Definition
An electronic filter that blocks a specific range of frequencies while allowing frequencies above and below that range to pass through unaffected. The blocked range is called the rejection band or stop band.
Plain English
A circuit that lets most signals through but cuts out a chosen middle slice of frequencies. Anything inside that slice is rejected; anything outside it passes normally.
Context Anchor
Seen in avionics, radio, and audio equipment when a circuit is designed to remove an unwanted range of signal, such as interference, without removing the whole signal.
Derivation
The name describes what it does: it rejects a band (a range) of frequencies. 'Band' here means a continuous span of frequencies, the same sense used in 'frequency band' or 'radio band.'
Why Pilots Care
It removes unwanted interference from aircraft radios and navigation receivers so the desired signals remain clear.
Grounding Statement
A band-reject filter leaves most of a signal alone and cuts down only the part it was designed to reject.
Intuition Check
Do not read “reject” as meaning the filter stops everything. A band-reject filter rejects only one selected frequency range and passes the frequencies outside that range.
Example Sentence 1
A band-reject filter in the navigation receiver removes a known interference frequency without affecting the surrounding signals.
Example Sentence 2
During the radio check the technician verified that the band-reject filter was tuned to suppress the local VOR harmonic.