A Novel Filtering Balun and Improvement of Its Isolation Performance

A novel filtering balun is proposed in this letter. The filtering balun consists of three half-wavelength uniform-impedance microstrip resonators. Each resonator is coupled to external port through tapped line. Two resonators (called output resonators) connected to balanced output ports are in parallel coupled to the third resonator (called input resonator) connected to the unbalanced input port. The coupling locations have equal-magnitude and out-of-phase voltages, which provides balanced output signals. To improve the isolation performance, a microstrip stub is used to introduce coupling between the output resonators and an isolation resistor is added between the symmetrical position of the microstrip stub and the ground. The isolation resistor can obviously improve isolation performance and has no effect on the balanced output signals. The design is discussed in detail and a prototype filtering balun with center frequency of 2.4 GHz is designed, fabricated, and measured. The measured results have good agreement with the simulated ones, which confirms the novel filtering balun and the design method.