A non-contacting sampled-line reflectometer for microwave scattering parameter measurements

A proof-of-concept non-contacting reflectometer is presented in this paper. The reflectometer utilizes a novel planar probing structure for sampling traveling waves and is based on the well known "sampled-line-reflectometer" architecture pioneered by Williams. Compared to coaxial probe methods that are commonly used for non-contacting measurements, the planar probing structure described in this work has the advantages of (1) simple fabrication, (2) precise probe positioning control and (3) capability of being scaled to the millimeter-wave band. To assess the performance of the reflectometer, scattering parameter measurements have been performed on a variety of different loads (namely, high, medium and low reflection loads) over an octave bandwidth from 0.75 GHz to 1.5 GHz. The s-parameters derived from these reflectometer measurements are compared with those obtained from a commercial HP 8720C network analyzer and show good agreement for medium and low-reflection loads, with measurement discrepancy less than 2%. However, measurement errors can be as large as 13% for high reflection loads. This can be attributed, in part, to the five-port "sampled-line" architecture adopted in this work. Although the simplicity of the design makes it readily amenable to scaling to higher frequency bands, precise measurement of the voltage standing-wave nodes associated with high-reflection loads limits the accuracy of the instrument for certain measurements. Nonetheless, the approach of combining the six-port measurement technique with planar probing structures shows promise for yielding a non-contacting measurement infrastructure for in-situ characterization of integrated microwave subsystems and modules.