Hybrid Microstrip T-Stub/Defected Ground Structure Cell for Electromagnetic Interference Bandpass Filter Design

In this paper, the hybrid microstrip T-stub/defected ground structure (HMT/DGS) cell is presented, which is composed of a microstrip T-stub and an interdigital DGS with the broadside-coupled (BC) transition. Finely adjusted resonance can be achieved by the proposed cell. Meanwhile, strong slow-wave effect is employed by the structure, which leads to wide stopband responses. Based on these HMT/DGS cells, the interdigital-coupled scheme is employed to implement the electromagnetic interference (EMI) bandpass filter with high performance. To verify the aforementioned mechanism, two filters are designed and fabricated. Specifically, the second-order bandpass filter with a fractional bandwidth (FBW) of 14.3% has a 29.1-dB stopband rejection up to 15 f₀ (where f₀ stands for the passband center frequency). The fourth-order bandpass filter with an FBW of 10.8% demonstrates a 30.1-dB stopband rejection up to 12.1f₀. Both types of filters show compact sizes with low insertion losses less than 1.1 dB.