Tunable filtering balun with enhanced stopband rejection

For the first time a tunable balun with bandpass response by loading tunable capacitors in the split ring resonators is proposed. By tuning the capacitors´ values, the operating frequency of the balun can be tuned continuously. A shunt stub is employed to realise an additional transmission zero in the higher stopband without any influence on the bandpass response. Theoretical analysis of the filtering balun is conducted and presented. The fabricated balun indicates that the frequency-tuned range of the passband is more than 400 MHz with acceptable insertion loss, and the differences between the two output ports is 180° ± 5° in phase and within 0.5 dB in magnitude in this tuning range.