Single-Ended and Balanced Bandpass Filters Using Multiple Pairs of Coupled Lines and Stepped-Impedance Stubs

Two different types of three pairs of coupled lines (TPCL) structures with bandpass and all-stop filtering (ASF) characteristics, respectively, are proposed and analyzed through impedance matrix deduction and input admittance calculation. Combined with the stepped-impedance stubs, high-selectivity single-ended and balanced filters using TPCL structures have been presented. Five transmission poles (TPs) within the passband and six deep transmission zeros (TZs) from 0 to 2f(0) (f(0): passband center frequency) can be realized in both of single-ended and balanced filters. For demonstration, two sharp roll-off single-ended and balanced filter prototypes with good in-band characteristics (insertion losses and return losses) and high common-mode suppression are designed, whose measured 3-dB fractional bandwidths are of 35% and 34.4% (differential mode), respectively. Good agreement between the simulations and measurements validates the design concept.