A low-loss and compact three-conductor balun with compensated artificial asymmetric transmission line in InP technology

This paper presents a low-loss and compact three-conductor balun utilizing transmission lines (T-Lines) in indium phosphide (InP) technology. A shorted-stub in the form of an artificial asymmetric T-Line (TL_a) is employed to enhance amplitude and phase consistency by mitigating the frequency dispersion in dielectric materials. The slotted-ground configuration broadens the operating bandwidth and indirectly contributes to the overall compact footprint. Electromagnetic (EM) simulation results indicate that the 1-to-4 balun achieves an amplitude imbalance of less than 0.25 dB and a phase imbalance below 3.5° across 110–150 GHz. Measured insertion loss (IL) for the 1-to-4 balun configuration ranges from 0.1 to 0.5 dB over 115−150 GHz. Measurements of multiple back-to-back (B2B) structures exhibit IL below 3 dB, consistent with simulation results within the D-band (110−170 GHz). The balun exhibits a measured return loss exceeding 10 dB from 110 GHz to 162 GHz, with a compact size of 380μm × 95μm, making it well-suited for monolithic microwave integrated circuits (MMICs).