## Phase distortion – filters

#### Just as filters introduce amplitude ripple in the passband, many also have phase variations across the passband and in the transition band. The effect of a non-flat phase response in a filter is to cause the various frequency components that make up the data signal to undergo slightly different amounts of phase shift. If the phase response is not flat, or does not increase linearly with frequency (termed a linear phase response), distortion will be introduced into the time waveform of the data pulse or symbol as shown here. If the component has a linear phase response, this translates into a fixed time delay of the signal passing through it, but otherwise the signal will not be distorted. |

#### Again, some filters have better phase responses than others, with the Bessel filter having a very good near linear phase response with frequency, and the Elliptic filter having a very poor response. Digital filters, on the other hand, can be implemented with perfectly linear phase response – so-called linear phase filters and hence it is possible to implement a raised cosine filter with no phase distortion using these techniques. (Note, not all digital filters have a linear phase response.)