Lesson 1/Learning Event 6
(2) FM signals. When two FM signals interfere, the stronger of the
two signals predominates, to the detriment of the weaker. When AFC is used,
the receiver will retune to pick up the stronger of the two signals.
Further, the limiters within the receiver will respond most readily to the
stronger of two input signals.
Heterodyning seldom occurs between FM
signals.
Learning Event 6: DISTORTION.
The amount of distortion appearing within the received signal is an
indication of system linearity. A system could have a low S/N and, at the
same time, be useless for communication because of high distortion levels.
Moreover, when distortion exceeds minimum allowable levels, it becomes
necessary to minimize it through a cooperative effort by system personnel
under supervision of the technical controller.
Further, the type of
distortion and its effects are related to the type of signal being
transmitted.
a. Hearing distortion.
The human ear is a poor indicator of
distortion, since the ear hears everything within the signal.
Electrical
instruments are therefore necessary to measure the distortion.
The
distortion is first measured at the transmitting end, and then the receiving
end.
The increase in measured distortion is the amount created by the
transmission path and the receiving equipment.
b. Harmonic distortion.
The presence of frequencies at the output
(receive) of a channel, caused by non-linearities within the channel, which
are related to a single applied frequency at the input (transmit) of a
channel.
When a single frequency is transmitted through a channel, if no
distortion of the signal occurs, the signal at the output of the channel
will contain only the original frequency transmitted.
But, if non-linear
distortion occurs in the channel, new frequencies are produced which are
harmonically related to the fundamental single frequency applied.
c. Intermodulation products. Intermodulation products (crosstalk) are
frequencies that are created by interaction of the distortion products in
the presence of a nonlinear circuit element. The test for intermodulation
products must therefore be performed by the use of two sine-wave signals
differing in frequency but having identical amplitude. If a third frequency
is created by this interaction, its level is a measure of the circuit
nonlinearity. The ratio of the third frequency signal level to either one
of the two equal level test tones is called
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