Allpass biquadratic switched-capacitor filters

Biquadratic allpass filters are of great importance for performing phase equalization in highly selective filters such as those employed in modems or other telecommunications equipment. This paper develops and examines topologies for implementing the switched-capacitor (SC) allpass biquad including all of the sampled data effects in the design equations. Several of the topologies allow the maximum capacitance ratios to be reduced over existing allpass biquads which makes the implementation of high sections simple and more accurate when integrated. A design example and experimental observations are presented for an allpass filter that is optimized for practicality and incorporated into a 40 pole modem filter chip.