In brief, the Passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.

Passband in terms of filters

In telecommunications, optics, and acoustics, a passband is the portion of spectrum, between limiting frequencies (or, in the optical regime, limiting wavelengths), that is transmitted with minimum relative loss or maximum relative gain by a filtering device.

Passband in terms of digital transmission

In digital communication transmission the frequency band is split up into two main parts: The baseband and the passband. The passband is all frequencies above a special limiting frequency, e.g. in radio communications one cannot transmit a signal near zero frequency. For transmission of near-zero-frequency-signals (e.g. human voice between 300Hz-3kHz) over a radio channel, one has to upconvert the signal to a suitable frequency for transmission. In other words, the signal is converted from the baseband to the passband. On receiving side a downconverter is used to retrieve the baseband signal.


Radio receivers generally include a tunable band-pass filter with a passband that is wide enough to accommodate the bandwidth of the radio signal transmitted by a single station.

Passbands are found in many systems outside of telecommunications. For example, most traditional musical instruments are tunable sonic band-pass filters with narrow passbands. Woodwind instruments such as the flute and penny whistle are good examples: the flute is stimulated by broad-band sonic noise at the mouthpiece but resonates only in a narrow passband around the fingered note. Overblowing a flute (that is, playing higher notes with the same fingering as a lower note) is possible because the flute has multiple passbands for any given fingering: the note that emerges is dependent on both the fingering and the spectrum of wind noise at the mouthpiece.

In general, there is an inverse relationship between the width of a filter's passband and the time required for the filter to respond to new inputs. Broad passbands yield faster responseFact|date=February 2008. This is a consequence of the mathematics of Fourier analysis.

"Note 1:" The limiting frequencies are defined as those at which the relative intensity or power decreases to a specified fraction of the maximum intensity or power. This decrease in power is often specified to be the half-power points, "i.e.", 3 dB below the maximum power.

"Note 2:" The difference between the limiting frequencies is called the bandwidth, and is expressed in hertz (in the optical regime, in nanometers or micrometers of differential wavelength).

"Note 3:" The related term "bandpass" is an adjective that describes a type of filter or filtering process; it is frequently confused with "passband", which refers to the actual portion of affected spectrum. The two words are both compound words that follow the English rules of formation: the primary meaning is the latter part of the compound, while the modifier is the first part. Hence, one may correctly say 'A dual bandpass filter has two passbands'.

See also

* Stopband

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