Lab 4 EEL 3552 Amplitude Modulation with MATLAB Simulations

1. Katrina Little
Max M.
Experiment 4: Amplitude Modulation

2. SINC
Function:
Frequency
Domain

3. SINC
Function:
Time
Domain

4. SIN
Function:
Frequency
Domain

5. SIN
Function:
Time
Domain

6. Intent
The purpose of the lab was to explore and understand amplitude modulation,
demodulation, and envelope detection. An AM signal is generated, modulated with a
carrier, and then demodulated with a low-pass filter. Then, another AM signal is
generated, modulated, and demodulated with a synchronous demodulator. The
specifics and differences are investigated.
Procedure
Amplitude Modulation
Once the circuit is built to the lab manual's specifications, the lab can begin.
The circuit is connected to the oscilloscope and the function generator and a sinewave
of frequency 100 Hz is generated.
The DC offset is adjusted to yield a DSB-LC waveform having modulation index m =
.75. In this laboratory, the function generator DC offset was malfunctioning and our
laboratory group could not obtain a screenshot of the LC signal.
Illustration 1: Input Sine

7. The circuit contains both the modulator and the carrier generator, all that is
needed is to input the message and measure the output. If the 100 Hz sine wave is
sent into the input, we receive the following DSB-LC signal:
If we place a triangle wave as the input, still at 100 Hz, we receive:
Illustration 2: Carrier Generator Output
Illustration 3: Modulated DSB-LC Sine
Illustration 4: Modulated DSB-LC Triangle

8. Frequency Spectrum of the modulated sine wave:
Frequency Spectrum of the modulated triangle wave:
Amplitude Demodulation
First we will use a low-pass filter to demodulate the modulated signal. A
simple filter is constructed from a diode, 1.6 kOhm resistor, and variable capacitor
and connected to the modulator. The same beginning input signals were used from
earlier in the lab, here is an example of a demodulated sine wave signal:
Illustration 5: Frequency Modulated Sine
Illustration 6: Frequency Modulated Triangle

9. The same circuit with a triangle wave signal input:
Now we'll try to demodulate the signal with a synchronous detector. The
detector must be built as per the laboratory manual and connected to the modulator
circuit. The yellow signal is the demodulated, the blue is the input. Using the same
Illustration 7: Sample Sine Demodulated Signal
Illustration 8: Sample Triangle Demodulated
Signal
Illustration 9: Demodulated DSB-SC signal

10. input signals from before:
The output signal is larger than the input in the SC signal, and smaller in the
LC signal. This is a consequence of the modulation index m. It varies and differs
from the laboratory manual because of the malfunctioning DC offset in our function
generator. The noise is very apparent in the DSB-LC signal, this is due to our DC
offset on the signal generator being very noisy and malfunctioning. In each of these
cases, ignoring the noise, the signal can be recovered.
Comments
Everything in the lab displayed results as expected, besides the variations in the
noise. The envelope detector and synchronous demodulator were able to recover the
original signals successfully. The variations in noise between the LC and the SC
were much larger than expected.
Questions and Calculations
1. m over 100% results in overmodulation by the envelope detector, the
signal will be distorted and completely different from the original signal.
2. Envelope detectors advantages include the fact that they're simpler and
cheaper to build. Disadvantages include sideband distortion because the
actual circuit is non-ideal.
Synchronous detectors advantages are that they can be more accurate
than envelope detectors because they are phase locked to the incoming
carrier, this is also the source for their disadvantage though, as they are
very sensitive to the phase of the carrier and must be tuned precisely.
They are also more complex and more expensive to build.
Illustration 10: Demodulated DSB-LC Signal