Measurement technique for the extraction of differential S-parameters from single-ended S-parameters

With the increasing implementation of communication devices and systems in the RF/low microwave ranges, along with the usage of differential signaling schemes, it has become necessary to develop practical methods to suitably characterize the operation of these devices. This paper describes a method of determining the four-port differential S-parameters of such networks with the use of two-port vector network analyzer (VNA) by the use of various terminating load combinations. With the aid of a general equation the single-ended S-parameters obtained using this technique can be readily manipulated to obtain the differential S-parameters. The device under test (DUT) chosen to validate this technique is a pair of coupled microstrip lines designed to each have an uncoupled characteristic impedance of 55 Ω and a differential impedance of 110 Ω at 1 GHz.