Crystal products are passive components most commonly used as time or frequency references in electronic systems. Consisting of a piezoelectric crystal that mechanically vibrates at a known frequency, their use requires provision of an external oscillator circuit with carefully designed capacitance, drive voltage, and series resistance characteristics, to generate a signal at the indicated crystal frequency that operates successfully over the operating temperature range, without damaging the crystal element. Active devices which integrate this additional necessary circuitry are considered oscillators and are catalogued separately.