Constant Phase Delay filter is

An active filter is a type of analog circuit implementing an electronic filter using active components, typically an amplifier. Amplifiers included in a filter design can be used to improve the cost, performance and predictability of a filter.

Types of active filters. 

• Butterworth 
• Chebyshev 
• Bessel
• Elliptic filters.

Bessel Filter:

• The Bessel filter provides ideal phase characteristics with an approximately linear phase re­sponse upto nearly cut-off frequency.
• Though it has a very linear phase response but a fairly gentle skirt slope.
• For applications where the phase characteristic is important, the Bessel filter is used.
• It is a minimal phase shift filter even though its cut-off characteristics are not very sharp. Hence it is known as a Constant Phase Delay filter.
• It is well suited for pulse applica­tions.