Radar System Engineering

SEC 10.9] THE HARD-lUBE PULSER 371
to only 1/10,000 of the peak current, or 2 ma, the power loss at 35 kv
anode voltage is 70 ~vatts. To reduce leakage current, the grid structure
of a switch tube must surround the emitting area of the cathode completely,
and must give a uniform field over the entire cathode. Figure
10.32 shows a group of high-vacuum switch tubes; Table 10.4 gives a set
of typical operating characteristics.
Except for the screen-grid and control-grid bias requirements of power
output and driver stages, the hard-tube pulser is rather simple, for (as is
shown in Fig. 10.33) the grid-driving circuit is not inherently complicated.
A single 829 or 3E29 tube at T2 will satisfactorily drive two Eimac
304TH’s or two Western Electric 715B tubes; ei~her complement of tubes
is adequate to deliver over 200 kw to the load. Circuit constants given
in Figs. 10.31 and 10.33 are representative and have been used in a pulser
produced in large quanties. With the constants shown, the output volt.
age pulse has a nominal duration of 1 psec at 12 kv and 12 amp. The
maximum duty-ratio limit, set by
Transformer ratio
the 715B tube, is 1/1000.
/ ~:,:,
If higher powers are required, ~ac~-C=200~P, EP=1200V
it is quite practical to use several
To g[ld of
715B’s in parallel. High-voltage
II II power output
‘*
q
stage
tubes such as the 6C21 can be op-
829
--- II
crated either singly or in parallel.
lpsec400n ‘-- ,01
in the case of parallel operation, Trigger
25 k
AT
A :: ,- ,
in >.
it is sometimes necessary to insert
TO=l,usec
-130
25 to 50 ohms into the plate and
+660 V\
screen leads in order to prevent
AWv E ,.,;:;”::,,.
parasitic oscillations. t-
750vfor715B
The Driver Circuit.—In the F1o. 1033.-Schematic diagram of regengrid-driving
circuit of a hard-tube erative pulsegenerator used as driver of hardtube
pulser.
pulser, all the requirements for
pulse shape, accuracy of pulse spacing, and flexibility of pulse rates must be
met. The important parts of the regenerative pulser circuit shown in Fig.
10.33 are the pulse-forming network at the left and the three-winding pulse
transformer which provides the necessary plate-grid feedback to make the
circuit self-driving.
Normally, all tubes are biased to cutoff; therefore, when a positive
pulse of 100 volts is applied to the “trigger in” point (Fig. 10.33), plate
current starts to flow in T2, inducing a further positive voltage on the
grid of Tj. Once regenerative action has commenced, the input trigger
has no further effect; the plate current increases rapidly until limited by
the eP-ip characteristic of Tz. As i, increases, eP decreases until it nears
the screen potential; from there on, i, increases only slowly, thus pre
ducing a relatively flat-topped wave,