Design of differential-mode wideband bandpass filters with wide stop band and common-mode suppression by means of multisection mirrored stepped impedance resonators (SIRs)

We present balanced microstrip wideband bandpass filters that simultaneously suppress the common-mode within the differential filter pass band and reject the differential-mode over a wide band above the pass band of interest. These filter characteristics are achieved by using multisection mirrored stepped impedance resonators (SIRs) transversally oriented to the filter axis and coupled through admittance inverters. Each mirrored SIR acts as a pair of independent shunt-connected series resonators for the common-mode. One of such resonators (the central one) is tuned through the patch capacitance in order to suppress the common-mode by providing a transmission zero for that mode within the differential filter pass band. For the differential-mode, the capacitances of the central resonators are virtually grounded, and the resulting inductances are determined according to the required differential-mode response. For the external resonators, the patch capacitances contribute to the differential-mode response and, combined with the inductors, provide transmission zeros useful to enhance the differential-mode stop band. The design of an order-5 quasi-Chebyshev prototype balanced filter with central frequency 2.4 GHz, 40% fractional bandwidth and 0.2 dB ripple is reported.