A Design of Balun Bandpass Filter for Wide Stopband Attenuation Base on Stepped Impedance Resonators

This paper presents a design of microstrip balun bandpass filter (BPF). Since the quarter-wavelength (λ/4) stepped impedance resonators (SIRs) in the proposed balun BPF are oppositely coupled to the main line of open-circuited half-wavelength (λ/2) microstrip transmission line, the same magnitude and out-of-phased signals can be obtained at two output ports of the balun BPF. The balun BPF is designed with two stage SIRs to achieve wide stopband attenuation. The balun BPF operated at 2 GHz with fractional bandwidth of 5% Chebyshev response was designed, fabricated, and measured. The measured magnitude and phase imbalances within the passband of 1.95 to 2.05 GHz are obtained ±0.3 dB and 180 ± 3.5° respectively. The measured input return loss is higher than 17 dB within the same passband. Indeed, the stopband attenuation is higher than 25 dB from DC to 1.72 GHz of the lower stopband and from 2.48 GHz to 6.58 GHz of the higher stopband.