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How Narrow Should FM Filters Be?

Electronic filters play an essential role in the operation of FM radio receivers. It selects a limited range of frequencies from the incoming radio signal. Radio engineers carefully choose the filter design as its frequency range, called bandwidth, determines the quality of the signal. If the bandwidth is too wide, the sound becomes noisy; if it is too narrow, speech and music become unrecognizable. Though consumer FM radios have a standard filter width, specialty radios have narrow filters for long-distance listening.

FM Bandwidth
- FM stands for ̶0;frequency modulation.̶1; An FM transmitter takes the audio program from the broadcaster and uses it to shift a radio signal̵7;s frequency rapidly up and down. A radio receiver detects these shifts and converts them back into sound. The greater the shift, the better the sound quality, though large shifts take up space on the FM band, limiting the number of stations that broadcast in a given area. In the 1940s, the Federal Communications Commission worked out a compromise, permitting FM radio stations to broadcast high-quality music while allowing dozens of stations to share the FM band. A radio̵7;s filter restricts the incoming signal to the frequency range for a station set by the FCC. Without the filter, you might hear interference from adjacent stations, especially if they have strong signals.
Fidelity Versus Selectivity
- An FM radio with a standard filter pulls in local stations with good sound quality, but it receives distant stations poorly or not at all. A filter with a narrower bandwidth has better selectivity to tune in weak signals from faraway stations, but it loses some audio quality. Also, a narrow filter prevents a radio from processing RDS data, which transmits text to your receiver for traffic reports and song titles. Most listeners are satisfied with a standard filter, though some radio enthusiasts use a narrow one to listen for distant stations, a hobby known as ̶0;DXing.̶1;
Standard
- In the U.S., the FCC spaces FM radio stations 200 kHz apart in frequency. This separation gives each station enough bandwidth to broadcast high-fidelity music while preventing overlap and interference from neighboring stations. A standard FM radio has a filter bandwidth of 180 kHz. The radio̵7;s tuner centers on a station̵7;s frequency, such as 93.1 MHz. A 180 kHz filter allows frequencies from 93.010 to 93.19 Mhz, or 90 kHz above and below the station frequency.
Long Distance
- Filters for DXing come in a variety of bandwidths, including 56, 80 and 110 kHz. The narrowest filters have the best selectivity, and therefore the best potential for receiving distant stations, though at a cost of compromised sound quality. The filters themselves are specialty components made by a handful of electronics manufacturers; some FM radio designs allow you to replace the standard filter with a narrow one for DXing.

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