Radar System Engineering

SEC.12.7] A UTOMA TIC FREQUENCY CONTROL 453
signals of 4 or 5 volts peak reach the limit level. It is wise to have a
limiter very early in the video amplifier, for then the following stages are
protected from excessively large signals.
Self-bias is frequently used in a video amplifier, but, since a condenser
of sufficient capacity to prevent degeneration at the lower frequencies
would be prohibitive y large, such a condenser is usually omitted entirely
in order to assure uniform frequency response.
When the indicator is some distance from the receiver, the two are
usually connected by a line terminated in its characteristic impedance.
Unterminated lines have been used, but the high-frequency response is
greatly impaired. Although lines of 1000-ohm characteristic impedance
have been made and are fairly satisfactory for lengths of 20 ft or less,
longer lines are usually standard 75- or 100-ohm coaxial cable. To drive
such a line from the plate of an amplifier requires a very large coupling
condenser; hence the cathode-follower circuit shown as the second stage
in Fig. 12-10 is ordinarily used (see Sec. 13.6). Its chief advantage is
that it can be direct-coupled. Although the gain is considerably less
than 1, this is not appreciably less than would be obtained with a platecoupled
amplifier working into such a low impedance.
Following the line and bridged across it are the video amplifiers
associated with each indicator. Since these amplifiers can be located
very near to the control electrode of the CRT, stray capacities can be
kept very small. These amplifiers can thus be either grounded-grid or
grounded-cathode, depending on the polarity of signal desired.
12.7. Automatic Frequency Control. ~The over-all bandwidth of a
radar receiver may be determined by the bandwidth of either the i-f or
the video circuits. However, for the purposes of this chapter we need
only be concerned with the i-f amplifier, since we are interested here in
questions of stability rather than in the quality of signals. As dictated
by the principles outlined in Sec. 2.22, bandwidths of 1 to 4 Me/see
are common. The narrowest r-f component, a high-Q TR switch, is so
much wider than this that, once adjusted, no drifts large enough to cause
serious detuning are likely to occur over a period of a day or so. The
problem of keeping a radar in tune, then, consists essentially in maintaining
the difference between the magnetron frequency and the localoscillator
frequency constant and equal to the intermediate frequency,
with an accuracy of 1 Me/see or better. There are two reasons for making
the tuning automatic. The first is that as the antenna scans, va@-
tions in standing-wave ratio arising from asymmetrical rotary joints or
reflections from near-b y objects can pull the magnetron several megacycles
per second. Manual tuning is so slow relative to scanning that it
brings in only part of the picture at a time. Second, whether or not
1By A. E. Whitford.