Radar System Engineering

S~C. 128] PROTECTION AGAINST EXTRANEOUS RADIATIONS 457
curve of the cavity. Under actual conditions of loading, the width Aj
between half-power points is commonly about + of the resonant frequency
itself.
Any control circuit that will lock the beacon local oscillator at the
top of the resonance curve must in some manner take a derivative of the
curve. The derivative, shown dotted in Fig. 12.13, has the same shape
as the usual discriminator curve. In the scheme shown in the block
diagram, a 1000-cycle sine wave from an a-f oscillator is added to the
slowly v’arying voltage from the sawtooth search oscillator. This sweeps
the local-oscillator frequency over a range that may be a tenth of the
half-power bandwidth of the cavity. The crystal output then has an
amplitude modulation of magnitude proportional to the slope of the
cavity resonance curve at the frequency in question. The phase of this
amplitude modulation depends on whether the slope is positive or negative.
The output signal from the crystal goes to one grid of a coincidence
tube, another grid of which receives the same sine wave used for modulating
the local-oscillator frequency.
Only when the two coincide in phase =
will the coincidence tube conduct. ~
As the frequency of the local oscillator
drifts across the cavity resonance
curve, the coincidence tube gives no !&.
1
output signal on the rising side of the
curve, and passes 1000-cycle pulses
increasing in amplitude as the top is
crossed and the slope becomes
l\ /’
\ 1’
‘Sk Derivative curve
FIG. 12.13.—Beaconcrystal current.
negative.
The pulse integrator uses these signals to stop the sawtooth sweep
at a voltage that produces a local-oscillator frequency very near the top
of the cavity resonance curve.
12.8. Protection against Extraneous Radiations. Antijamming.—
Although the sensitivity of a radar repeiver is normally limited by the
noise produced in the receiver, extraneous radiation may be occasionally
picked up on the radar antenna. Such radiation may, for example, be
the result of the operation of other microwave equipment in the vicinity.
It is good design practice to protect against such interference, and, as
will be seen later, the same provisions are sometimes of value in the
absence of interference. These AJ (antijamming) provisions may
merely be precautions taken in the design of the receiver which do not
affect its normal operation, or they may be special AJ circuits that can be
switched in and out of use.
Interference may be any of several types such as CW, amplitudemodulated
CW, frequency-modulated CW, r-f pulses, or noise-modulated
C!W, The more nearly the interference is like the echoes being received