# 4.2 Gain, phase and group delay distortion

## Gain distortion – filters

#### Most communications hardware – filters, mixers, amplifiers, and so on, and most channel types, cable, fibre, radio, infra red, introduce amplitude distortion into a signal, usually with a frequency-dependent response. Filters, for example, are never perfectly 'flat' in the passband but all exhibit some degree of 'amplitude ripple'. Some filter types such as Elliptic or Chebychev filters have very high passband ripple, but also achieve very fast roll-off. Other filters such as Butterworth or Bessel filters have much less ripple but also much slower roll-off. The raised cosine filter also exhibits passband ripple, depending on the length (number of taps) used in the filter implementation. The degree of ripple can be made arbitrarily small with very long filter length, but at the expense of processing delay and complexity. In practice, the amplitude ripple in a digital raised cosine filter does not significantly degrade modem performance; however, other filtering within the communications system such as the crystal or ceramic filters used in radio IF (intermediate frequency) circuits can exhibit gross levels of amplitude distortion and this effect cannot be ignored. |