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Crystal-Controlled Logic Oscillator
Crystal-controlled logic oscillator circuit.

It is not necessary to use a single transistor as the amplifier in a crystal-controlled oscillator. For example, consider the circuit to the left, which uses a CMOS inverter as its amplifier. The CMOS gate is effectively a very high-gain amplifier, so its output is a square wave rather than a sine wave. This makes this circuit a very handy and accurate clock signal generator for digital circuits.

In this circuit, C2 and C3 form a tapped capacitor with its tap grounded. Thus, the crystal provides the 180° phase shift required between output and input. The 1M resistor causes the gate to bias itself at half the supply voltage. Therefore both transistors in the gate are turned on and operating in their linear regions. This value is not critical, and any higher resistance — such as 10M — will work fine here.

The 1K resistor is also not critical. Its purpose is to limit the current through the crystal, so that the square wave output doesn't overstress the crystal. The output inverter serves to isolate the oscillator from all external circuits, so that external capacitances and other components cannot affect the oscillator frequency or output waveform.

The capacitor values are selected according to the requirements of the crystal, to provide accurate matching. If the precise frequency is not an issue, C1 isn't used at all, C2 might be about 30 pf, and C3 might be 82 pf. If the exact frequency is important, as it would be for a frequency reference, C2 is reduced to about 10 pf, and C1, a 2-20pf trimmer capacitor, is added. The trimmer capacitor is then adjusted to place the oscillator exactly on frequency.

The crystal-controlled logic oscillator can operate at any frequency the inverter can handle. Frequencies up to 10 MHz are quite reasonable and practical for oscillators of this type, and specialized versions can operate even faster.

One word of caution if you decide to try building and testing this circuit: do not use a breadboard socket. The capacitances associated with the socket itself, while very minor at audio frequencies, are too much for operation at radio frequencies. You could use perfboard, IC sockets, and point-to-point wiring to get much better results. Even better would be a printed circuit board designed specifically for the circuit.

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