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Bandwidth is the width, usually measured in hertz, of a frequency band f2 - f1. It can also be used to describe a signal, in which case the meaning is the width of the smallest frequency band within which the signal can fit.

It is usually notated B, W, or BW. 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 bandwidth of an electronic filter is the part of the filter's frequency response that lies within 3 dB compared to the center frequency of its peak.

In signal processing and control theory, the bandwidth is the frequency at which the closed-loop system gain drops to -3 dB.

In discrete time systems and digital signal processing, bandwidth is related to sampling rate according to the Nyquist-Shannon sampling theorem.

The term bandwidth is also used, informally, and by extension from the above, to mean the amount of data that can be transferred through a connection in a given time period. In such cases, bandwidth is usually measured in bits or bytes per second. Bandwidth is normally based on the frequencies used and the spectral spread of the information carried on the frequency. The maximum digital bandwidth for a given analog channel is determined by the Shannon-Hartley theorem. Typical digital communications now come close to the entire possible bandwidth.