Pierce Oscillator

In addition to the description elsewhere in this magazine of HC and HCT based R-C/L-C RI oscillators for use up to 20 MHz, this design brief concentrates on quartz-controlled oscillators whic find applications in digital equipment and micro- procesior systems. Such oscillators can only be made with HCU gates, because HC and HCT ones have buffered outputs that make them unsuit- able for uze as analogue ampli- fiers. mim The circuit‘ diagram shows a Pierce oscillator set up around a single gate in a Type 2rnA 74HCU04 package. The inverter functions as an inverting ampli- fier with a phase shift of 180°. The circuit can be modified into a Collpits oscillator by replacing the quartz crystal with an inductor. It should be noted, however, that the use of a quartz crystal is more appro- priate because it ensures minimum current consumption and adequate suppression of the third harmonic frequency. Finally, R2 must be replaced with a 33p capacitor if the oscil- lator is operated above 4...