A New CFOA-Based Shadow Analog Comb Filter

A novel electronically tunable shadow comb filter with N notches is presented. The proposed filter is built around the commercially available current-feedback operational-amplifier (CFOA). The bandwidth of each notch can be independently controlled, without disturbing the gain of the comb filter and the notch frequency, by adjusting the gain of an external voltage amplifier built around a CFOA. The gain of this CFOA-based voltage amplifier is controlled by adjusting a variable grounded resistance. This grounded resistance can be implemented, if desired, using an operational transconductance amplifier (OTA) configured as a grounded resistance. The proposed shadow comb filter enjoys the attractive feature of low output impedance. This would facilitate cascading of comb filters to obtain any number of notch frequencies. Experimental results obtained from a shadow comb filter with 4 notch frequencies using the AD844 CFOA are presented. These results confirm the functionality of the proposed comb filter both for low as well as high notch frequencies.