passband

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 (not to be confused with band-pass), 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

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 pulse amplitude modulated (PAM) signal. This is typically utilized over non-filtered wires such as fiber optical cables and short-range copper cables, for example serial cables and LAN networks.

• The passband transmission, digital modulation methods are utilized in view to only utilize a limited frequency band over a passband filtered channel. Passband transmission is typically utilized in wireless communication and in passband filtered channels such as the analogue telephone access network. It also allows frequency division multiplex. The digital bit-stream is converted first into an equivalent baseband signal (often a complex-valued signal), and then to a Radio Frequency (RF) signal. On the receiver side a demodulator is used to detect the signal. A combined equipment for modulation and demodulation is called modem.

Overview

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 response^{[citation needed]}. 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'.

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

See also

• Band-pass filter
• Stopband
• 10PASS-TS