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Definition: SAW filter

(Surface Acoustic Wave filter) A semiconductor device that is used to filter out desired frequencies. Widely used in mobile phones to filter both RF and IF frequencies, a SAW filter uses the piezoelectric effect to turn the input signal into vibrations that are turned back into electrical signals in the desired frequency range.

Two sets of metal electrodes like teeth on a comb are adhered to a quartz crystal and spaced apart (in microns) based on the required frequencies. As few as five and as many as 5,000 electrodes are used. An intriguing analogy from Bruce Thomas at Sawtek, a subsidiary of TriQuint Semiconductor, is that the first set of electrodes is like throwing a large log into a pond, which makes huge waves that cause little sticks to bounce out at the other side (second set).

SAW filters are sensitive to high temperatures, and temperature-compensated SAW (TC-SAW) filters were designed to retain their integrity under such conditions. However, TC-SAWs do not support the higher LTE frequencies, but bulk acoustic wave (BAW) filters do. As a result, SAW filters are often used for lower frequencies, while BAWs, which are twice as costly, are used for the high end. See RF filter.