Frequency-Tunable Constant-Absolute-Bandwidth Single-/Dual-Passband Filters and Diplexers With All-Port-Reflectionless Behavior

An approach to design RF all-port-reflectionless quasi-elliptic-type single-/dual-band bandpass filters (BPFs) and diplexers with frequency-adaptive constant-absolute-bandwidth (ABW) response is reported. The tunable reflectionless BPF case is first addressed, where a balanced-circuit scheme composed of two identical wideband 3-dB quadrature couplers and two equal reflective-type BPF units with synchronous center-frequency agility is exploited. The input signal is split into the two BPF units with equal amplitude and 90° phase shift by the first coupler and then recombined at the output node of the second coupler after being processed by the balanced-circuit branches. In this manner, the signals reflected at the stopband regions of the BPF units are mutually canceled at the coupler input port and absorbed at the isolated port by its loading resistor. This allows a symmetrical low-reflection behavior to be attained in the overall circuit, whose filtering transfer function is the one determined by the BPF units. Furthermore, by using dual-band BPFs in the balanced-circuit branches, this concept is readily extended to frequency-tunable constant-ABW dual-band BPFs whose passbands can be separately switched OFF (i.e., absorptive switched-OFF states) as an added reconfiguration capability. For practical-demonstration purposes, starting from theoretical design guidelines followed by an optimization step, the third- and second-order microstrip prototypes of reflectionless frequency-controllable single- and dual-band BPFs with predefined constant ABW, respectively, are developed and tested. Afterward, inspired by the devised tunable absorptive BPF structure, an all-port-reflectionless diplexer with frequency-adjustable constant-ABW channels is shown. This controllable multiport-absorptive filtering device is also verified with the fabrication and testing of a microstrip prototype. All the constructed circuits employ varactor diodes for electronic tuning, showing close agreement between simulation and measurements.