Editing Passband

{{short description|Frequencies or wavelengths that can pass through a filter}}
{{refimprove|date=December 2009}}

A '''passband''' is the range of [[frequency|frequencies]] or [[wavelength]]s that can pass through a [[Filter (signal processing)|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 when it is tuned into the desired frequency as in a radio station or television channel.

A [[band-pass filter|bandpass-filtered]] signal (that is, a signal with energy only in a passband), is known as a '''bandpass signal''', in contrast to a [[baseband signal]].<ref>
{{cite book
 | title = Introduction to digital signal processing and filter design
 | author = Belle A. Shenoi
 | publisher = John Wiley and Sons
 | year = 2006
 | isbn = 978-0-471-46482-2
 | page = 120
 | url = https://books.google.com/books?id=37g8oUqaS_AC&q=%22bandpass+signal%22&pg=PA120
 }}</ref> The bandpass filter usually has two [[stopband|band-stop]] filters.

== Filters ==
<div>[[File:Passband schematic3.png|class=skin-invert-image|thumb|right|upright=1.75|Unrestricted signal (upper diagram). Bandpass filter applied to signal (middle diagram). Resulting passband signal (bottom diagram). ''A(f)'' is the frequency function of the signal or filter in arbitrary units.]]</div>

{{modulation techniques}}

In [[telecommunications]], [[optics]], and [[acoustics]], a passband (a [[band-pass filter]]ed signal) is the portion of the [[frequency spectrum]] that is transmitted (with minimum relative loss or maximum relative [[Gain (electronics)|gain]]) by some filtering device. In other words, it is a ''band'' of frequencies which ''pass''es 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 (signal processing)|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 baseband transmission, [[line coding]] is utilized, resulting in a pulse train or digital [[pulse amplitude modulated]] (PAM) signal. This is typically used over non-filtered wires such as fiber optical cables and short-range copper links, for example: [[ITU-T V.29|V.29]] ([[EIA/TIA-232]]), [[ITU-T V.35|V.35]], [[IEEE 802.3]], [[SONET/SDH]].
* In passband transmission, [[digital modulation]] methods are employed so that only a limited frequency range is used in some bandpass filtered channel. Passband transmission is typically utilized in wireless communication and in bandpass filtered channels such as [[DSL filter|POTS]] lines. It also allows for [[frequency-division multiplexing]]. The digital bitstream is converted first into an equivalent baseband signal, and then to a [[radio frequency|RF]] signal. On the receiver side a [[demodulator]] is used to detect the signal and reverse the modulation process. A combined equipment for modulation and demodulation is called a [[modem]].

== Details ==
{{Unreferenced section|date=August 2018}}
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.{{Citation needed|date=February 2008}} 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 (physics)|intensity]] or [[Power (physics)|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 [[decibel|dB]] below the maximum power.

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).

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 (linguistics)|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'.

==References==
{{reflist}}

* {{FS1037C MS188}}

[[Category:Filter frequency response]]
[[Category:Physical layer protocols]]

[[sv:Passband]]