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

In this paper, a new class of compact Chebyshev differential-mode (DM) bandpass filters with intrinsic commonmode (CM) rejection is proposed and demonstrated on λ/4 coplanar stripline (CPS) resonators with an improved unloaded quality factor (Qu). The distinct advantage of the proposed λ/4 CPS resonator with higher Qu than slotline resonator, together with intrinsic CM rejection of CPS line, can demonstrate that λ/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 higher Qu than 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 on λ/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.