Design of a new balun bandpass filter with single-band balance and dual-band filtering characteristics

In this article, a new type of balun bandpass filter with single-band balance and dual-band filtering function is presented. For this purpose, a novel compact dual-mode open loop resonator with short-circuited stubs and interdigital capacitors is proposed. Two identical resonators are coupled to two output ports in order to obtain 180 degrees phase difference between the output ports. By means of the changes in the short-circuited stubs and interdigital capacitors, control of the passbands at two output ports can be achieved. In addition, as the short-circuited stubs are located at the same wave traveling paths, phase difference cannot be obtained in the first passband and only the second passband has balun function. The designed balun bandpass filter was fabricated and tested for the experimental verifications. Phase difference and magnitude imbalance have been measured within 180 degrees +/- 2 degrees and 0.5 dB, respectively. The measured results exhibit a very good agreement with the predicted results.