Compact Wideband Differential Bandpass Filter Using Coupled Microstrip Lines and Capacitors

A compact wideband differential bandpass filter (BPF) is achieved by loading two pair of capacitors at different positions of the half-wavelength-coupled microstrip line. The capacitances and loaded positions of capacitors combining with the even-odd-mode impedance of the coupled microstrip line can produce and control the three differential-mode (DM) transmission poles (TPs), four DM transmission zeros (TZs), and three common-mode (CM) TZs. Comparing with the state-of-the-art wideband differential BPFs, the proposed design has the advantages of compact size and low insertion loss. A prototype with the size of 51 mm x 4 mm (0.5 lambda(g) x 0.04 lambda(g), lambda(g) is the microstrip guided wavelength at the center frequency f(0)) is designed for demonstration, which exhibits the 3-dB fractional bandwidth of 105% centered at 1.8 GHz and the minimum insertion loss of 0.16 dB.