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2-6 There are two transistors in an astable multivibrator. The two output voltages are equal in amplitude, but are 180 degrees out of phase. The output of the astable multivibrator consists of positive and negative rectangular waves. Positive rectangular waves are applied to an RC differentiator and converted into positive and negative trigger pulses. As in the sine-wave synchronizer, the negative trigger pulses are removed by means of a negative-lobe limiter, and the positive pulses are applied to the transmitter. Both positive and negative rectangular waves from the astable multivibrator are applied to the indicator. One set of waves is used to intensify the cathode-ray tube electron beam for the duration of the sweep. The other set of waves is used to gate (turn on) the range marker generator. Single-Swing Blocking Oscillator Synchronizer In the single-swing, blocking-oscillator synchronizer, shown in view C of figure 2-2, a free-running, single-swing blocking oscillator is generally used as the master oscillator. The advantage of the single- swing blocking oscillator is that it generates sharp trigger pulses without additional shaping circuitry. Timing trigger pulses of only one polarity are obtained by means of a limiter. Gating pulses for the indicator circuits are produced by applying the output of the blocking oscillator to a one-shot multivibrator or another variable time delay circuit (not shown). Crystal-controlled oscillators may be used when very stable frequency operation is required. Q7. What basic circuits meet the requirements of an externally synchronized master oscillator? Q8. Name a disadvantage of sine-wave oscillator synchronizers. Q9. Which of the basic timing circuits produces sharp trigger pulses directly? TRANSMITTERS The TRANSMITTER produces the short duration high-power rf pulses of energy that are radiated into space by the antenna. Two main types of transmitters are now in common use. The first is the KEYED-OSCILLATOR type. In this transmitter one stage or tube, usually a magnetron, produces the rf pulse. The oscillator tube is keyed by a high-power dc pulse of energy generated by a separate unit called the MODULATOR (discussed in the following section). The second type of transmitter consists of a POWER-AMPLIFIER CHAIN. This transmitter system begins with an rf pulse of very low power. This low-level pulse is then amplified by a series (chain) of power amplifiers to the high level of power desired in a transmitter pulse. In most power-amplifier transmitters, each of the power-amplifier stages is pulse modulated in a manner similar to the oscillator in the keyed-oscillator type. Because the modulator is common to both types of transmitter systems, the operation of a typical modulator will be discussed first. RADAR MODULATOR The modulator controls the radar pulse width by means of a rectangular dc pulse (modulator pulse) of the required duration and amplitude. The peak power of the transmitted rf pulse depends on the amplitude of the modulator pulse. Figure 2-3 shows the waveforms of the trigger pulse applied by the synchronizer to the modulator, the modulator pulse applied to the radar transmitter, and the transmitted rf pulse.