Radar System Engineering

454 THE RECEI VIA-G SYSTEM-RADAIi RECEIVERS [SEC. 127
pulling exists, the magnetron and local oscillator will not hold constant
frequency to one part in several thousand for several hours, nor will they
vary together in some predictable manner. Variations in voltage, temperature,
and pressure all produce frequency changes. Frequent tune-up
by the operator, using echoes, is not dependable since serious deterioration
in the sensitivity of a radar, such as that caused by drifting out of
tune, is not immediately obvious on the scope. Also, in a ship on the
open ocean or in an airplane over the ocean, there may often be no
‘Wtargets
For these reasons, an automaticfrequency-control
circuit, AFC, has
come to be a standard part of all radar
sets. It is an electronic servo-mechanism
that tunes the local oscillator in
FIG. 12.11 .—Height of discriminator
such a way that the proper difference
output pulses, where fo is the transmitter
frequency and f; is the intermediate frequency between it and the magfrequency.
netron is maintained. The speed of
tuning can usually be great enough to follow any pulling of the magnetron
that may occur during the scanning cycle.
In the case of beacon reception, automatic frequency control maintains
the local oscillator at a predetermined frequency that is higher or
lower than the frequency of the distant beacon transmitter by just the
intermediate frequency.
Radar AFC,—A1l forms of radar AFC work on the same basic principle.
Part of the local-oscillator power is mixed, in a crystal, with a
small fraction of the magnetron power drawn out during transmission
of the pulse. The difference frequency is applied to a frequencY-discriminator
circuit whose crossover is set at the intermediate frequency.
rhe variation of the height of discriminator output pulses as a function
af local-oscillator frequency is shown in Fig. 12.11.
These pulses are integrated or otherwise converted to a voltage, which
is applied to a control electrode of the local oscillator in the proper sense
to push the frequency toward the crossover of the discriminator curve.
This makes a degenerative feedback loop, and the frequency of the local
oscillator is held very near the crossover point on the curve.
As shown in Fig. 11.22 (Sec. 11.7), the voltage on the reflector of a
reflex klystron of the type used as a local oscillator tunes the tube over a
range of about 30 to 70 Me/see. Some local oscillators are also thermally
tuned by means of a control electrode which is the grid of an auxiliary
triode. This provides slow tuning over a much wider frequency range
(see Vol. 7 of this series)..
A block diagram of a typical AFC system is shown in Fig. 12.12.
The dotted portions are the additions necessary to include beacon AFC