Page 58 - Complete Wireless Design

Modulation

Modulation 57

As an important aside to AM power measurement, increasing the trans-
mitter’s range by increasing its power is not linear. In other words, to
extend a transmitter’s distance by 2, multiply the transmitter’s power, in
2
watts, by 2 (or 4 power). To increase the distance by 3, multiply the power
2
by 3 (or 9 power).

2.1.4 Disadvantages
The disadvantages of AM are many: The bandwidth of an AM signal is twice
that actually required for the reception of the intelligence being sent, since
only one sideband is absolutely necessary to convey the baseband information;
a significant amount of power is in the carrier, which is not even required to
furnish the intelligence; the phase relationship between the carrier and the
sidebands must be precise, or severe fading will result within the demodulat-
ed signal, and this is quite difficult to maintain under most atmospheric and
multipath conditions.

2.2 Frequency Modulation
2.2.1 Introduction
Frequency modulation (FM) was originally invented as an answer to the many
deficiencies inherent in AM, primarily that of excessive noise sensitivity. Since
noise is normally produced by undesired amplitude variations in a signal, this
is removed in frequency-modulated receivers by amplitude limiters.
Two techniques can be employed to generate an FM signal. The first, direct
FM, directly alters the frequency of the carrier in step with the baseband’s
amplitude variations; the second method, indirect FM, changes the phase of
the carrier, which creates phase modulation. However, both of these tech-
niques produce the end effect of frequency modulation of the RF carrier. Both
methods are classified under the designation of angle modulation.

2.2.2 Fundamentals
Modulation is the method we use to insert baseband information on an RF car-
rier wave. The baseband information can be voice, digital data, analog video,
etc. Demodulation is the procedure of extracting this baseband information,
which is then sent to a speaker to reproduce the original voice and music, or
on to digital circuits for processing or storage.
FM accomplishes this modulation process by altering the carrier’s frequency
in step with the baseband signal’s changes in amplitude. When this frequency-
modulated RF carrier arrives at the receiver, the frequency variations created
by the original baseband modulations are changed back into amplitude varia-
tions. The baseband is then amplified and inserted into an appropriate trans-
ducer. As stated, in FM the baseband’s amplitude alters the frequency of the
RF carrier, and not the amplitude as it does with AM, while the amount of the

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