Digital phase bit for microwave operation

Abstract

A digital phase bit is provided for microwave operation, comprising a pair of FET switches and at least three transmission lines. The FETs when operated in a digital switching mode, present a small impedance when on and a high impedance when off. Each of two of the transmission lines exhibits a series inductive impedance over the operating frequency band and shunts a FET switch, two shunt combinations being interconnected by the third transmission line. When the switches are on, the signal path is effectively through the FET switch alone (and not branched) and a reference phase shift is produced. When the FET switches are off, a signal applied to the phase bit branches at each shunt combination. The inductive reactance of the transmission line and the capacitive reactance of the FET switch of each shunt combination then jointly produce a resonantly enhanced reactance over the band, causing a reflection and a maximum differential phase shift. The reflections are cancelled at the input port by a suitable choice of length and impedance for the third transmission line. The phase bit is suitable for monolithic fabrication on a common semiconductor substrate and is bidirectional when symmetrical FET switches are used.