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Passband

Passband

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A passband is the range of frequencies or wavelengths that can pass through a filter
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...

 without being attenuated.

A bandpass filtered signal (that is, a signal with energy only in a passband), is known as a bandpass signal, as opposed to a baseband signal.

Passband in terms of filters


In telecommunications, optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

, and acoustics
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...

, a passband (not to be confused with band-pass
Band-pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....

) is the portion of the frequency spectrum
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...

 that is transmitted (with minimum relative loss or maximum relative gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...

) by some filtering device. In other words, it is a band of frequencies which passes through some filter or set of filters.
The accompanying figure shows a schematic of a waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...

 being filtered by a bandpass filter consisting of a highpass and a lowpass filter.

Radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 receivers generally include a tunable band-pass filter
Band-pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....

 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
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...

 and passband.
  • In baseband transmission, line coding is utilized, resulting in a pulse train or 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: V.29 (EIA/TIA-232), V.35, IEEE 802.3
    IEEE 802.3
    IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control of wired Ethernet. This is generally a local area network technology with some wide area network applications...

    , SONET/SDH.

  • In passband transmission, digital modulation methods are employed so that only a limited frequency range is used in some passband filtered channel. Passband transmission is typically utilized in wireless communication and in passband filtered channels such as POTS
    DSL filter
    A DSL filter is an analog low-pass filter installed between analog devices and a plain old telephone service telephone line, in order to prevent interference between such devices and a digital subscriber line service operating on the same line...

     lines. It also allows for frequency-division multiplexing
    Frequency-division multiplexing
    Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...

    . The digital bit stream is converted first into an equivalent baseband
    Baseband
    In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...

     signal, and then to a RF
    Radio frequency
    Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

     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
    Modem
    A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...

    .

Characteristics


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. This is a consequence of the mathematics
Mathematics
Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

 of Fourier analysis.