Design of balanced and balun diplexers using stepped-impedance slot-line resonators

A balanced diplexer and a balun diplexer, both designed for the two channels of 2.45-GHz WLAN and 3.55-GHz WiMAX, are proposed with high common-mode (CM) suppression and differential-mode (DM) isolation in this paper. For the balanced diplexer, balanced BPFs for the two desired channels were first designed using stepped-impedance slot-line resonators (SISLRs). Through two T-junctions, the two BPFs were subsequently combined to form a three-balanced-port diplexer having six single-ended ports. Such a balanced diplexer can be easily converted into a balun diplexer with one single-ended and two balanced ports. For that purpose, one of the T-junctions and its associated two feeding lines are removed, and the feeding lines connected to the other T-junction are modified. For both diplexers, the measured CM suppressions (DM isolations) are larger than 54.3 (39.5), 52.2 (44.5), and 40.7 (38.7) dB, respectively, in the lower channel, the upper channel, and the displayed frequency range of 1-6 GHz. A satisfactory agreement has been validated between the simulated and measured results.