Effects of Termination Impedance and Cell Topology on the Linearity of Gallium Nitride 180° High-Pass/Low-Pass Phase Shifters

Phase shifters (PSs) are fundamental components of phased-array antennas, but they can also be a major contributor to intermodulation distortion. This article investigates how the internal impedance and cell topology of gallium nitride (GaN) 180° PSs affect their linearity. A theoretical analysis, supported by a simplified, yet accurate electrical model of the series FET in the switch and by measured data, demonstrates that selecting an internal impedance other than$50~\boldsymbol {\Omega }$can significantly improve the input-referred third-order intercept point. In addition, the influence of the PS cell topology (T versus$\boldsymbol {\Pi }$) on linearity is examined. To validate the theoretical results, a comparative experimental analysis is carried out, considering both internal impedance and topology. To this aim, we designed and characterized three demonstrator PS cells. Two demonstrators employ the standard 50-$\boldsymbol {\Omega }$impedance but are implemented using a T topology and a$\boldsymbol {\Pi }$topology, respectively. A third demonstrator uses instead a 120-$\boldsymbol {\Omega }$internal impedance, together with a matching network (MN), to prove the analyzed relationship between internal impedance and switch-induced distortion. All demonstrator cells were designed for operation at 10 GHz and manufactured in a 0.12-$\mu $m gate-length GaN HEMT MMIC process. As expected, the best linearity performance is exhibited by the 120-$\boldsymbol {\Omega }$PS, achieving an IIP3 of +47 dBm.