AN826 Datasheet | Microchip Technology

Crystal Oscillator Basics and Crystal Selection for rf PICTM and PICmicro® Devices Author: Steven Bible Microchip Technology Inc. - - - - - - - - What temperature stability is needed? What temperature range will be required? Which enclosure (holder) do you desire? What load capacitance (CL) do you require? What shunt capacitance (C0) do you require? Is pullability required? What motional capacitance (C1) do you require? What Equivalent Series Resistance (ESR) is required? - What drive level is required? To the uninitiated, these are overwhelming questions. What effect do these specifications have on the operation of the oscillator? What do they mean? It bees apparent to the product design engineer that the only way to answer these questions is to understand how an oscillator works. This Application Note will not make you into an oscillator designer. It will on ly exp lain th e ope ration of an oscillator in simplified terms in an effort to convey the concepts that make an oscillator work. The goal of this Application Note is to assist the product design engineer in selecting the correct crystal and external capacitors re quired fo r th e rf PI CTM or PI Cmicro® device. In order to do this th e designer needs a cl ear understanding of the interrelationship of the various circuits th at ma ke up an oscillator ci rcuit. Th e p roduct design engineer should also consult with the crystal manufacturer about the needs of their product design. INTRODUCTION Oscillators are an im portant c omponent of radio f requency (RF) and digital devices. Today, product design engineers often do not find themselves designing oscillators because the oscillator circuitry is provided on the device. However, the circuitry is not plete. Selection of the crystal and ex ternal capacitors hav e bee n left to the product design engineer. If the incorrect crystal and external capacitors are selected, it can lead to a product tha t does not op erate properly, fails prematurely, or will...