3.6M north10.pdf - Dean-O's Toy Box
3.6M north10.pdf - Dean-O's Toy Box
3.6M north10.pdf - Dean-O's Toy Box
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192 High-Power Microwave-Tube Transmitters<br />
RC time-constant, because 27varies continuously with voltage.<br />
The rise-and-fall waveforms for a constant-current source am identical but<br />
inverted, as shown in Fig. 10-32. They have variable slopes like RC time-constant<br />
exponentials, except that the closer the exponential come to the asymptotes, the<br />
greater are the instantaneous values of R and the more slowly they approach<br />
asymptotes, much like exaggerated RC time-constants. It is possible, however, to<br />
evaluate the length of time required for the pulse voltage to change from one<br />
normalized value to another by using the formula shown in the equation<br />
TF=t2–tl=<br />
%(7%-+<br />
where al and a2 are two such normalized values, typically 0.1 and 0.9. The three<br />
important . Parameters .<br />
are the perveance of the diode gun, k; the flat-top pulse<br />
cathode voltage, Vb and the value of the stray capacitance, CS. In the example<br />
shown, a tube that operates at 50-kV beam voltage and 1-Abeam current and has<br />
a perveance of 0.9x10 -G, has a stray capacitance of 200 pF. Given these parameters,<br />
the time between the 10% and 90% points of the rise-and-fall waveforms<br />
would be 38 ~ if there was a constant-current source of 1 A. This would be<br />
unacceptably long for many, if not all, applications.<br />
However, this is a rare case because most cathode-pulsed tubes have higher<br />
perveances and operating voltages. Moreover, switch tubes are usually overdriven<br />
during the rise time to provide an excess of current in order to charge the stray<br />
High-voltaga<br />
““’~’ti<br />
-v*---- ----- -----<br />
Figure IO-32. Determination of pulse fall time of a hard-tube modulator.