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CRT Screen Persistence
Range-Marker Generator

Neets Module 18-Radar Principles
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3-12 on A-scopes and as concentric circles on ppi scopes. The distance between range marks is generally determined by the type of equipment and its mode of operation. In a weapons systems radar that requires extremely accurate target-range data, a movable range marker may be used. The range marker is obtained from a range-marker generator and may be a movable range gate or range step. When a ppi scope is used, a range circle of adjustable diameter may be used to measure range accurately. In some cases, movement of the range marker is accomplished by adjusting a calibrated control from which range readings are obtained. The following discussion describes the operation of three types of range-marker generators: the RANGE-GATE GENERATOR, the RANGE-MARKER GENERATOR, and the RANGE-STEP GENERATOR. The range-gate generator, used in conjunction with a blocking oscillator, generates a movable range gate. The range-marker generator and the range-step generator, used in conjunction with an astable multivibrator, generate fixed range marks and a movable range step, respectively. Range-Gate Generator Figure 3-11 shows a simplified block diagram of a typical range-gate generator. The pulse-repetition frequency is controlled by a master oscillator, or multivibrator, in which the output is coupled to a trigger thyratron (both in the synchronizer). The output of the trigger thyratron is used to trigger the radar modulator and the scope sweep circuits, thus starting the transmitter pulse and the range sweep at the same instant, referred to as time T0. Figure 3-11.—Range-gate generator. The PHANTASTRON in the sweep circuits is a variable timing circuit that supplies a sweep sawtooth to the sweep amplifier. The width of the gate and sawtooth is dependent upon the range selected by the radar operator. The range-gate circuit receives its input pulse from the trigger thyratron and generates a delayed range-gate pulse. The delay of this pulse from time T0 is dependent on either the range of the target when






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