Distortion Inherent To VNA Test Port Cable Assemblies

By Paul Pino, W. L.Gore & Associates

Today's modern vector network analyzers (VNAs) are the product of evolutionary advances in technology that literally span decades - a marrying of the latest developments in microwave, integrated circuit, and software technologies. To a great extent, the precise nature of vector network analysis results from its highly developed calibration algorithms and methodologies. In many VNA applications, a flexible test port cable assembly or "test port extension" may be required. The test port extension is used to establish a through connection for calibration purposes and for making measurements of the Device Under Test (DUT). Generally, a single port extension is used, but this is by no means the rule. Port extensions may be employed on port 1, port 2, or both VNA ports as well. Capable of very precise and accurate results, a VNA's performance is predicated upon the stability and repeatability of the measurement system, i.e., the DUT to VNA interface, which includes the test port extension, if used.

A test configuration using a flexible test port extension on port 2 is illustrated. The resulting calibrated measurements of a given DUT will contain notable inconsistencies. When observing the analyzer display, s22 return loss will differ from s11 return loss, appearing somewhat "noisy" in nature. If the test port extension is flexed, changes in both s22 and s11 return loss data will be observed. If measuring a low-loss DUT, the aforementioned behavior is exaggerated. The intent of this article is to offer a qualitative explanation for this behavior and its underlying mechanisms.