Chapter 4 Wireless Technology
151
demodulation
the process of sepa-
rating a data signal
from a carrier wave.
channel
the bandwidth of a
carrier wave.
The combined wave is transmitted across the atmosphere. When the combined
wave reaches the receiver and is accepted, the electromagnetic energy is converted
into electrical energy. The receiver separates the voice wave from the carrier wave.
This process is known as demodulation. After the combined wave is demodulated,
the transceiver discards the carrier wave and then amplifi es the voice wave and
sends it to a speaker. The speaker converts the electrical energy into a voice wave.
While this is a simple, nontechnical explanation, it is important to remember that
the carrier wave and the voice wave are combined, or modulated, before they are
transmitted and are separated, or demodulated, after they are received.
The same principle is used to transmit digital data signals. A carrier wave
establishes the transmitter/receiver relationship. The carrier wave is modulated
with a wave pattern resembling the digital data signal. The two waves are
combined before transmission and then separated at the receiver.
To modulate data, the carrier wave must be at a much higher frequency than
the digital data. In the example of the voice wave and carrier wave frequency,
the carrier wave is 104.5 MHz while the voice wave fl uctuates between 400 Hz
and 4,000 Hz. Based on an average voice frequency of 2,000 Hz, an approximate
500:1 ratio exists between the two frequencies. The carrier wave is only slightly
distorted when combined with the voice wave. The same principle applies when
a carrier wave is combined with a digital data signal. If the two signals do not
have a high ratio, the digital data signal distorts the carrier wave to a point where
the transmitter cannot recognize it.
A 104.5-MHz carrier wave is 200 kHz in width. Technically, a 104.5-MHz
carrier wave has a bandwidth of 200 kHz. The bandwidth of a carrier wave
is referred to as a channel. Technically, a channel is a small portion of the
electromagnetic spectrum and is used to designate a set of frequencies for a
particular electronic application. The FCC assigns channels and bandwidths for
electromagnetic waves.
A channel is identifi ed by the assigned frequency that represents the starting
point of the band. For example, 104.5 is the identifi cation of the channel even
though the channel spans the next 200 kHz. Look at Figure 4-4. The 145 MHz is
the designated channel for the carrier wave. This channel is a single position in
the entire radio frequency spectrum represented by the 145-MHz designation.
The carrier wave channel is 200 kHz wide starting at exactly 145 MHz and
ending at 145.2 MHz. The carrier wave must stay within the 200-kHz band as
specifi ed by FCC regulation.
The exact center of the bandwidth area is 145.1 MHz. It is nearly impossible
for the carrier wave to remain exactly at 145.1 MHz. The electronic components
that are used to create the circuit that produce the exact frequency change
directly with temperature. The components heat up because of the electrical
energy passing through them and are also infl uenced by environmental
temperatures. Electronic transmitters are enclosed and air-conditioned to keep
the components at a predetermined temperature. The enclosure compensates
for environmental changes in temperature and the heat effects of the electronic
components. A change in component temperature causes a direct change in the
carrier wave frequency produced by electronic components.
Infrared Transmission
Infrared uses a series of digital light pulses. The light is either on or off. A typical
television remote control uses infrared technology rather than radio wave technology
to transmit to the television receiver. There are two distinct disadvantages to infrared