Compact dual-mode balun bandpass filter with improved upper stopband performance

A compact dual-mode balun bandpass filter with improved upper stopband performance is proposed by loading a quasi-T shaped open stub on a square ring resonator. By stretching the length of the loaded open stub, one of the second-order degenerate modes moves down quickly and builds up a dual-mode passband together with one of the first-order degenerate modes. Owing to the long electrical length of the open stub, more phase oscillations between the input and output ports are introduced, thus generating one more transmission zero above the passband. Measured results confirm both a good in-band filtering balun and improved upper-stopband performance.