Radar System Engineering

SEC. 16.21] SPECIAL TEST EQUIPMENT 677
extra half-length line generates triggers at double the PRF. These are
divided by a counting circuit before delivery to the radar transmitter.
16.21. Special Test Equipment. Operating Tests.—The minimum
additional equipment required to check the components peculiar to an
MTI system can be incorporated in a single A-scope chassis. This
chassis should include video amplifiers, expanded and delayed sweeps,
and a vacuum-tube voltmeter, together with switches and permanent
connections to other parts of the system. In addition, the receiver
chassis s’hould include the locking-test channel shown in Fig. 16.30, and
the two-channel amplifier may contain a delay-line attenuator.
By means of a rectifying crystal permanently connected to the r-f
transmission line, together with an expanded and suitably phased sweep,
the transmitted pulse envelope can be inspected for hum modulation
and mode jumping. This test can be used whenever the more general
coherent oscillator locking test shows trouble.
The locking-pulse mixer current should be monitored. This can be
done by switching the meter normally used for the signal-mixer current.
As can be seen from Fig. 1630, the locking-test channel delays a
sample of the locking pulse by means of a short auxiliary supersonic delay
line, and then mixes this sample with the received signal. If reflecting
end cells are used, multiple reflections within the delay line will give rise
to a number of equally spaced locking pulse echoes. These echoes will
beat with the reference signal. The presence of cycles within the echoes,
when viewed on an expanded A-scope, will show when the local oscillator
is out of tune. Since the coherent oscillator is unlikely to drift, the
tuning of the local oscillator to give maximum receiver response needs to
be checked only rarely.
The principal function of the locking-test channel, as its name implies,
is to reveal any unsteadiness in the original locking pulse or any failure of
the coherent oscillator to lock properly.
An attenuator and appropriate switches can be built into the input
circuits of the two-channel amplifier to permit measurement of delay-line
attenuation.
Checking and adjustment of cancellation can be done while the radar
is operating by mixing a delayed video pulse with the signals before
cancellation. The delay can be chosen to bring the test pulse to a range
position near the outer edge of the PPI, and adjustments can then be
made for the best cancellation of the video pulse.
Because of the several gain controls in series in the receiving train,
a d-c vacuum-tube voltmeter is necessary to check the carrier level at
the cancellation detectors.
Testing MZ’I Oscillator Stability.—It has been shown in Sec. 16.4 that
free-running frequency stability of a high order is required of the local