A Wideband Differential Bandpass Filter Based on T-shaped Stubs and Single Ring Resonator

A wideband differential bandpass filter (BPF) with differential mode passband and common mode suppression is proposed and implemented on microstrip lines for wideband application in this letter. The initial BPF is similar to a single ring resonator with two unequal feed lines which have 180◦ separation and T-shaped stubs are loaded on the ring resonator to form an improved one for better performances. The lengths and widths of these stubs can be adjusted to produce a highly selectivity under the differential mode and improved attenuation under the common mode. This simple, compact structure is easy for construct without any coupling structure. Finally, a microstrip differential wideband BPF is designed, simulated, fabricated, and measured. The presented differential BPF has a 3dB fractional bandwidth (FBW) of 38% for the differential mode and insertion loss greater than 17 dB for common mode. Good agreement between simulated and measured results is obtained.