Radar System Engineering

700 RADAR RELAY [SEC. 176
train. The remainder of the circuit is concerned with coding and mixing
the various signals and providing synchronization with the modulator
(which has been assumed to be triggerable). The basic puke and the
pulse train are passed through the coder, as is a pulse formed at the end
of flip-flop b. This last pulse, delayed by a time equal to that of the coding,
serves as the modulator trigger. In order to separate the pulse
signals from the video signals at the receiver, if amplitude selection cannot
be used, the two are passed alternately through video switch b, which is
controlled by flip-flop c. In its normal position, the latter causes the
switch to pass signals from the coders. When the flip-flop is triggered by
the pulse to;the modulator, the switch is reversed and video signals are
passed until the flip-flop spontaneously returns to its initial condition,
shortly before the next basic pulse, and opens the channel to pulses again.
If the transmitter is such that the pulses can be transmitted at several
times the level of the video signals, this switching need not be done since
amplitude discrimination can then be used at the receiver.
At the receiving station, the various signals must be separated and
the data abstracted and put into usable form. The signals from the
receiver are received by a switching and decoding circuit similar to that
of Fig. 17,3 excspt for the source of the input signal to V5b, Whose function
will be explained later. For the moment assume that the switch is open.
When the basic pulse is decoded it starts a chain of events through flip-flop
a, the switched oscillator, the phase shifter, and the pulse former on the
one hand; and another chain through the delay circuit and flip-flop b’ on
the other. Both culminate in switch a’ and produce at its output terminal
G’ a train of pulses like that at point G at the transmitter. Meanwhile,
the transmitted pulse train is decoded and passed through switch
c, whose purpose is to exclude the basic pulse and the modulator pulse.
The two pulse trains are brought together in a “comparison” circuit
which produces a polarized error voltage if they do not coincide. (If the
two pulse trains have slightly different frequencies, because of slight
differences in the oscillators, the error voltage will refer to their “centers
of gravity.”) The amplified error voltage controls a motor which turns
the phase shifter in such a way that the error voltage is kept very small,’
and the phase shifter and the data transmitter rotate in synchronism
with the radar scanner. The modulator pulse is selected by switch d
which is ~tivated by fip-flop a. The latter is triggered at the moment
of recovery of flip-flop b’ and endures until after the arrival time of the
modulator pulse. In addition to its function at the indicators this pulse
also triggers the ilip-flop ( V~, and VW Fig. 17”2) controlling the switch
(V,. and Vlb, Fig. 17”2) ahead of the decoder. When this switch is
1 See Vol. 20 of this series for details of the comparison circuit and motor drive.