Radar System Engineering

SEC. 17.13] 10&MC/SEC F-M EQUIPMENT 721
an insertion loss of less than 2 db from 290 to 320 Me/see, and a loss of
more than 40 db above and below this band. The design is conventional:
three T-sections are matched to the line with a ir-section at each end.
Inductors are used as series elements, and combinations of lines as the
shunt elements.
Figure 17. 17b shows a block diagram of the receiver. The r-f amplifier
is a miniature triode (6J4) connected as a grounded-grid amplifier. A
dual triode (6J6) is used in a push-pull oscillator circuit tuned 30 Me/see
below the carrier frequency. The 30-Mc/sec i-f signal from the converter
is amplified by six stages, gain control being applied to the first three.
The i-f bandwidth is about 3 Me/see between half-power points. The
output of the detector, a 6AC7 connected as a diode, is applied to the first
video stage. A choice of two time constants is available in the grid
circuit of this stage, 0.47 sec and 2.4 ~sec. The longer one is normally
used; it gives good response to very low video frequencies. The short
time constant, when used, serves the same function against extended or
c-w interference as similar circuits do in a radar receiver (Sec. 12.8).
The automatic gain control is actuated by the synchronization pulses.
A small signal is taken from the plate of the final cathode follower,
amplified, and passed to another cathode follower. Because of inverse
feedback, the output signal of this cathode follower is a sharp spike,
rather than a flat-topped 2-gsec pulse. If the synchronization pulses are
coded, they pass through a delay line to a coincidence tube, the combination
acting as a decoder. The coincidence tube is so biased that only
pulses will actuate it, the video signals being biased out at this point by
the video-level control. The output signal of the coincidence tube is
applied to the cathode oi a diode, whose plate potential is set by the AGC
level control. Thus, if the signals from the coincidence tube are sufficiently
negative to cause the diode to conduct, the grid of the cathode
follower which is also connected to the plate of the diode will change
potential and thereby change the grid potential on the first three i-f
stages. A long time constant in the cathode-follower grid circuit holds
the grid potential essentially constant between pulses. There is thus a
loop in which strong pulses produce a more negative potential on the i-f
grids to reduce the receiver gain, and vice versa.
It may happen that operation of a radar in the vicinity will overload
the receiver during transmission. Such interference can be overcome
by introducing a portion of the interfering radar trigger at the interfeience-suppression
terminals shown. This reduces the receiver gain at the
instant of radar transmission, with the loss of only one or two microseconds
of video-signal reception.
17.13. A 100-Mc/sec Frequency-modulated Equipment.—A second
type of equipment which has been used for air-to-ground or air-t~ship