Lesson 1/Learning Event 4
(b) Peak limiters.
Peak limiters reduce the effect of noise on
the signal by clipping off the noise peaks above the desired peak level of
signal.
However, peak limiters can be used only on signals that use
constant amplitude, such as frequency-modulated (FM), frequency-shift-keyed
(FSK), or pulse-code-modulated (PCM) signals. Peak limiters cannot be used
on voice or amplitude-modulated (AM) signals because peak clipping distorts
the quality of AM signals.
c. Meaning of S/N. The ratio of the amplitude of the desired signal
level to the amplitude of noise level at a given point in time. A favorable
ratio is one in which the desired signal level always exceeds the noise by a
comfortable margin.
(1) The human ear is a poor judge of the S/N because the ear is not
frequency sensitive; it hears all frequencies within its range. If you were
to listen to the output of a wide-band radio receiver with a pair of
earphones, you would hear both signal and noise together. Your ear cannot
separate signal from noise.
(2) To isolate the signal from background noise, frequency-selecting
devices must be used, the simplest being a filter.
In this way a
communication channel using a narrow-band input filter hears only that noise
which passes through the filter along with the signal. The S/N in this case
is the ratio of the desired signal level passing through the filter compared
with the noise falling within the bandpass filter.
(3) There are times when it would appear to the receiving operator
that the sound of the received signal is so badly smothered in noise that
communication seems almost hopeless, yet the telegraph terminal may be
producing perfect copy on the teletypewriters.
A terminal can perform in
this manner because of the succession of narrow-band filters and limiters in
each channel circuit.
The presence of these narrow-band filters explains
the superiority of VF telegraph channels for reliable communication as
compared with telephone communication under noisy operating conditions.
(4) If the received signal is strong, the noise must likewise be
strong to affect it. If the signal is weak, a weak noise can affect it. In
other words, S/N expresses the relationship between the signal level and the
noise level, and has nothing to do with the individual values of signal
level or noise level.
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