Compact 7–23-GHz Bandpass Filter with High Selectivity and Wide Stopband Using Hybrid Microstrip/SIDGS Scheme for 6G Application

In this paper, a miniaturized 7–23-GHz bandpass filter based on hybrid microstrip/substrate-integrated defected ground structure (SIDGS) scheme is proposed to meet the 6G wireless. Firstly, the via-hole loaded stepped-impedance SIDGS resonator and short-stubs loaded microstrip patch with broadside-coupled scheme allocate the multiple resonances with flexible adjustment. Secondly, a pair of microstrip L-stubs acting as input/output (I/O) feed-lines can enhance the coupling around aforementioned resonances to achieve the broad passband response with spurious suppression. Finally, to further improve the stopband performance, the lowpass units (LPUs) are employed and tapped at the I/O feed-lines. To verify the mechanism mentioned above, a prototype broadband bandpass filter operating at 15 GHz (i.e., center frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{0}$</tex>) with minimum 1.2-dB insertion loss and 3-dB fractional bandwidth (FBW) of 106.7% is implemented and fabricated. The measured stopband extends to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$4.7 f_{0}$</tex> with a rejection level more than 25 dB.