Contents I. Introduction
Differential phase shifters are four-port passive microwave devices providing, in the specified bandwidth, a constant phase difference between the signals at their output ports. Ideally, such devices should produce proper phase shift values, exhibit relatively small attenuation, and operate effectively over a wide frequency band. They are used, for example, in wideband phased-array antennas and find application in various microwave equipments and measurement systems. In principle, broadband phase shifters mainly use coupled lines sections. Schiffman [1] was the first who applied such a section to design an octave 90° differential phase shifter. This configuration included a reference transmission line and the two edge coupled striplines that were connected together at one end. When this circuit is fabricated in microstrip technology [2], [3], its operation deteriorates. The operation bandwidth decreases and deviation of the phase-shift characteristic increases. These deteriorations are due to unequal phase velocities of the even and odd modes in the coupled microstrips. In recent years, one can find papers describing the design of modified phase shifter circuits with broader bandwidth, acceptable insertion losses, and low phase-shift deviation. These requirements are met in the circuits of the phase shifters proposed in [1]–[11], which used different arrangements of the edge coupled microstrip sections. In [1]–[4], the main circuit is designed as a cascade of the coupled sections, while in [6]–[8], the series or mixed connections are applied. These configurations provide acceptable phase-shifter performance over a wide bandwidth greater than 100%. However, the insertion losses, as well as the size of their circuits, increase with an increasing number of the interconnected sections.
