(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
(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
cathode follower circuits.