Basic synchronizer circuits should meet the following three 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.
The picture below shows the block diagrams and waveforms of these three basic synchronizer circuits 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.
Timers used in externally synchronized radar systems.
Sine-Wave Oscillator Synchronizer
In the sine-wave oscillator is another one of the basic synchronizer circuits (view A above), a sine-wave oscillator is used for the basic timing device (master oscillator). The oscillator output is applied to both an overdriven amplifier and the radar indicator. The sine waves applied to the overdriven amplifier are shaped into square waves. These square waves are then converted into positive and negative timing trigger pulses by means of a short-time-constant RC differentiator.
By means of a limiter, either the positive or negative trigger pulses from the RC differentiator are removed. This leaves trigger pulses of only one polarity. For example, the limiter in view A of the figure above is a negative-lobe limiter; that is, the limiter removes the negative trigger pulses and passes only positive trigger pulses to the radar transmitter.
A disadvantage of a sine-wave
oscillator synchronizer is the large number of pulse-shaping circuits
required to produce the necessary timing trigger pulses.
Master Trigger (Astable) Multivibrator Synchronizer
In a master trigger (astable) multivibrator synchronizer (view B of the figure above), the master oscillator generally is an astable multivibrator. The multivibrator is either ASYMMETRICAL or SYMMETRICAL. If the multivibrator is asymmetrical, it generates rectangular waves. If the multivibrator is symmetrical, it generates square waves. In either case, the timing trigger pulses are equally spaced after a limiter removes undesired positive or negative lobes.
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 the figure above, 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.
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.