A Balanced Wideband Filtering Phase Shifter With High Frequency Selectivity

ABSTRACT A balanced wideband filtering phase shifter is achieved by utilizing a cascaded structure composed of two half‐wavelength transmission lines, two T‐type short‐circuited stub‐loaded transmission lines, and a ring structure constructed by a capacitor and a terminal short‐circuited coupled line. Four differential‐mode (DM) transmission zeros (TZs), five DM transmission poles (TPs) and five common‐mode (CM) TZs can be obtained by controlling the capacitance of the capacitor, the impedance of the short‐circuited stub, and the even‐odd‐mode impedance of the coupled microstrip lines. Thus, high frequency selectivity, wideband DM impedance matching, and CM suppression, as well as consistent filtering passbands for different phase shifts, can be achieved. In comparison to the existing balanced filtering phase shifters, the proposed design simultaneously obtains wider operating bandwidth and higher frequency selectivity. A comprehensive theoretical analysis and design procedure are conducted. Two prototypes with a center frequency of 1 GHz are designed, featuring a fractional operating bandwidth of 71% for 45° ± 3.5° and 90° ± 5° phase shifters, along with CM suppression fractional bandwidths of 200% and 180%, respectively.