Crystal-Oscillator Circuits

Posted on Feb 7, 2014

The frequency of a crystal-controlled oscillator is held constant to a high degree of accuracy by the use of a quartz crystal. The frequency depends almost entirely on the dimensions of the crystal (essentially its thickness); other circuit values have comparatively negligible effect. However, the power obtainable is limited by the heat the crysta

Crystal-Oscillator Circuits
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l will stand without fracturing. The amount of heating is dependent upon the r. f. crystal current which, in turn, is a function of the amount of feedback required to provide proper excitation. Crystal heating short of the danger point results in frequency drift to an extent depending upon the way the crystal is cut. Excitation should always be adjusted to the minimum necessary for proper operation. The simplest crystal-oscillator circuit is shown in Fig. 6-2A. An equivalent circuit is shown in Fig. 6-2B. , where C4 represents the grid -cathode capacitance and C5 indicates the plate-cathode, or output capacitance. The ratio of these capacitors controls the excitation for the oscillator, and good practice generally requires that both of these capacitances be augmented by external capacitors, to provide better control of the excitation. The circuit shown in Fig. 6-2C is the equivalent of the tuned-grid tuned-plate circuit discussed in the chapter on vacuum-tube principles, the crystal replacing the tuned grid circuit. The most commonly used crystal-oscillator circuits are based on one or the other of these two simple types, and are shown in Fig. 6-3. Although these circuits are somewhat more complicated, they combine the functions of oscillator and amplifier or frequency multiplier in a single tube. In all of these circuits, the screen of the tetrode or pentode is used as the plate in a triode oscillator. Power output is...

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