Passband


A passband is the range of frequencies or wavelengths that can pass through a filter. For example, a radio receiver contains a bandpass filter to select the frequency of the desired radio signal out of all the radio waves picked up by its antenna. The passband of a receiver is the range of frequencies it can receive.
A bandpass-filtered signal, is known as a bandpass signal, in contrast to a baseband signal.

Filters

In telecommunications, optics, and acoustics, a passband is the portion of the frequency spectrum that is transmitted by some filtering device. In other words, it is a band of frequencies which passes through some filter or a set of filters.
The accompanying figure shows a schematic of a waveform being filtered by a bandpass filter consisting of a highpass and a lowpass filter.
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.

Digital transmission

There are two main categories of digital communication transmission methods: baseband and passband.
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 response times. This is a consequence of the mathematics of Fourier analysis.
The limiting frequencies of a passband 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.
The difference between the limiting frequencies is called the bandwidth, and is expressed in hertz.
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. These 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'.