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

Cathode Pulsers: Hard-Tube Modulators (10) 151
The phase slope with time is40,000°/s. -Because there are360° per cycle, the
frequency shift, which is exactly the same as a Doppler shift, is 40,000/360, or 111
Hz. This effect has been referred to by some as the “serrodyne” effect because of
its serrated, or sawtooth, waveform, which is caused in this case by voltage
removal from a capacitor. Before there were modern frequency synthesizers with
phase-locked loops and other means of assuring phase coherency between transmitter
and receiver local-oscillator frequencies, the serrodyne effect was actually
used to achieve the frequency offset between them. This was done by applying a
linear ramp of voltage with its short-duration reset timed so that the period of
the resulting sawtooth waveform was the same as the desired receiver intermediate
frequency. This sawtooth waveform was applied between helix and cathode
of a low-power, continuous-wave TWT that had a relatively high phase-pushing
factor. With proper adjustment of the amplitude of the sawtooth waveform, the
RF output would be at the desired local-oscillator frequency.
Figure 10-1 showed the most gradual of topological changes that could ease a
designer from the line-type, half-control-switch modulator technology into the
hard-tube, full-control-switch modulator technology. Figure 10-3 shows four further
refinements. The first refinement, illustrated in Fig. 10-3a, simultaneously
replaces the soft-tube switch with the hard-tube switch and the artificial transmission
line PFN with a capacitor energy store, which is coupled with a series faultcurrent-limiting
resistance. In the case of a switch-tube arc, however, all of the
charge and energy stored in the capacitor will be dumped. Charge and energy
will be transferred to the load connected to the puk+transformer secondary until
the transformer core saturates, which, in a well-engin~md application, will not
be much longer than the longest normal pulse. Up until this time, the cur~nt in
the discharge loop will only be about 15!40 greater than the normal load current,
assuming that the normal switch-tube voltage drop was approximately 10% of
the initial capacitor voltage and that the load is a 3/2-power, space-charge-limited,
diode-type electron gun.
When the pulse transformer core saturates, the transfer of energy and charge
to the load terminates, but things get much worse for the faulted switch tube.
Remember from the discussion on half-control switches that an arc in vacuum is
a very good conductor with almost negligible voltage drop. Current is now
limited only by the series resistance of the capacitor. What is more, the action
integral of the discharge, or
J i2dt,
is equal to the stored energy divided by the value of series resistance. (joules/
ohm is the equivalent of ampere~seconds.) If the arc in the switch tube was
between the &ode and one OFthe small-diameter wires that makeup the control
grid, the action integral determines whether the grid wire will fuse. If it does
and if enough grid wire is involved, the voltage on the grid will no longer
effectively cut off anode current in the interpulse interval.
For low intrapulse voltage droop, the capacitor’s stored energy should be