ABSTRACT

Filter (n.) is an electrical engineering term having a broad meaning. Filters are generally considered to operate on signals in the frequency domain; that is, they attenuate, stop, pass, or boost certain frequency regions in the input signal’s power density spectrum. These operations are called filtering (v.). Filter operations are classified as low-pass (LP), bandpass (BP), high-pass (HP), band-reject (BR), or notch. Bandpass and band-reject filters can have narrow (tuned) pass-bands or reject bands. One descriptor of such sharply tuned filters is their “Q.” Q can be defined as the BP filter’s center frequency, fc, divided by its half-power bandwidth, Δf; the narrower the bandwidth, the higher the Q. (Δf is generally measured as the frequency span between half-power frequencies, where the filter’s frequency response function is down to 0.7071 times the peak pass gain, or –3 dB from the peak gain.) Thus, mathematically, Q ≡ fc/Δf.