Proposal and Efficient Design of Wideband Balanced Reflectionless Filters With Intrinsic Common-Mode Suppression

Balanced reflectionless filters have been extensively studied to meet the stringent noise constraints of modern communication systems. This article introduces a highly efficient design method for balanced reflectionless filters that avoids the complexity of integrating multiple modules into conventional filters. The method focuses on developing absorptive circuits and their balanced structures to leverage the intrinsic common-mode suppression of the balanced transmission lines. Notably, the composite absorptive stubs (CASs), which have the absorptive properties and composite structures on coplanar stripline (CPS) and double-sided parallel-strip line (DSPSL), demonstrate their advantages in terms of circuit and structural design. The symmetrical filters, assembled flexibly by CASs, are equivalent to bandpass transmission-line circuits with out-of-band absorption, and implemented with resistors on the balanced strips. High-order prototypes can be achieved by adding a few connecting-line sections and are suitable for wideband operation. For the experiment, the prototype, which utilizes the microstrip-to-CPS transitions at the absorptive CPS ports, demonstrates differential out-of-band absorption and common-mode suppression through simulation. Furthermore, the design table, curves, and equations based on the wideband synthesis are provided to determine all the parameters according to the desired working frequency band and out-of-band impedance matching. The design process is in final evidently validated by the fabricated prototype and measurement results.