Cavity Oscillators

The simplified circuit of a triode oscillator with grounded grid and tuning in both the cathode and plate circuits is shown in Fig. 39a. Feedback for sustaining oscillations is through the plate to cathode capacitance. A cavity-type oscillator is shown in Fig. 396. The electronic portion and the cavity portion are in reality part of the same tube structure. The relationship between the two circuits of Figs. 39a and 396 is evident. The cavities provide the inductances, and the electrodes provide the capacitances, giving an oscillator that will generate frequencies of thousands of megacycles.^31,32,33

Figure 39. A simplified oscillator with grounded grid is shown in (a). A cavity-type oscillator that is equivalent is shown in (b).

A vacuum tube, called a disk-seal tube³¹ or lighthouse tube, is constructed with parallel closely spaced "disk" electrodes such as are shown in Fig. 39b. The close spacing reduces the transit time, or time required for the electrons to pass from the cathode to the plate. This makes possible high-frequency oscillations. If the transit time is not short and if the frequency is high, an electron may start out toward the plate, only to arrive there under unfavorable phase conditions. Although the preceding discussion is for the use of disk-seal tubes as oscillators, they are also used at ultrahigh frequencies for amplification and for other purposes, much like ordinary vacuum tubes.^34,35