Bessel Filters

Bessel filters are optimized for maximally-flat time delay (or constant group delay). This means that they have linear phase response and excellent transient response to a pulse input. This comes at the expense of flatness in the pass-band and rate of roll-off. The Bessel filter is only available with low pass selectivity.

Group delay measures how long it takes a signal to traverse a network or its transit time. Constant group delay in the pass band, well into the transition band, indicates a very good step response. Group delay with peaking indicates overshoot and ringing. The group delay of a filter is nearly proportional to its order. Also, filter group delay is inversely proportional to filter bandwidth (small percentage bandwidth filters have a large group delay). From the Bode plot shown in Figure 1, it shows that the attenuation beyond the –3 dB cut-off frequency is not as steep as Butterworth.