Miniaturized Stepped Impedance Transmission Lines for D-Band Wideband Power Divider With Isolation Capacitor

In this letter, a broadband Wilkinson power divider (WPD) with small size and low loss using round-shaped stepped impedance transmission lines (RS-SITLs) is proposed in both differential and single-ended structures. Miniaturization was achieved through the SITLs with an electrical length smaller than 90°. The insufficient length for odd-mode matching is addressed by introducing an isolation capacitor. Physical parameters are determined considering feasible characteristic impedances through respective mode analyses. Chips are fabricated using a 40-nm RF CMOS process, resulting in a 25% reduction in area compared to the conventional WPD with a $\boldsymbol{\lambda }$ /4 transmission line, with a core size as small as $0.002~\lambda ^{2}$ . In the 110–170-GHz band, the proposed single-ended and differential SITL WPDs, respectively, have low-insertion losses (ILs) of 0.91 and 0.69 dB and high isolations (ISOs) of 14.7 and 15.3 dB.