Filters are two-port networks with a frequency selective behaviour. A filter shall reject (or attenuate) signals in the stop band while signals in the pass band are unaffected. Figure 6.17 shows four basic filter types with distinctive pass and stop band schemes: low-pass filter, high-pass filter, band-pass filter, and band-stop filter [3, 7].
The desired frequency response (maximum attenuation in the pass band and minimum attenuation in the stop band ) is graphically displayed in tolerance schemes. Pass and stop band frequency limits are given by cut-off frequency fc and stop band frequency fS. The filter transfer function is represented by the transmission coefficient s21 or the insertion loss (equal to attenuation A).
In order to ensure low distortions in the pass band the phase of the transfer function should be as linear as possible. The group delay as the derivative of the phase should be constant .