2-41
one gas tube is required to discharge the pulse-forming network, and a low amplitude trigger pulse is
sufficient to initiate discharge. A damping diode is used to prevent breakdown of the thyratron by
reverse-voltage transients. The thyratron requires a sharp leading edge for a trigger pulse and depends on
a sudden drop in anode voltage (controlled by the pulse-forming network) to terminate the pulse and cut
off the tube.
As shown in figure 2-39, the typical thyratron modulator is very similar to the spark-gap modulator.
It consists of a power source (Eb), a circuit for storing energy (L2, C2, C3, C4, and C5), a circuit for
discharging the storage circuit (V2), and a pulse transformer (T1). In addition this circuit has a damping
diode (V1) to prevent reverse-polarity signals from being applied to the plate of V2 which could cause V2
to breakdown.
With no trigger pulse applied, the pfn charges through T1, the pfn, and the charging coil L1 to the
potential of Eb. When a trigger pulse is applied to the grid of V2, the tube ionizes causing the pulse-
forming network to discharge through V2 and the primary of T1. As the voltage across the pfn falls below
the ionization point of V2, the tube shuts off. Because of the inductive properties of the pfn, the positive
discharge voltage has a tendency to swing negative. This negative overshoot is prevented from damaging
the thyratron and affecting the output of the circuit by V1, R1, R2, and C1. This is a damping circuit and
provides a path for the overshoot transient through V1. It is dissipated by R1 and R2 with C1 acting as a
high-frequency bypass to ground, preserving the sharp leading and trailing edges of the pulse. The
hydrogen thyratron modulator is the most common radar modulator.
Pulse modulation is also useful in communications systems. The intelligence-carrying capability and
power requirements for communications systems differ from those of radar. Therefore, other methods of
achieving pulse modulation that are more suitable for communications systems will now be studied.
Q-18.
What is the primary component for a spark-gap modulator?
Q-19.
What are the basic components of a thyratron modulator?
COMMUNICATIONS PULSE MODULATORS
To transmit intelligence using pulse modulation, you must provide a method to vary some
characteristic of the pulse train in accordance with the modulating signal. Figure 2-40 illustrates a simple
pulse train. The characteristics of these pulses that can be varied are amplitude, pulse width,
pulse-repetition time, and the pulse position as compared to a reference. In addition to these three
characteristics, pulses may be transmitted according to a code to represent the different levels of the
modulating signal. To ensure maximum fidelity (accuracy in reproducing a modulating wave), the
modulating signal has to be represented by enough pulses to restore the original wave shape. Logically,
the higher the sampling rate (the more often sampled) of the pulse modulator, the more accurately the
original modulating wave can be reproduced. Figure 2-41 illustrates the effectiveness of three
pulse-sampling rates. View (A) shows a sampling rate of more than two times the modulating frequency.
As you can see, this reproduces the modulating signal very accurately. However, the high sampling rate
requires a wide bandwidth and increases the average power required of the transmitter. If less than two
samples per cycle are made, you are not able to reproduce the original modulating signal, as shown in
view (B). View (C) shows a sampling rate that is two times the highest modulating frequency. This is the
minimum sampling rate that will give a sufficiently accurate reproduction of the modulating wave. The
standard sampling rate is 2.5 times the highest frequency that is to be transmitted. This ensures the ability
to accurately reproduce the modulating waveform. In military voice systems the bandwidth for voice
signals is limited to 300 to 3,000 hertz, requiring a sampling frequency of 8 kilohertz. Although the pulse
characteristic that is changed may vary for each type of pulse modulation, the sampling frequency will
remain constant. We will now briefly discuss common types of pulse modulation.
