Radar System Engineering

SEC. 122] A TYPICAL RECEIVING SYSTEM 439
before. The latter must be provided with a delayed trigger which may
come from a continuously variable timing circuit or may be one pulse of a
discrete set, often the range-marker pulses themselves. The continuous
type of delay is frequently associated with a continuous precision range
marker (Sec. 13. 12).
31any deviations from Fig. 12.1 are possible if the timer provides the
synchronizing pulse. In particular, the discrete range markers can then
be derived from a high-precision c-w oscillator, one of the marker pulses
being selected as the modulator trigger by a scaling-down process. A
continuous marker can also be provided from this oscillator by means of a
continuous phase-shifting device. Other advantages of the timer-controlled
synchronization entail the use of a ‘‘ pre-trigger” by means of
which the actions of certain circuits can. be initiated somewhat in advance
of the firing of the modulator. h~any other functions, some of which
will appear in later sections, are performed by the timer in complex
situations.
The functions of those indicator circuits not included in the timer
vary widely with the different display types. For illustrative purposes,
Fig. 12.1 has been arranged to illustrate a simple A-scope or, alternatively,
a simple B-scope. The former requires no equipment beyond that
already described. The connections to the cathode-ray tube are shown
as position A of the switch. The range-sweep voltage is applied to one
pair of deflecting plates and the signal and range-mark voltages to the
other. The square wa~-e controlling the cathode-ray-tube intensity is
applied to the cathode in proper polarity to brighten the tube during the
range sweep.
In the B-scope display, the range sweep is applied to a deflection plate
just as before. Signal modulation is applied to the control grid, and the
second set of deflecting plates receives a ~-oltage that produces the azimuthal
deflection. This may be furnished, in simple cases, by a linear
potentiometer geared to the scan axis. simplification may sometimes be
necessary before applying the azimuth sweep voltage to the cathode-ray
tube. The circuits shown are equally applicable to a magnetic tube, the
only appreciable changes being in the deflection amplifiers.
The timer of Fig. 121 can be used in the production of many other
types of indication. For example, Fig. 122 shows the additional pai-ts
necessary to generate one type of PPI, and Fig. 12.3 those necessary for a
(technically) different PPI or, alternatively, for one form of RHI.
The PPI of Fig. 12.2 is of the so-called “rotating coil” type. A single
deflection coil driven by the range-sweep amplifier produces a radial
range sweep. This is made to take the direction on the tube face that
corresponds to the instantaneous antenna orientation by some form of
electromechanical repeater. Except for this modification in the de flee-