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

148 High-Power Microwave-Tube Transmitters
Soft-tubs line-type modulstor
Shun_thsrd-lube mcdulstor
(topo!qical descendant of line-type moduhtor)
Figure 10-1. Progression of sofi-tube, line-type moduhtor into shunt-type, hard-tube modus’utor.
combination of screen and anode voltages.
Because of cut-off grid bias, the hard tube can be turned off as well as on.
Tube current, therefore, can be fully controlled by the voltage applied to the
control grid from zero to the maximum. (The maximum curxw-tt is determined
by cathode emissive capability, a factor that varies with cathode area, cathode
material, and cathode temperature.) This feature, then, makes the tube a “proportional
and reciprocal” full-control modulator switch. It is called proportional
and reciprocal because the factors that will reduce current are simply the reverse
of the factors that can increase it. More will be said about hard-tube technology
later.
10.2 Hard-tube modulator topology
Figure 10-1 shows how the replacement of the half-control switch in the linetype
modulator with a full-control switch allows the pulse-forming network to
be replaced with a quasi-infinite energy store. This is now possible because the
time delay of an artificial transmission line is no longer required to externally
turn the switch tube off through commutation. Initiated by a trigger, the pulse
duration in the line-type modulator was set by network constants: the PRF was
limited by the rate of rise of charging voltage across the soft switch tube, while
the pulse-repetition interval (PRI) had to exceed
The pulses of current removed from the hard-tube modulator are now replicas of
the voltage applied to the grid of the hard switch tube. The output voltage V. of
the hard-tube modulator is the inverted replica of the circuit’s switch tube grid
drive voltage. Unlike the line-type modulator, the output-pulse duration and