Filters

Filters are frequency selective circuits. The filter used in audio loudspeakers to direct high and low frequency signals to the appropriate transducers is called a cross-over network. There are numerous applications of filters including radio receivers, television receivers, noise reduction systems, and power supply circuits to name just a few.

Circuits that pass certain frequencies and attenuate (eliminate) other frequencies are called filters. Filters are categorized into three general types. Circuits that pass low frequency signals are called low-pass filters (LPF). Circuits that pass signals with frequencies between lower and upper limits (ie. a band of frequencies) are called bandpass filters (BPF). Circuits that pass high frequency signals are call high-pass filters (HPF). Often filters are described in terms of ideal characteristics. An ideal lowpass filter, for example, might pass frequencies below 1 kHz and completely stop (reject, eliminate) all frequencies above 1 kHz. A physical LPF filter, however, might pass all frequencies below 1 kHz but also partially pass frequencies above 1 kHz. A physical filter can be designed to closely approximate an ideal filter if needed. However, the better the approximation, the higher the cost because more components are required.