Radar System Engineering

704 RADAR RELAY @EC. 17.7
illustrates such a method. The stators of the scanner synchro are excited
by equal-amplitude signals of frequencies f, and f,, as are the two equal
primaries of a transformer whose secondary is in series with the synchro
rotor. The voltage across the combination is then
jl(A + B sin 0) + ~@ + B cos o),
where fl and fzrepresent the audi~frequency input voltages and A and B
are constants. Since A is greater than B, the amplitude terms are always
positive. This voltage is mixed with a constant-amplitude a-f signal of
frequency fj (which is used for AGC at the receiver), and the combination
is used to modulate a subcarrier of somewhat higher frequency than the
maximum required for video signals. The modulated carrier, the video
signals, and the trigger pulse are then mixed and passed on to the transmitter
proper. At the receiving station, the various component signals are
separated by appropriate filters, as indicated in Fig. 17.11, and the sine
and cosine signals passed through detectors. The resulting voltages contain
a d-c component of magnitude C (with respect to the detector bias
point) and a varying component of amplitude D, the sum being always
positive. The d-c component can be prevented from affecting the final
device by relating its bias level properly to that of the detector. The
scheme of adding a constant a-f component to the signal is intended to
preserve the phase sense of the components varying sinusoidally with the
scanner rotation; in this case it is considerably simpler than transmitting
a reference signal to be used in keying a phase-sensitive rectifier.
Pulse-timing Methods.—The simplest method of using pulse-timing
techniques to relay sine and cosine information can be understood by
referring to the timing diagram of Fig. 17.12. A basic pulse occurs once
each radar cycle. The delay of a second pulse is varied with respect to it
in accordance with the expression A + B sin O,where A must be greater
than B in order that the delay shall never become negative. A third
pulse is delayed with respect to the “sine” pulse by an amount proportional
to A + B cos 0. In the interpretation of the data it is only necessary
to provide for each function a circuit that will develop a voltage
proportional to the time lapse between the members of a particular pulse
pair.
The essentials of such an equipment are illustrated in Fig. 17.12.
Slowly varying sine and cosine potentials are furnished by a data transmitter
(which can be either a d-c excited sine-cosine potentiometer or a
two-phase synchro whose output signals are rectified in a phase-sensitive
manner). Each of these potentials controls the operation of a linear
delay circuit. The sine delay circuit is triggered by the basic pulse, and
the cosine circuit is triggered by the sine pulse produced by the sine delay
circuit. Each. gives a finite delay A when the controlling voltage is zero