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. The cut-off frequency in a simple RC passive filter can be accurately controlled using just a single resistor in series with a non-polarized capacitor, and depending upon which way around they are connected.
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.
Bandpass filters are used primarily in wireless transmitters and receivers. The main function of such a filter in a transmitter is to limit the bandwidth 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. 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.