Lesson 1/Learning Event 4
distortion
that
occurs
when
the
carrier
fades
during
selective
fading
periods.
(b) Increase in power output.
Although an increase in radiated
output power does improve the S/N, the improvement is not spectacular.
Moreover, when the fading conditions create an absence of signal at the
receiving station, no increase in power can improve the S/N.
(c) Reduction of bandwidth.
By
reducing the signal bandwidth at
the transmitter, the receiver bandwidth
can be narrowed a like amount,
improving the S/N by the numerical value
of bandwidth decrease.
In other
words, if the signal bandwidth is reduced
to one-half its former value, the
S/N is improved by a factor of two.
(d) Change of transmitter frequency.
The change of transmitter
frequency in line with radio propagation predictions is a very effective
method of improving the S/N. The frequency-changing process is a completely
coordinated activity under control of the two communicating technical
controllers, who are alerted to the need for frequency change by the radio
receiving operators.
(3) Radio receiver.
(a) Circuit.
A triple-conversion superheterodyne circuit is
normally used in HF radio receivers to achieve the necessary selectivity and
sensitivity.
(b) AGC. Automatic gain control (AGC) helps the radio receiver to
maintain a relatively constant output in the presence of fading.
In SSB
reception, the reduced pilot carrier normally furnishes the input signal for
the AGC action.
The AGC voltage is proportional to the strength of the
carrier.
(c) AFC.
Automatic frequency control (AFC) helps to keep the
receiver tuned precisely to the received pilot carrier in SSB reception,
thus minimizing the effects of transmitter frequency drift as well as
stabilizing the value of AGC voltage developed from the pilot carrier. When
radio receivers and transmitters use compatible frequency synthesizers, the
importance of AFC is minimal.
(4) Terminal equipment.
(a) Narrow-band filters.
The bandpass of each filter in the
terminal equipment is designed to pass only the signal components needed to
convey the information desired and to ignore other frequencies on either
side of the bandpass.
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