Page 286 - Satellite Communications, Fourth Edition

266 Chapter Nine

intermediate frequency bands, with the bandwidth of the IF stage
determining the total noise power at the input to the demodulator. The
IF bandwidth has to be wide enough to accommodate the FM signal, as
described in Sec. 9.6.2, but should be no wider.

9.6.1 Limiters
The total thermal noise referred to the receiver input modulates the
incoming carrier in amplitude and in phase. The rf limiter circuit
(often referred to as an instantaneous or “hard” limiter) following the
IF amplifier removes the amplitude modulation, leaving only the
phase-modulation component of the noise. The limiter is an amplifier
designed to operate as a class A amplifier for small signals. With large
signals, positive excursions are limited by the saturation characteris-
tics of the transistor (which is operated at a low collector voltage), and
negative excursions generate a self-bias which drives the transistor
into cutoff. Although the signal is severely distorted by this action, a
tuned circuit in the output selects the FM carrier and its sidebands
from the distorted signal spectrum, and thus the constant amplitude
characteristic of the FM signal is restored. This is the amplitude-lim-
iting improvement referred to previously. Only the noise phase mod-
ulation contributes to the noise at the output of the demodulator.
Amplitude limiting is also effective in reducing the interference pro-
duced by impulse-type noise, such as that generated by certain types of
electrical machinery. Noise of this nature may be picked up by the
antenna and superimposed as large amplitude excursions on the carrier,
which the limiter removes. Limiting also can greatly alleviate the inter-
ference caused by other, weaker signals which occur within the IF band-
width. When the limiter is either saturated or cut off by the larger
signal, the weaker signal has no effect. This is known as limiter capture
(see Young, 1990).

9.6.2 Bandwidth
When considering bandwidth, it should be kept in mind that the word
is used in a number of contexts. Signal bandwidth is a measure of the
frequency spectrum occupied by the signal. Filter bandwidth is the fre-
quency range passed by circuit filters. Channel bandwidth refers to the
overall bandwidth of the transmission channel, which in general will
include a number of filters at different stages. In a well-designed system,
the channel bandwidth will match the signal bandwidth.
Bandwidth requirements will be different at different points in the
system. For example, at the receiver inputs for C-band and Ku-band
satellite systems, the bandwidth typically is 500 MHz, accommodating