A Method for Broad-Band Matching of Microstrip Differential Phase Shifters

Meander-line phase shifters in microstrip are not well matched, because of the phase-velocity difference between the odd and the even mode in the coupled-line region. A stepped impedance design is introduced which allows one to realize, for example, a quarter-wavelength 90° Schiffman phase shifter with three sections, having, theoretically, a return loss better than 30 dB over a 1:5 bandwidth. Design equations are given and are confirmed by measurements on microwave integrated circuits in the frequency range 2-10 GHz.