Crystal Oscillators

Author: J.B. Hoag

Fig. 29 E. A beam-tetrode crystal oscillator circuit

It is advantageous to use pentode or beam-tetrode tubes in crystal oscillator circuits (Fig. 29 E) because they have a high power sensitivity. This means that the power output will be large for a comparatively small grid voltage. The small grid voltage is necessary in order not to damage the crystal. With pentode tubes having a plate voltage of about 300 volts, 4 or 5 watts output will be obtained. Beam-tetrodes, such as the 6L6, will deliver about 15 watts when maximum plate voltage is used. The screen voltage is usually made from one-third to one-half that of the plate. Because the r.f. is grounded through a bypass condenser, the screen plays no part in the operation of the oscillator other than to set the operating characteristics of the tube. A feedback condenser C₅ (Fig. 29 E), of lowest possible value for stable oscillation, will probably be required to give sufficient feedback for oscillation at the lower frequencies.

In order to adjust a crystal oscillator, a d.c. milliammeter is connected in series with the plate supply. The tuning condenser C₁ (Fig. 29 E) is then varied until the plate current dips as in Fig. 29 F.

Fig. 29 F. Adjustment of a crystal oscillator

Maximum power is developed at the point A, but it is best to operate somewhere between B and C, where the oscillator is more stable and the crystal current is smaller. The dotted curve of Fig. 29 F is for the case when the oscillator is delivering power. The dip will be less pronounced the greater the load. The crystal current, and hence its heating, the cause of breakdown, will be less when the oscillator is loaded, than when it is unloaded.

Fig. 29 G. The Pierce crystal oscillator

Figure 29 G shows another type of quartz-crystal oscillator which does not require tuning control. It is the equivalent of the ultraudion circuit, with the tuned circuit replaced by the crystal; or of a Colpitts oscillator with the crystal displacing the tank coil inductance. Condensers C₁ and C₂ are in series with the crystal, from the plate to the cathode. Hence these condensers determine the feedback. As tuned tanks are not used in this simple circuit, a wide range of frequencies, one for each crystal used, can be obtained without serious change in the circuit values. The circuit is limited to low power outputs, and must be carefully adjusted to avoid damaging the crystal.