Master oscillator (V1)

(a) Master oscillator (V1). The circuit used to control the

pulse repetition frequency is usually a crystalcontrolled

oscillator. The sinewave output of the oscillator should have

(b) Cathode follower (V2). To prevent the limiter stage from

loading down the master oscillator stage, a cathode follower is used

to isolate the two stages from each other. The cathode follower will

appear as a constant load to the master oscillator, and as a result,

the limiter will not load down the oscillator.

(c) Limiter (V3). The first step in producing trigger pulses

from the sinewave voltages is to convert the sine waves into square

waves. To make this conversion, the amplitudes of both the positive

and negative alternations of the sine waves must be limited to

specific values.

(d) Overdriven amplifier (V4). Although the sinewave

voltages have been squared by the limiter, the sides of the square

waves are not as vertical as is desired for the production of sharply

peaked pulses. Therefore, the square waves produced by the limiter

stage are applied to an overdriven amplifier to steepen the sides of

the square waves.

(e) Peakers (D1 and D2). To produce sharp pulses from the

square waves, the time constants of the coupling circuits are made

very short. By making the time constants short, the highfrequency

components of the square waves will be coupled to the following

stages and the low frequencies will be rejected. The output

waveforms will be sharp positive and negative pulses. The sharp

pulses are called triggers.

(f) Clipper (V5). To ensure that the negative pulses do not

interfere with the blocking oscillator circuit operation, they are

removed by a clipper circuit.

(g) Blocking oscillator (V6). To obtain the desired

frequencies for the transmitter and indicators, the master oscillator

frequency must be reduced or divided. The blocking oscillator

divides the incoming synchronizing triggers by a predetermined

amount. Figure 13 shows that the blocking oscillator requires four

input triggers to produce one output pulse. In this example, the

blocking oscillator divides the incoming frequency by four. A

blocking oscillator can be designed to divide by a different amount.

A timer may use one or more blocking oscillators as frequency

dividers to obtain the desired timing signals.

(h) Cathode followers (V7, V11, V13, and V14). To prevent

reflections in the connecting cables and ensure maximum power

transfer, the low impedances of the cables are matched to the high

impedances of the multivibrators and the blocking oscillator by

cathode follower circuits.