Radar System Engineering

SEC. 109] THE HARD-TUBE P ULSEh? 367
Figure 10.28 shows how differing internal impedances can affect mode
stability: operation in the unwanted % mode is impossible in the case of a
pulser load line such as B, but may occur if the load line has the form of
A or C.
We must also pay attention to the reverse voltage V~, which most
pulsers put across the magnetron after the main-power pulse has passed,
for in some cases V~l may be larger than V~O and lead to breakdown of
the magnetron insulation. This same reverse voltage may also appear
across various parts of the pulser circuit itself unless steps are taken to
suppress it. Most pulsers keep the reverse voltage within safe limits by
means of a resistance or, better still, a nonlinear element such as a diode.
The power wasted in backswing da’mping is rarely more than a few per
cent of the total useful power output of the pulser, and the duration of
the backswing is seldom greater than 2 to 10 .psec. Some pulsing circuits
produce a secondary forward pulse, V~,, which can cause trouble by
feebly exciting the magnetron to give a few microwatt of power. In
many applications this would cause co
no concern because of the much ().1 , A’
?
greater amplitude of the main power T, I
pulse. In most radar systems, : Rz
however, a few microwatt, directly t 6k ~
Load
from the magnetron, would swamp From
; L, ;
driver
any but the strongest echoes which circuit
5mhj
++ *
might happen to return coinciden- L2
1 2 mh
tally with the appearance of VA. LA
SFl 1‘~ 4 *
As a matter of practical experience,
E, EP=+13kv
Vm* should be kept below 10 per
1200v
cent of V-O if all possibility of ~IG. 1031.-Schematic diagram of output.
stage of hard-tubepulser.
trouble is to be avoided.
10c9. The Hard-tube Pulser.—A simplified diagram of the poweroutput
stage of the hard-tube pulser is given in Fig. 10,31. The energystorage
element COis recharged through the isolation inductance L1. In
the quiescent state, the point A’ is at ground potential while the highvoltage
side of Co is at the full d-c supply voltage along with the anode of
vacuum tube T1. The control grid of TI is biased to cutoff and the screen
grid is held at a normal positive value. If a sufficiently large positive
pulse be applied to the control grid of Tl, the cathode will emit electrons
vigorously and the anode will drop from its high positive pot ential to just
that potential required to pass the plate current demanded by the load.
Since the voltage across Co cannot change instantaneously, the point A’
will assume a high negative voltage, which is applied to the load. Currert
will continue to flow out of CO,and around the load circuit, until the
driving pulse on the grid of T, is removed. The system will return to its