Selection
Selection is the ability to distinguish a particular
station’s frequency from all other station frequencies
appearing at the antenna.
Detection
Detection is the extraction of the modulation from
an rf signal. Circuits that perform this function are
called detectors. Different forms of modulation require
different detector circuits.
Reproduction
Reproduction is the action of converting the elec-
trical signals to sound waves that can be interpreted by
the ear.
RECEIVER CHARACTERISTICS
Understanding receiver characteristics is manda-
tory in determining operational condition and for com-
paring receivers. Important receiver characteristics are
sensitivity, noise, selectivity, and fidelity.
Sensitivity
Sensitivity is a measure of receiver’s ability to re-
produce very weak signals. The weaker the signal that
can be applied and still produce a certain signal-to-
noise (S/N) ratio, the better that receiver’s sensitivity
rating. Usually, sensitivity is specified as the signal
strength in microvolts necessary to cause a S/N ratio of
10 decibels, or 3.16:1.
Noise
All receivers generate noise. Noise is the limiting
factor on the minimum usable signal that the receiver
can process and still produce a usable output. Ex-
pressed in decibels, it is an indication of the degree to
which a circuit deviates from the ideal; a noise figure of
0 decibels is ideal.
Selectivity
Selectivity is the ability of a receiver to distinguish
between a signal at the desired frequency and signals at
adjacent frequencies. The better the receiver’s ability
to exclude unwanted signals, the better its selectivity.
The degree of selectivity is determined by the sharp-
ness of resonance to which the frequency determining
components (bandpass filters) have been engineered
Figure 1-7.—AM superheterodyne receiver and waveforms.
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