3.6M north10.pdf - Dean-O's Toy Box

Cathode Pulsers: Hard-Tube Modulators (10) 153
with time. Any changes consist mostly of the capacitor-bank droop and the
voltage drop across the surge-current-limiting resistors caused by the normal
peak-load current. Before there were dependable high-performance optically
coupled signal links, special pulse-transformer designs and even pulsed RF links
were used to transmit low-level timing signals across the high-voltage gap, examples
of which we will see later. As complicated as these circuits are, they are
in most cases preferable to solving the problem of insulating the pulse-transformer
primary winding for the full switch-tube operating voltag~specially
because the performance of the pulse transformer is either the most important
issue or second only to that of the switch tube itself.
Circuit complications am also unavoidable in the final configuration, which
is shown in Fig. 10-3d. This is the direct-drive hard-tube modulator—the top of
the line, as it were. In this design, there is no pulse transformer at all. This
feature eliminates the limitations on pulse duration, rise-and-fall times, intrapulse
droop, and leading-edge overshoot &d ringing. The price paid for this imp~ovement
is increased circuit complexity and the fact that the switch tube itself must
handle the full operating voltage and current of the microwave tube load as
separate and independent ratings-not as a peak-power product, where lack of
voltage hold-off can be compensated for by increased current-handling (or viceversa)
with any mismatch handled by an appropriate pulse-transformer turns
ratio. This direct-drive connection is capable of the highest performance of all
modulator types. Given enough excess capability, or head-room, it can be programmed
to compensate for almost every shortcoming of the rest of a transmitter,
including power-supply ripple, noise, and capacitor-bank droop. In fact,
there is a class of switch tube, which will be described later, that will do much of
this with no programming at all. Not surprisingly, therefore, this class of modu-
I
Figure 10-4. The back-swing clipper circuit as used with transformer-coupled unidirectional! [o&.