Low-pass filters pass all frequencies below the cutoff frequency defined by the -3-dB point (Fig A). These filters are useful for removing the harmonic content of signals or eliminating interfering signals above the cutoff frequency.
(A) low-pass
High-pass filters pass all frequencies above the -3-dB cutoff point (Fig.B). These filters are useful for eliminating interference from strong signals below the cutoff point. For example, a person using a shortwave receiver might wish to install a 1800-kHz high-pass filter to eliminate signals from strong AM broadcast stations.
(B) high-pass
Bandpass filters pass all frequencies between lower (FL) and upper (FH) -3-dB points, while rejecting those outside the FH–FL range. Bandpass filters are either wideband (low Q) or narrow band (high Q), as shown in Fig.C and D, respectively.
(C) bandpass
(D) narrow bandpass
(E) notch or band reject
Q is the quality factor of the bandpass filter and is defined as the ratio of the center frequency to the bandwidth. For example, if a filter is centered on 10,000 kHz, and it has a 25-kHz bandwidth between the lower and upper -3-dB points, then the Q is 10,000/25 =400
Notch filters (band reject filters), pass all frequencies except those between lower and upper cutoff frequencies (Fig E). Some notch filters are made broad, but many are very narrow. The latter are designed to suppress a single frequency. For example, a local FM broadcast signal will often interfere with television or two-way radio services. A notch filter tuned to that frequency will wipe it out. Similarly, on an AM band receiver, if it is being desensitized by a strong local station, reducing reception of all other frequencies on the AM band, a single-frequency notch filter can be used to suppress that one station’s signal, leaving the other frequencies untouched.
