Radar System Engineering

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360 THE MAGNETRON AND THE PULSER [SEC. 10.7
Now that a practical and highly efficient discharging circuit has been
shown to be feasible, there remaimthe problem ofreplenkhing the energy
of the pulse-forming network. This could bedoneby using a resistance
as isolating element. The very low efficiency of this schemel makesit
unsuitable for all but a very few special applications, such as systems
requiring variable interpulse intervals. Accordingly, inductance charging
is almost always used. It will be shown later that, if a d-c supply
voltage is available, the network voltage at the time of discharge is double
the supply voltage, except for losses in the inductance; it will also be
shown that the network can be recharged from a source of a-c voltage,
provided the repetition frequency is a multiple of one-half the supply
frequency.
Comparison oj the TWO Types of Pulsers.—The basic circuit of Fig.
10.26 applies, as has been shown, to two types of pulse generators. In
one type a small amount of the energy stored in a condenser is allowed to
be dissipated in the load during each pulse. The switch, which must be
able to interrupt the pulse current, is always a vacuum tube, and pulsers
of this type are commonly called “hard-tube pulsers. ” Pukers of the
other type, where exactly the correct amount of energy is stored before
the switch is closed, and the pulse is shaped by the discharge circuit itself,
are referred to as ‘‘ line-type pulsers, ” since the pulse-shaping elements or
pulse-forming networks have been derived from the electrical characteristics
of transmission lines.
The two types of pulsers have different characteristics, and it is of
interest to analyze briefly some of the considerations involved in the
design.
For instance, it is easier to change the pulse duration in a hard-tube
than in a line-type pulser, since it is sufficient to change the time during
which the switch is conducting. This can be done easily at low voltage
in the driver stage, instead of using a high-voltage switch to change pulseforming
networks in a line-type pulser.
Methods for turning on the switch in a hard-tube pulser, discussed
more fully later, generally involve a small regenerative pulser which
applies a positive pulse ‘~drive” to the control grid of the vacuum-tube
switch. This small pulser nearly always requires auxiliary voltage supplies
and, in addition, the switch-tube control grid must be maintained
beyond cutoff during the interpulse interval. In line-type pulsers, the
‘‘ triggering” of the switch (or initiation of the discharge) is usually
accomplished with much less power than is necessary to drive the grid
of the hard tube; in most cases, the driver power output is only a few per
I Simple considerations show that, when chargi~g a condenser from zero to the
power supply voltage through a resistance, as much energy is dissipated in the resistance
as is stored in the condenser.
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