An Ultra-Compact Common-Mode Bandstop Filter With Modified-T Circuits in Integrated Passive Device (IPD) Process

Based on multi-cell modified-T circuits, this paper proposes a novel circuit topology to realize a common-mode bandstop filter with differential-mode all-pass characteristics for high-speed digital differential circuits. The circuit behaviors for both differential and common modes are analyzed based on symmetrical network analysis techniques. Two transmission zeros at common mode can be synthesized by the derived formulas. The proposed design flow is used to realize a test sample in the integrated passive device process. Simulated and measured results are in good agreement. The common-mode suppression band (vertical bar S-cc21 broken vertical bar < -10 dB) is from 1.6 to 4.7 GHz, whose fractional bandwidth is nearly 100%. The cutoff frequency of the differential mode can maintain up to 10 GHz with a constant group delay. Moreover, eye diagrams under different signaling rates all perform well. Compared with the other literature, the proposed common-mode filter has a very wide common-mode stopband, the highest differential-mode cutoff frequency up to 10 GHz, and the most compact circuit size of 0.017 lambda(g) x 0.016 lambda(g).