Page 63 - Complete Wireless Design

Modulation

62 Chapter Two

to be adequate to pass enough sideband power for ample intelligibility of voice
signals. However, as mentioned above, for FM broadcast use the maximum
deviation is set at 75 kHz, with the maximum audio frequency being 15 kHz
and the deviation ratio at 5, or 75 kHz/15 kHz. This allows for high-fidelity
music to pass without significant loss of quality.

2.2.3 FM and AM comparisons
Frequency modulation holds many benefits over amplitude modulation: superior
noise immunity, helped by amplitude limiting to eliminate AM noise; a decrease
in high-frequency noise constituents, due to preemphasis circuits, which boost the
higher frequencies at the transmitter, and deemphasis circuits, which attenuate
the now overemphasized frequencies at the receiver; FM’s capture effect, which
forces any undesired signal that is near, or at the same, frequency as the desired
signal to be rejected. And, since FM does not have a delicate modulation envelope,
as does AM, it does not require Class A linear amplifiers, but instead can utilize
the far more efficient Class C types in both its RF and IF sections. Also, trans-
mitter efficiency in FM is quite high, since the transmitter itself can be modulat-
ed by low-level techniques, needing little baseband modulation power.
Frequency modulation does have its drawbacks: Increased bandwidth is
necessary because of the additional sideband production over AM; the FM
transmitter and receiver are more expensive to design and construct because
of their higher frequencies of operation, along with higher stability require-
ments; bouncing the FM signal off the atmosphere’s ionosphere creates distor-
tion of the FM wave, so it’s normally (unless repeaters are used) a line-of-sight
communications medium.
A reprise of some of the more important FM terms:
Center frequency, sometimes referred to as the rest frequency, is the FM
transmitter’s carrier frequency with zero percent modulation.
Frequency deviation is the amount the RF carrier shifts from its center
frequency in a single direction when modulated.
Frequency swing is the movement of the modulated carrier on both sides of
the center frequency, or twice the frequency deviation.
Modulation index, which is employed when one tone, at a steady deviation,
is transmitted, is the ratio of the carrier’s instantaneous frequency deviation
to the instantaneous frequency of the modulation.
Deviation ratio is the ratio between the maximum frequency deviation—
with 100 percent modulation—to the maximum audio modulation frequency.

2.3 Single-Sideband Modulation

2.3.1 Introduction
As a result of the highly stable oscillators and phase-locked loops available
today, along with advances in mass production and RF integrated circuits,

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