Scattering parameter characterization of differential four-port networks using a two-port vector network analyzer

A common problem faced in the areas of packaging, signal integrity and interconnect technology is the inability to adequately characterize networks incorporating differential signaling schemes. The main challenge faced by such a need is the lack of any instrument capable of simultaneously stimulating dual input ports with signals synchronized with each other and then accurately determining the power waves entering and exiting the network, i.e., a four-port vector network analyzer (VNA) with dual synthesized frequency sweepers. This study proposes a low cost method of characterizing differential networks with existing two-port VNAs by using a technique of measuring the scattering parameters (s-parameters) of two ports at a time while the other two are appropriately terminated, in this work with standard 50Ω terminations. The device under test (DUT) chosen is a pair of symmetrical coupled microstrip lines, designed for operation at 1 GHz. The single-ended s-parameters obtained are converted to the required differential s-parameters, using appropriate linear combinations, and these results are compared with two sets of simulation data from electromagnetic field solver softwares. The port impedances of the microstrip lines, for the differential and common-mode cases, from the measurements as well as the simulations are at most 8.35% from the theoretical values. The frequency range addressed in this paper is up to 3 GHz.