Using folded open-loop ring resonator to design a common-mode suppression and frequency adjustable balun-bandpass filter

A compact and simple balun-bandpass filter was investigated by using a pair of half-wavelength open-loop ring resonators (OLRRs) and two microstrip lines. The balun-bandpass filter could be designed easily with a different center frequency by changing the path length of OLLRRs. The design methodology was adopted based on EM simulation to obtain these optimally designed parameters of the OLRRs, and microstrip lines and the filters were fabricated on FR4 substrate. The filters with the frequencies of 2.6 GHz and 5.2 GHz were designed to prove the characteristic of frequency adjustable and they were fabricated with the properties of wide bandwidth and low insertion loss. The balun-bandpass filters presented an excellent inband balanced performance with common-mode rejection ratio over than 40 dB and 34 dB in the passbands of 2.6 GHz and 5.2 GHz, respectively. Good correlation was seen between simulation and measurement, and the result showed that first run pass had been achieved in the majority of our design.