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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.

RF filter

(Radio Frequency filter) An analog circuit that detects and filters out a single carrier frequency, such as a radio or TV channel. RF filters are increasingly a major component in smartphones, which are required to tune into a dozen or more frequency bands. While the cost to the phone manufacturer for RF circuits in a modern smartphone is considerably less than the storage or display screen, it is nevertheless as much as 30 times that of the RF filters in the cellphones of the 1980s. See SAW filter.

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