A function ( for example an electrical signal ) is band-limited, if it does not contain frequency components above a certain frequency. The band limitation is the frequency spectrum, all frequencies above the cutoff frequency of zero. In practice, a band-limited signal obtained by filtering with a low- or band-pass.
For signals in the baseband in this case, the lower limit frequency are filtered by a low pass filter. Real signals at baseband always exhibit negative frequency components, as exemplified in the illustration of the frequency response of a real-valued band-limited with the cut-off frequency B signal.
For signals in a so-called band-pass position, the band limitation is performed by a bandpass filter. Signals in bandpass situation arise, for example by modulating a baseband signal. They occur in radio receivers, among others, in the intermediate frequency circuit. Band-limited signals in the band-pass situation are characterized in that the lower cut-off frequency has a positive value.