A bandpass filter is a circuit or a device which allows the transmission of signals at frequencies within a particular range and rejects signals at frequencies outside that range. Bandpass filters consist of two types: active bandpass filters and passive bandpass filters.
Active filters are provided with an external power source and active elements such as integrated circuits and transistors. Passive bandpass filters, on the other hand, have no external power source and consist of only passive elements such as inductors and capacitors.
The key parameters associated with the characterization of bandpass filters include bandwidth, gain and center frequency. Center frequency is the frequency with maximum amplitude. Gain is the maximum amplitude response at the center frequency. Bandwidth is the frequency limit between the -3 dB points present on the either side of center frequency. A typical bandpass filter will have a flat passband to completely attenuate all frequencies outside the passband.
Bandpass filters are primarily used in wireless receivers and transmitters. In transmitters, bandpass filters are used to reduce the bandwidth of the output signal in order to transmit data at a desired speed and form. The function of such filters in receivers is to decode signals within a selected range of frequencies, thereby preventing the transmission of signals at unwanted frequencies. Additionally, the bandpass filter optimizes the signal-to-noise ratio of the receivers.
Well-designed bandpass filters are generally used for both transmitting and receiving applications by using optimum bandwidth for the speed and mode of communication. This also maximizes the number of signals to be transferred in a system and minimizes the interference among the signals.
Sources and Further Reading