Differential microstrip bandpass filter with dual-band responses using parallel-coupled line structure

In this paper, a novel differential bandpass filter (BPF) with dual-band filtering response is proposed. The equivalent circuits of proposed dual-band filter are illustrated and analyzed in both differential-mode and common-mode. By controlling the electrical length of an open stub in the differential-mode, two passbands operating at the desired frequency ratio can be designed. Also, the spurious response within the passbands can be suppressed by the over-coupling technique. In the common-mode operation, the short-ended meander line enhances the common-mode suppression. From the measurement results, the dual-band BPF operates at 854 MHz and 2.40 GHz, with 3-dB fractional bandwidth 7.17% and 3.71% respectively. A transmission zero is recorded at 1.79 GHz to suppress the spurious passband. The measured common-mode suppression is better than 18.5 dB in two passbands is achieved.