Compact Chebyshev Differential-Mode Bandpass Filter on$lambda$/4 CPS Resonator With Intrinsic Common-Mode Rejection

摘要

In this paper, a new class of compact Chebyshev differential-mode (DM) bandpass filters with intrinsic common-mode (CM) rejection is proposed and demonstrated onn$lambda$n/4 coplanar stripline (CPS) resonators with an improved unloaded quality factor (n$Q_{u}$n). The distinct advantage of the proposedn$lambda$n/4 CPS resonator with highern$Q_{u}$nthan slotline resonator, together with intrinsic CM rejection of CPS line, can demonstrate thatn$lambda$n/4 CPS resonator is more suitable for the design of narrowband DM filter than others on slotline resonator. Then, the underlying physics why CPS resonator possesses highern$Q_{u}$nthan slotline resonator is numerically studied and further interpreted by the radiation loss. To facilitate the design process, an analytical design approach is introduced by using alternative J/K inverters and a tailored method is used to extract J/K inverter values with their associated effective electrical lengths based on their equivalent circuit models. Finally, two design examples, a second-order and a fourth-order DM Chebyshev bandpass filter, are designed and implemented, operating at the center frequency of 1.8 GHz with the fractional bandwidths of 6% and 19%, respectively. The simulated and measured DM/CM frequency responses match well with each other. The proposed DM filters based onn$lambda$n/4 CPS resonators have demonstrated the claimed superior performances in terms of intrinsic CM rejection, improved unloaded quality factor, easy implementation of J/K inverter, inherent suppression of first DM harmonic, and compact size.