Differential bandpass filter with high common-mode rejection ratio inside the differential-mode passband using controllable common-mode transmission zero

Controllable common-mode transmission zero is proposed to improve the common-mode rejection ratio (CMRR) inside the differential-mode passband of differential bandpass filters. With the controllability of the common-mode transmission zero, the common-mode transmission zero can be moved into the frequency range of the differential-mode passband, and then the CMRR inside the differential-mode passband can be greatly improved. Two two-order filters using different resonators are designed to demonstrate the existence and controllability of the common-mode transmission zero. A four-order filter with two resistor-loaded stepped-impedance resonators (SIRs) and two resistor-loaded uniform impedance resonators (UIRs) are designed to get good selectivity of differential-mode passband and wide-band common-mode suppression besides the high CMRR inside the differential-mode passband. The design methodology and experimental results of the proposed filters are presented.