Bandwidth is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context. Passband bandwidth is the difference between the upper and lower cutoff frequencies of, for example, an electronic filter, a communication channel, or a signal spectrum. In case of a lowpass filter or baseband signal, the bandwidth is equal to its upper cutoff frequency. The term baseband bandwidth refers to the upper cutoff frequency. Bandwidth in hertz is a central concept in many fields, including electronics, information theory, radio communications, signal processing, and spectroscopy.
In computer networking and other digital fields, the term bandwidth often refers to a data rate measured in bits per second, for example network throughput. The reason is that according to Hartley's law, the digital data rate limit (or channel capacity) of a physical communication link is related to its bandwidth in hertz, sometimes denoted frequency bandwidth, analog bandwidth or radio bandwidth. For bandwidth as a computing term, less ambiguous terms are bit rate, throughput, maximum throughput, goodput or channel capacity.
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Bandwidth is a key concept in many telephony applications. In radio communications, for example, bandwidth is the frequency range occupied by a modulated carrier wave, whereas in optics it is the width of an individual spectral line or the entire spectral range.
In many signal processing contexts, bandwidth is a valuable and limited resource. For example, an FM radio receiver's tuner spans a limited range of frequencies. A government agency (such as the Federal Communications Commission in the United States) may apportion the regionally available bandwidth to licensed broadcasters so that their signals do not mutually interfere. Each transmitter owns a slice of bandwidth, a valuable (if intangible) commodity.
For different applications there are different precise definitions. For example, one definition of bandwidth could be the range of frequencies beyond which the frequency function is zero. This would correspond to the mathematical notion of the support of a function (i.e., the total "length" of values for which the function is nonzero). A less strict and more practically useful definition will refer to the frequencies where the frequency function is small. Small could mean less than 3 dB below (i.e., less than half of) the maximum value, or more rarely 10 dB below, or it could mean below a certain absolute value. As with any definition of the width of a function, many definitions are suitable for different purposes.
For analog signals, which can be mathematically viewed as functions of time, bandwidth, BW or Δf is the width, measured in hertz, of the frequency range in which the signal's Fourier transform is nonzero. Because this range of nonzero amplitude may be very broad, this definition is often relaxed so that the bandwidth is defined as the range of frequencies where the signal's Fourier transform has a power above a certain amplitude threshold, commonly half the maximum value, or −3 dB.^{[1]} This same "half attenuation" convention is also used in spectral width, and more generally for extent of functions as full width at half maximum (FWHM).
The word bandwidth applies to signals as described above, but it could also apply to systems, for example filters or communication channels. To say that a system has a certain bandwidth means that the system can process signals of that bandwidth.
A baseband bandwidth is synonymous to the upper cutoff frequency, i.e. a specification of only the highest frequency limit of a signal. A nonbaseband bandwidth is a difference between highest and lowest frequencies.
As an example, the (nonbaseband) 3dB bandwidth of the function depicted in the figure is , whereas other definitions of bandwidth would yield a different answer.
A commonly used quantity is fractional bandwidth. This is the bandwidth of a device divided by its center frequency. E.g., a device that has a bandwidth of 2 MHz with center frequency 10 MHz will have a fractional bandwidth of 2/10, or 20%.
The fact that real baseband systems have both negative and positive frequencies can lead to confusion about bandwidth, since they are sometimes referred to only by the positive half, and one will occasionally see expressions such as B = 2W, where B is the total bandwidth, and W is the positive bandwidth. For instance, this signal would require a lowpass filter with cutoff frequency of at least W to stay intact.
The 3 dB bandwidth of an electronic filter is the part of the filter's frequency response that lies within 3 dB of the response at its peak, which is typically at or near its center frequency.
In signal processing and control theory the bandwidth is the frequency at which the closedloop system gain drops 3 dB below peak.
In basic electric circuit theory when studying Bandpass and Bandreject filters the bandwidth represents the distance between the two points in the frequency domain where the signal is of the maximum signal amplitude (half power).
In photonics, the term bandwidth occurs in a variety of meanings:
A related concept is the spectral linewidth of the radiation emitted by excited atoms.
