Radar System Engineering

356 THE MAGNETRON AND THE P ULSER [SEC. 107
It is common practice to begin the design of a pulser around a resistance
load equal to the static resistance of the actual load to be used at its
required operating point. Although final evaluation of the pulser performance
requires consideration of the whole system and particularly of
the oscillator used, much information about pulser behavior can be
obtained by considering a pure resistance load. The following discussion
of the types of pulsers in use at present and their characteristics is based
on the assumption of a resistance load. The general characteristics of
the magnetron load will be considered later, and their effect on pulser
behavior and design will be studied in greater detail for the two main
types of pulsers.
10.7. Pulser Circuits.—To obtain substantially rectangular pulses
of short duration and high pulse power requires that energy stored in
some circuit element be released quickly upon demand and be replenished
from an external source during the interpulse interval. Either electrostatic
or electromagnetic means of energy storage can be used.
In the latter case, energy stored in the magnetic field of an inductance
through which current is flowing is released to the load by suddenly interrupting
the current. This can be achieved by biasing to cutoff a highvacuum
tube in series with the inductance, the resulting inductive voltage
rise being applied to the load. To restore the energy to the magnetic
field, it is necessary either to pass large steady currents through the
inductance between pulses, or to use a fairly complicated grid-modulating
circuit whose function is to start the current flowing through the inductance
a short time before the moment of interruption.
In either case, power losses in the switch and auxiliary equipment are
much larger than those in pulsers of comparable output using electrostatic
storage of energy. .4s a result, the use of pulsers employing electromagnetic
energy storage has been limited to special applications (such
as trigger circuits) where it is necessary to obtain a very high ratio
between the pulse voltage and the supply voltage or where the load is
mostly capacitive. The following discussions in this chapter will refer
to pulsers whose energy is stored in an electrostatic field.
The basic circuit of most practical pulsers so far designed is given in
Fig. 10.26. Assume for the present that the energy-storage element is a
condenser, Co, charged to a potential Vc so that the energy stored is
~ COV~. 1$’hen the switch S’ is closed at L = O, the condenser will begin
discharging exponentially through the load resistance R.. If the switch S
can now be opened suddenly at a time tOvery small compared to the time
constant RLCO of the circuit, the voltage appearing across the load, for
O < t < h, will be given by