14190_62

2-4 cathode-ray tube trace occurs only during sweep time and is eliminated during the flyback (retrace) time. The negative portion of the range-marker gate pulse also occurs during the indicator sweep time. This negative gate pulse is applied to a range-marker generator, which produces a series of range marks. The range marks are equally spaced and are produced only for the duration of the range-marker gate pulse. When the range marks are combined (mixed) with the receiver output signal, the resulting video signal applied to the indicator may appear as shown at the bottom of figure 2-1. Q3. A self-synchronized radar system obtains timing trigger pulses from what source? Q4. What type of multivibrator can be used as a radar master oscillator? Q5. In an externally synchronized radar, what determines the prr of the transmitter? Q6. In figure 2-1, what causes the initial and final pulses on the receiver output signal? BASIC SYNCHRONIZER CIRCUITS The basic synchronizer circuit should meet the following three basic requirements: 1. It must be free running (astable). Because the synchronizer is the heart of the radar, it must establish the zero time reference and the prf (prr). 2. It should be stable in frequency. For accurate ranging, the prr and its reciprocal, pulse-repetition time (prt), must not change between pulses. 3. The frequency must be variable to enable the radar to operate at different ranges. Three basic synchronizer circuits can meet the above mentioned requirements. They are the SINE- WAVE OSCILLATOR, the SINGLE-SWING BLOCKING OSCILLATOR, and the MASTER- TRIGGER (ASTABLE) MULTIVIBRATOR. Figure 2-2 shows the block diagrams and waveforms of these three synchronizers as they are used in externally synchronized radar systems. In each case, equally spaced timing trigger pulses are produced. The prr of each series of timing trigger pulses is determined by the operating frequency of the associated master oscillator.