is the desired frequency of the oscillator. For any given crystal cut, lower frequency crystals exhibit superior stability. For a given frequency, the highest possible overtone will provide the better stability. Here the rule is, “the greater the mass of quartz, the greater the stability”. The only constraint here is crystal package, and ultimately oscillator package. Frequency ranges down to 10 MHz. Below this frequency dividers and CMOS outputs work best. Above 100 MHz phase-locked loops and frequency multipliers are used to take advantage of the stability of low frequency crystals.