Communication systems consist of an input device, transmitter, transmission
medium, receiver and output device; the input device may be a computer, sensor or
oscillator depending on the application of the system, while the output device could be a speaker
or computer (Xiaodong and Poor, 2003). The source section produces two types of
signals, namely, the information signal, which may be speech, video or data, and a signal of
constant frequency and constant amplitude called the carrier (Derickson and Muller, 2007).
The information signal mixes with the carrier to produce a complex signal which is
transmitted; this combination is effected by a modulator. The destination section must be able
to reproduce the original information, which is carried out by separating the
information from the carrier signal. The transmission medium may be a copper cable, such as
a coaxial cable, a fiber-optic cable or a waveguide (Goldsmith, 2005).
Both the transmitter and receiver blocks incorporate many amplifier and
processing stages; one of the most important is the oscillator stage, which is generally referred to
as the master oscillator as it determines the channel at which the transmitter functions
(Black et al., 2008). This generates a constant-amplitude and frequency signal which is used
to carry the audio or intelligence signal. The receiver oscillator is called the local
oscillator as it produces a local carrier within the receiver which allows the incoming carrier
from the transmitter to be modified for easier
processing within the receiver (Glisic, 2004). The receiver
amplifies the incoming signal, extracts the intelligence and passes it on to
an output transducer such as a speaker. The local oscillator, in this case, causes the
incoming RF signals to be translated to a fixed lower frequency, called the Intermediate
Frequency (IF), which is then passed on (Horowitz and Hill, 1980). |
