Lesson 1/Learning Event 7
the signal-to-distortion ratio (S/D). The frequency of the third signal is
chosen to reflect the interaction between the second harmonic of one
frequency with the fundamental of the other.
This test is especially
important for high-powered SSB radio transmitters.
Example:
Two frequencies, 1,575 Hz and 1,000 Hz, are fed into the
input of an SSB transmitter.
If distortion is present,
intermodulation will occur among all frequencies developed.
Among these frequencies is the second harmonic of 1,000 Hz,
which is 2,000 Hz. The difference between the 1,575-Hz and
the 2,000-Hz harmonic is now 425 Hz. If we filter out the
425-Hz signal and measure its level, we will have an
indication of how badly the input signals are being
distorted and intermixed.
Learning Event 7: LONG-DISTANCE COMMUNICATIONS NETWORKS.
Long-distance communications networks are combinations of long-lines
cable, line-of-sight radio and satellite communication systems. An example,
a message traveling from Washington, D.C., to Frankfurt, GER may travel any
one of a number of ways. During its travel the message must pass through a
number of points where the systems join.
These junctures are generally
called interfaces. Since each of the communication systems has a different
set of characteristics, each circuit must be conditioned prior to
interconnection.
Failure to properly condition the circuit results in
distorted and/or noisy signals. And once signals are distorted, nothing can
be done to remove the distortion products; the only remedy is to compensate
for their effects.
Technical controllers have the task of directing the
circuit conditioning procedures, and for checking the distortion of
communication circuits that are routed through their facilities.
Learning Event 8: CIRCUIT CONDITIONING.
When audio signals travel over a communication channel, they may be
affected by variation in amplitude and phase.
Circuit conditioning
therefore is an attempt to return the received signal to as nearly its
original condition as possible. Variation in the original frequency is not
likely, but the amplitude may vary at different frequencies.
Circuit
conditioning involves processes of equalization, amplification, delay
compensation, and frequency translation.
The techniques of circuit
conditioning always involve the measurement of signal levels.
This is
usually accomplished by measuring the level with
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