Crystal frequency multiplier

Crystals for operation above 20 MHz are invariably cut to an overtone (harmonic), which may be the third, fifth, or seventh. In other words, the fundamental frequency of such a crystal is a third, fifth or seventh of the operating frequency. If, however, an overtone crystal is used in an oscillator designed for operation on the fundamental, it is doubtful whether the wanted frequency will be generated. In most such cases, there are difficulties that make it necessary for the oscillator circuit to be modified.