Linear-Circuits in U.H.F. Oscillators

Posted on Feb 7, 2014

The use of a quarter-wave parallel-wire line as a tuning unit has been discussed in the chapter on Short-Lines, where it was pointed out that these ` circuits ` have comparatively high Q even at the higher frequencies. Their great length ( »/4) prevents their widespread use at lower frequencies. But when the wave-length is only 1 meter or thereabo

Linear-Circuits in U.H.F. Oscillators
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uts, the line has reasonable physical dimensions. And it is just in these regions that the requirement of high Q becomes difficult to attain with ordinary circuits. Our purpose now is to learn how to connect a quarter-wave line to a vacuum tube to form an efficient oscillator for the production of ultra-high frequencies. The student should now turn back to Fig. 14 E and study the parallel-fed ultraudion oscillator circuit for a few minutes. Series feed is also possible, as in (a) of Fig. 37 F. The lumped L and C of this circuit is replaced by the quarter-wave line of (b) for u. h. f. Compare (a) and (b) part by part. In general, the LC circuits used in the various types of oscillators of Chapter 14 can be replaced by quarter-wave lines. In the u. h. f. oscillator of Fig. 37 F, large copper or brass tubes may be used for the line. The lead wires to the tube are made as short as possible. The shorting bar can be shifted along the line (within limits) to change the frequency. The plate and grid leads are tapped onto the line as close to the shorted end as possible in order that, for a given voltage from the tube, the open-end voltage may be as high as possible. The effect is shown in Fig. 37 G. High voltage at the open end means strong electrostatic fields, and this means large electrostatic energy. This is similar to the action of a large flywheel in stabilizing the rotation of a machine. It results in frequency stability in...

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