Arbitrary-Order Balanced BPFs and Common- and Differential-Mode Quasi-Reflectionless BPFs Using Out-of-Phase Coupled DSPSL

A class of novel arbitrary-order balanced bandpass filters (BPFs) and balanced quasi-reflectionless BPFs (QR BPFs) using out-of-phase coupled double-sided parallel-strip lines (DSPSLs) are presented in this article. The proposed out-of-phase coupled DSPSL structure consists of a pair of double-sided parallel-strip transmission lines with opposite phases and resonators set in the center. With this architecture, only differential-mode (DM) signals can excite the resonators, while common-mode (CM) energy has little effect on them. The first designed balanced BPF consists of a bandpass section (N-order coupled-line filter) in the main pass and a pair of reflective bandstop (RBS) sections (M-order coupled $\lambda $ /4 short-circuited resonators) terminated in the bypasses. Furthermore, by terminating the RBS sections with resistive elements, absorptive bandstop (ABS) sections can be formed, which can constitute a complementary duplexer structure together with the bandpass section. Thus, the balanced filters are easily extended to QR BPFs. High-order balanced filters and reflectionless filters are designed by network functions analysis, and detailed design guidance is given. Three balanced prototypes with center frequencies set at 3 GHz are manufactured and measured. Good DM selectivity and CM rejection are achieved in the balanced filter. The balanced reflectionless filters have the advantages of absorption in stopband DM and all-range CM energy, while in-band flatness can be improved by order extension. The experimental features are in good agreement with the theoretical and simulation results.