TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK
In practice, the frequency translator (multiplexer) uses single sideband modulation of
radio-frequency (RF) carriers. A different RF carrier is used for each channel to be mul-
tiplexed. This technique is based on mixing or heterodyning the signal to be multiplexed,
typically a voice channel, with an RF carrier.
An RF carrier is an unmodulated RF signal of some specified frequency. In theory,
because it is not modulated, it has an indefinitely small bandwidth. In practice, of course,
it does have some measurable bandwidth, although very narrow. Such a carrier derives
from a simple frequency source such as an oscillator or a more complex source such as
a synthesizer, which can generate a stable output in a range of frequencies.
A simplified block diagram of an FDM link is shown in Figure 4.18.
. The heterodyning or mixing of signals of frequencies
A and B is
shown as follows. What frequencies may be found at the output of the mixer? Both the
original signals will be present, as well as the signals representing their sum and their
difference in the frequency domain. Thus at the output of the illustrated mixer we will
have present the signals of frequency
A, B, A + B, and A - B. Such a mixing process
is repeated many times in FDM equipment.
Let us now look at the boundaries of the nominal 4-kHz voice channel. These are
300 Hz and 3400 Hz. Let us further consider these frequencies as simple tones of 300 Hz
and 3400 Hz. Now consider the following mixer and examine the possibilities at its output:
First, the output may be the sum or
+ 300 Hz + 3,400 Hz
A simple low-pass filter could filter out all frequencies below 20,300 Hz.
Simplified block diagram of an FDM link.