A Band-Pass Filter (BPF) is a device that passes frequencies within certain range and rejects (attenuates) frequencies outside that range.
They are also known as BandPass Filters and they play a crucial role in usage of wireless radio frequency Receivers and Transmitters in radio electronics.
When used in a Transmitter, their primary function is to restrict the Bandwidth of the output signal to the allocated transmission Band.
This ensures that the Transmitter does not interfere with other stations operating in different frequency Bands.
Conversely in a Receiver a BandPass Filter permits signals falling within a particular frequency range.
They are then received and decoded, while blocking out unwanted signals at other frequencies.