Radar System Engineering

382 THE MAGNETRON AND THE PULSER [SEC. 10.10
high enough for breakdown. Current flows from 4’ to the disk pin and
out through a second spark at pgint A. The simplicity of this device is
partially offset by an uncertainty of up to ~ 50 psec in firing time, and by
the narrow voltage range over which satisfactory operation occurs. The
display-tube sweeps must be triggered by the appearance of the power
pulse because of the great uncertainty in firing time. If j is the speed of
the disk in rpm, and n is the number of pins, the resulting pulse rate is
nf/60. For a given power output, there is an optimum spacing between
~otating and fixed pins, and a minimum permissible spacing between pins
in the rotating disk. These quantities vary somewhat with disk speed,
so that it is not easy to design a rotary spark-gap pulser for variable pulse
rate.
‘I’he Recharging Circuit.—In all forms of network pulsers it is necessary
to recharge the network between pulses. This should not be done at too
rapid a rate. A slow rate of charge is easily obtained by using a large
inductance LO, which is also needed to prevent a virtual short circuit
across the energy source every time the network is discharged. Inductance
charging is used in practically all line-type pulsers, because it has
the advantage of high efficiency and permits charging the pulse-forming
network to a voltage nearly double that of the power supply, as shown
below.
Consider first a d-c power supply voltage of negligible resistance, in
series with an inductance, a switch, and a capacity C originally discharged,
The energy supplied by the source in a time T after closing
the switch is V.S ~~ i dt. If Q. is the charge on the condenser, as long as
I
the energy in the inductance at time T equals that at time O,
But
Then
or
T
v.
~ o
i (it= VSQC = + CV&
Q. = CVc.
T“,(7VC = * Cv;
v. = 2V,
and the voltage on the condenser C (or pulse-forming network) will be
twice the supply voltage. It must be noted that this result is independent
of the value of inductance used.
The network voltage obtainable with inductance charging in practice
lies between 1.8 and 1.95 times the d-c supply voltage, because of resistance
losses in the charging reactor. If LOand C are the values of charging
inductance and network capacity, t4resonanm” charging is obtained when
the repetition rate, f, = l/(r V“LOC). If j, > l/(r m, current in