Novel Unbalanced-to-Balanced Quad-Channel Diplexer With Controllable Bandwidth and High Isolation

A novel unbalanced-to-balanced (UBTB) quad- channel diplexer consisting of two balun filters sharing a common single-ended microstrip feeder is proposed. By loading a stepped-impedance quad-mode resonator (SIQMR) into the balun filter, not only each mode can be flexibly adjusted, but also the size of the circuit can be reduced. In addition, motivated by the internal hybrid coupling, two adjacent modes are utilized to form a passband filter with a transmission zero. On the basis of which, two channels with flexibly adjustable bandwidth and good frequency selectivity are implemented on Rx or Tx path. Finally, a U-shaped microstrip slot-line structure is utilized as the balanced port to achieve good common-mode (CM) suppression. To verify the proposed diplexer, a quad-channel prototype is designed, fabricated and tested. Measured results of the fabricated circuit agree well with the simulated ones.