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3.2 Modulation and Demodulation
Modulation occurs in the transmitter, where the input signal (also known as the modulating
signal) is intentionally ‘mixed’ with a sinusoidal carrier wave (a wave of constant frequency,
amplitude and phase). Through this process, one of the three parameters (i.e. amplitude,
frequency or phase) of the carrier wave is modified by the input signal according to a
predefined method known as the modulation technique. This encodes the carrier wave
with the required information from the input signal before the carrier wave is transmitted
to the receiver. The information contained in the input signal is able to travel over a much
larger distance when encoded into the carrier wave.
After the modulated carrier wave is received and amplified by the receiver, the transmitted
information is extracted from the carrier wave through a process known as demodulation.
The type of demodulation used to remove the carrier wave depends on the technique
used during modulation. It is important to note that the receiver must be able to identify
the received modulated carrier signal from other signals which may be using the same
channel.
For a two-signal input into a modulator (or non-linear mixer or amplifier), an input signal
of frequency f1 is mixed with a carrier signal (or other competing signal) of frequency
f2. These signals will be present at the output along with additional signals called
intermodulation products which are at the sum and difference and integer multiples of the
original frequencies. The frequencies of the intermodulation products can range in order
from second order (e.g. 2f1, 2f2, f1-f2 or f1+f2), third order (e.g. 3f1, 3f2, 2f1-f2, 2f1+f2, 2f2-f1,
2f2+f1), fourth order (e.g. 4f1, 4f2, 2f2+2f1,2f2-2f1,2f1+2f2, 2f1-2f2) up to the nth order and
usually arise from non-linearity in the signal processing equipment.
When many transmitters are placed at the same location, intermodulation may become
an issue. Typically, it is the third odd order intermodulation products which are of most
concern because they are the closest in frequency to the fundamental frequencies and
therefore cannot be effectively filtered out of the system (although this is not limited to
only third order products). This results in a spectrum of products that are spread out either
side of the fundamental frequencies which increases the bandwidth of the signal. In some
cases, the bandwidth of the signal is increased to a degree where it causes sideband
splatter in which the channels either side of the operating frequency are impinged by the
wider bandwidth of the signal. This causes interference with other systems operating in
these directly adjacent channels, otherwise known as co-channel interference.
3.3 Intermodulation
