EE3550 - Communications Systems Analysis
Laboratory Exercise 3
This assignment may be done individually or in groups of two or three.
Partners may turn in a combined report and will receive the same grade.
In this exercise, you will use the TIMS system to generate amplitude-modulated signals
and then demodulate them.
The goals of the laboratory exercise are:
1. To observe the process of amplitude modulation and demodulation
2. To observe the bandwidth of a DSBSC and AM signal
3. To understand the concept of modulation factor and over-modulation
To turn in
Certain tasks in the labs involve measurements or have questions to answer. Please
answer each of these in your lab report. Note that all required questions, observations,
and sketches are underlined. A quality report will include neatly organized answers to
questions and sketches in a readable document.
Amplitude Modulation
Modulation is the process of shifting a signal’s center frequency to a different value.
Modulation allows an engineer to move a baseband signal to any desired portion of the
RF spectrum. For instance, audio data in the range of 0-20KHz may be moved to a more
useful band such as 100.010 – 100.030 MHz. There are many reasons for modulation, but
the most basic is to move RF signals to different portions of the spectrum to allow many
users to share the airwaves.
The simplest form of modulation is amplitude modulation (AM). In AM, a source signal
is used to modulate a carrier frequency in such a way that the amplitude of the carrier
changes in response to the amplitude of the source signal. This is most easily seen
visually, as in Figure 1. Mathematically, AM results by multiplying the signal by the
carrier. For instance:
signal A cos(xt) and carrier B cos(yt)
product = AB cos(xt) cos(yt)
= AB/2 cos((x+y)t) + AB/2 cos((x-y)t)
Thus, the result of multiplying these waves is a waveform with ½ power at the sum and
difference of the frequencies. Now, assume that the souce wave is a wave with frequency
5kHz and the carrier is a wave with frequency 100KHz. Multiplying these together gives
a wave with frequency components at 95kHz and 105KHz – we have moved the 5kHz
wave to a higher frequency. Notice that we have energy in two bands – these are called
the lower and upper sidebands. Also notice that there isn’t any energy at the carrier
frequency (100KHz). Thus, this modulation is called Dual Sideband Suppressed Carrier,
or DSBSC. It is commonly diagrammed as in Figure 2.
