envelope detector. For each modulation depth:
- Observe, calculate the
modulation depth and digitally capture the input and output
waveforms.
- Perform a spectral
analysis of the input and output waveforms.
Single-Tone
AM Detection: With added Gaussian Noise
Repeat
the single-tone AM detection experiment by digitally synthesizing single-tone
AM signals with varying SNR.
At
each stage, note down your observations and conclusions.
Part
2: AM and DSB-SC Communication Systems
The objective is to observe (in time and spectral
domains) & listen as audio tones progresses along a modulation-demodulation
system, both in the presence and absence of added Gaussian noise.
Develop a Matlab Simulink
simulation of an AM system that includes a source, modulator, channel,
demodulator and sink. Test the system with single and multi-tone audio
frequency message inputs. Note down observations in the time and spectral
domains, and listen to the audio signals and the input and output.
Repeat the exercise for a DSB-SC system. Bonus: Demonstrate the
Quadrature-Null effect.
Click here for required lab project
report format.
Click here for suggestions for a good lab
report.