Highly Selective Differential-Mode Wideband Bandpass Filter for UWB Application

A novel highly selective wideband differential-mode bandpass filter (BPF) with good common-mode suppression is proposed on microstrip line for ultra-wideband (UWB) application. The initial BPF is similar to a two-stage branch-line structure in geometry and its improved one is formed by stretching the two vertical arms on the input and output ports from λ_g /4 to 3 λ_g /4. The aim is to achieve better filter selectivity by adding two additional transmission zeros from the extended arms. In our design, a symmetrical filter with two pairs of external ports is firstly decomposed into two bisections with two single ports under differential- and common- mode excitations. Our study shows that the two-port bisection circuits achieve bandpass and bandstop filtering performances under the differential- and common-mode operation in the lower UWB band, i.e., 3.1-5.1 GHz. The wideband differential-mode BPF is then designed and fabricated. All the three sets of results from the simulations based on the circuit model, fullwave software, and actual measurement are found in good agreement with one another. A highly selective differential-mode passband is achieved in the frequency range of 2.8-5.3 GHz, with a common-mode suppression of about 20 dB.