band-stop

A band-stop filter removes or attenuates electromagnetic energy within a defined frequency band while allowing signals outside that band to pass through with minimal loss. It is the opposite of a band-pass filter. The filter works by introducing impedance mismatch or destructive interference across its stopband, typically achieving attenuation of 20 to 60 decibels depending on design and component quality.

Band-stop filters are essential in industrial settings where unwanted signals must be suppressed without disrupting the broader frequency spectrum. Common applications include removing 50 or 60 hertz line noise from measurement instruments, eliminating interference from switching power supplies in data acquisition systems, and protecting sensitive radio receivers from local broadcast transmitters. They are also used in audio equipment to suppress hum from AC mains, and in telecommunications to reject out-of-band interference that degrades signal quality.

Construction and Performance

Band-stop filters can be implemented as passive circuits using resistors, capacitors, and inductors, or as active designs incorporating operational amplifiers for sharper frequency response. A simple passive notch filter (a type of band-stop filter) typically consists of a series resonant LC circuit, which presents high impedance at the center frequency and blocks that signal. The width of the stopband, called the rejection bandwidth, is determined by the quality factor (Q) of the circuit; higher Q values produce narrower, deeper notches. In contrast, wideband band-stop filters may use multiple stages or active feedback to suppress broader frequency ranges.

The term "notch filter" is often used interchangeably with band-stop filter in practice, though notch filters usually refer to narrower rejection bands. Performance degrades when component tolerances are loose or when capacitor and inductor values drift with temperature; industrial designs often specify matched pairs or temperature-compensated components. The insertion loss outside the stopband should be minimized, typically under 3 decibels at frequencies far from the center frequency.

Band-stop filters sit alongside band-pass, high-pass, and low-pass filters in the engineer's toolkit for signal conditioning. Unlike low-pass or high-pass filters, which are inherently simpler and cheaper to implement, band-stop filters demand precision because they must surgically remove one frequency range while preserving everything else. This makes them more common in measurement and test equipment than in simple power distribution systems.