Radar System Engineering

456 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC.12.7
for the beginning of the pulse and to accept a signal only from the latter
part. This requires accurate timing and becomes very difficult for
short pulses.
A satisfactory operating level for the AFC crystal is 0.5 mw c-w
power from thelocal oscillator and 1 to2mwpulse powerfromthemagnetron.
Reduction of the magnetron power level by 65 to 90 db below
the transmitted power is obtained by a combination of weak coupling out
of the main r-f line and an attenuator, preferably dissipative. It is
important that the r-f system be tight in order to prevent leakage power,
possibly many times that desired, from reaching the crystal by stray
paths. This becomes easier if the operating level of the AFC crystal is
made as high as is permissible. A high operating level also requires less
i-f gain ahead of the discriminator and thereby reduces the effect of gain
variations.
Too high an operating level is, however, undesirable because as the
crystal is driven up to 10 mw or above, it saturates. The harmonics of
the difference frequency then become stronger relative to the fundamental.
For example, if the discriminator crossover is set at 30 Me/see,
corresponding to an i-f amplifier centered at that frequency, a second
harmonic at 30 Me/see large enough to actuate the control circuit and
cause locking may appear when the local oscillator is only 15 Me/see
away from the magnetron frequency, under conditions of AFC crystal
saturation.
The gain of the AFC feedback loop should be high enough to insure
tight locking but not so high that the second harmonic can also cause
locking. At the recommended level of 1 to 2 mw of magnetron power
the harmonics are at least 20 db below the fundamental. A gain control
is undesirable; therefore, care in controlling the r-f power levels and in
amplifier design is required for a foolproof AFC. The most frequent
sources of trouble are usually on the r-f side rather than in the electronic
circuits, and arise from high power leakage into the crystal, faulty
coupling to the main line, or a wrong amount of attenuation.
Beacon Al’C.-To hold the beacon local oscillator at a given absolute
frequency, some r-f reference standard must be provided, since the magnetron
involved is in the beacon, distant from the radar. At microwave
frequencies the reference standard is a resonant cavity. With proper
attention to details such as temperature compensation and moisture
sealing, production-line cavities can be depended upon to maintain a
specified frequency to 1 or 2 parts in 10,000.
The dotted portion of Fig. 12.12 shows the beacon local oscillator
attached both to the radar crystal and, through the reference cavity, to a
beacon crystal. The output of the beacon crystal as a function of frequency
is shown in Fig. 12.13. It is, of course, just the simple resonance