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Bandpass Filter

July 19, 2010

A bandpass filter is an electronic device or circuit that allows signals between two specific frequencies to pass, but that discriminates against signals at other frequencies. Some bandpass filters require an external source of power and employ active components such as transistors and integrated circuits; these are known as active bandpass filters. Other bandpass filters use no external source of power and consist only of passive components such as capacitors and inductors; these are called passive bandpass filters.

The illustration is an amplitude-vs-frequency graph, also called aspectral plot, of the characteristic curve of a hypothetical bandpass filter. The cutoff frequencies, f1 and f2, are the frequencies at which the output signal power falls to half of its level at f0, the center frequency of the filter. The value f2 f1, expressed in hertz (Hz), kilohertz (kHz), megahertz (MHz), or gigahertz (GHz), is called the filter bandwidth. The range of frequencies between f1 and f2 is called the filter passband.

Bandpass filters are used primarily in wireless transmitters and receivers. The main function of such a filter in a transmitter is to limit the band width of the output signal to the minimum necessary to convey data at the desired speed and in the desired form. In a receiver, a bandpass filter allows signals within a selected range of frequencies to be heard or decoded, while preventing signals at unwanted frequencies from getting through. A bandpass filter also optimizes the signal-to-noise ratio (sensitivity) of a receiver.

In both transmitting and receiving applications, well-designed bandpass filters, having the optimum bandwidth for the mode and speed of communication being used, maximize the number of signals that can be transferred in a system, while minimizing the interference or competition among signals.

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