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LABORATORY 2:
AMPLITUDE MODULATION AND DETECTION
EEE 186: COMMUNICATIONS SYSTEMS LABORATORY
Department of Electrical & Electronic Engineering
College of Engineering & Computer Science
California State University, Sacramento
FALL 2015
Amplitude Modulation and Detection
Amplitude modulation (AM) is perhaps the easiest type of analog modulation to
understand, and some aspects of it are also used in systems where the main principle is
one of the other kinds of modulation, perhaps even a digital type. It is the easiest to
understand because any analog multiplication of two signals produces a spectrum that
contains the sum and the difference of the two frequencies.
It is often easier to get a multiplication of signals than to avoid its occurrence.
The reason is that any non-linearity in an electronic device produces a multiplying effect
on the signals. To illustrate how easy, even inevitable, such multiplication is, we first
simply feed two signals into a diode having a resistor load. When we look at the result
using both an oscilloscope and spectrum analyzer, we find that we have a lot of
unexpected junk besides what we wanted, but we can filter out much of it, so that the
remainder may even be recognizable as amplitude modulation.
Part I. Simulation of Amplitude Modulation using Simulink
Simulate the AM system as shown below in Figure 1, using Simulink software.
Scope
Carrier Signal
(50 mV pp, 200
KHz)
X
Modulating
Signal (10 V pp,
1 KHz)
+
Spectrum
Analyzer
Figure 1. Block diagram of Amplitude Modulation (AM) System
Obtain the power spectrum of the conventional AM signal (Double Sideband with
Carrier, or DSB-C), which includes the carrier signal. The power contained in the Fourier
coefficients is given as follows:
Pcn, comp = |cn|2 (mW)
(a) Note down the frequency and amplitude of the carrier and two sidebands.
(b) Repeat part (a) by removing the link connecting the carrier to the adder in Figure 1.
This is the Double Sideband Suppressed Carrier, or DSB-SC).
Part II. Fabrication and Testing of AM transistor circuit
Two signal generators are used in this circuit, one representing a high frequency (200
KHz) RF carrier, VG2, the other signal generator is used to inject a 1KHz audio signal
VG2,. The two signals are mixed and amplified by the transistor and an amplitude
modulated signal appears at the collector of the BC548.
Figure 2. AM Modulation circuit
(Courtesy: http://www.zen22142.zen.co.uk/spice/ammod.htm)
Fabricate the circuit shown in Figure 2 above.
Measure the carrier signal, audio signal
and the modulated signal across the output resistor load. Write down the frequency and
amplitude of each peak of the signals.
The signal you just arrived at is called “double-sideband suppressed carrier”,
which is abbreviated to DSB-SC. Next produce conventional amplitude modulation
(DSB-C) by adding carrier to the DSB-SC. Do this by connecting the carrier signal to the
modulated signal output. Note down the carrier and sideband amplitudes and frequencies
on the dynamic signal analyzer.
Part III. Comparison of Simulated and Measured data
(a)
Compare the measured and computed power spectrum of the carrier and two
sidebands in mW, after normalizing the peak values of the fundamental to 1 mW (0
dBm).
(b) Compute the % error between the computed and measured power spectrum (mW).
The % error is defined as:
|Pcn, comp - Pcn, meas|
% error = ------------------------------Pcn, comp
(c) Repeat the steps (a) and (b) using a DSB-C system, by adding the carrier signal.