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
telecommunications, optics, and acoustics, a passband is the portion of spectrum, between limitingfrequencies (or, in the optical regime, limiting wavelengths), that is transmitted with minimum relative loss or maximum relative gainby a filtering device.
Passband in terms of digital transmission
In digital communication transmission the frequency band is split up into two main parts: The
basebandand 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 basebandto the passband. On receiving side a downconverteris used to retrieve the baseband signal.
Radioreceivers generally include a tunable band-pass filterwith 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. Woodwindinstruments such as the fluteand penny whistleare 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. Overblowinga 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
mathematicsof Fourier analysis.
"Note 1:" The limiting frequencies are defined as those at which the relative
intensityor 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'.
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