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Anything that resonates can be used as a filter. A crystal filter is a quartz crystal cut to the right dimensions to resonate at a precisely controlled frequency. It is resonating mechanically, but because of the piezoelectric effect of quartz, the mechanical motion can be detected (or created) electrically. Quartz crystals are exceptionally narrow-band filters, easily having a quality factor (Q) of 1000 or more. The quality factor is approximately the ratio of the passband bandwidth (low-side corner to high-side corner) over the center frequency. Quartz crystals are also cheap. The most common usage of quartz crystal filters is in radio receivers, often used for an IF-to-baseband anti-imaging filter. Glen Ragan |