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New for February 2006! In April 1958 the IRE MTT published a paper by Bernard M. Schiffman titled A New Class of Broadband Microwave 90 Degree Phase Shifters. Schiffman's contribution to phase shifter science was that the phase difference between a quarterwave coupled section, compared to a 3/4 wave straight section, would provide a nearly flat 90 degree phase differential. Schiffman went on to publish a second article extending the analysis to multi-section coupled lines. We've added Bernard Schiffman to the Microwave Hall of Fame!
Although Schiffman's original analysis was for a three-port network, we prefer to think of a Schiffman phase shifter as a fourport network to which you can add switches to achieve a "digital" phase shift, or cascade it with a Wilkinson power divider to achieve a very broadband 90 degree coupler (better than a branchline!)
Schiffman basics
Below is a picture of two paths of the Schiffman 90 degree phase shifter:
Here's the phase difference that is calculated by MeasEqn "Meas1". There is only about ten degrees of error over xxx bandwidth.
Schiffman 90 degree power divider
This is a great application of the Schiffman technique. You can get performance that exceeds the bandwidth of a branchline quadrature hybrid, although you need a little more area.