Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both. Electronic filters can be:
Radio frequency (RF) and microwave filters represent a class of electronic filter, designed to operate on signals in the megahertz to gigahertz frequency ranges (medium frequency to extremely high frequency). This frequency range is the range used by most broadcast radio, television, wireless communication (cellphones, Wi-Fi, etc...), and thus most rf and microwave devices will include some kind of filtering on the signals transmitted or received.
Such filters are commonly used as building blocks for duplexers and diplexers to combine or separate multiple frequency bands.