A 200–325-GHz wideband, low-loss modified Marchand balun in SiGe BiCMOS technology

A modified version of the Marchand balun based on three folded symmetric coupled lines is presented in this paper. The S-parameters of a quasi-TEM coupler are derived in the case when even- and odd- mode phase velocities are unequal. The effects on the performance of the Marchand balun are then investigated by analyzing its calculated S-paramaters under different phase-velocity-ratios of both the modes. Measurements show a minimum insertion loss of 2.3 dB, and a 1-dB insertion loss bandwidth from 200-325 GHz. The amplitude imbalance is less than 1.5 dB upto 325 GHz, while the phase imbalance is below 10 deg upto 273 GHz. Through out the operating range the input return loss remains higher than 15 dB. The balun is fabricated in a 130nm SiGe BiCMOS process and occupies only 0.015mm2, excluding the pads.