8 - Two-port network analyzer calibration

Summary

Introduction

Although VNAs are probably the most advanced microwave systems, with broadband sources, high-speed and high-dynamic-range receivers, the intrinsic property of distribute components makes a calibration procedure mandatory to obtain reasonable results due to an enormous systematic error. To stress this fundamental problem, imagine weighing 300 g of ham with a one-ton plate scale! This is more or less the same influence as systematic phase error introduced by a 1 meter teflon cable in front of a VNA port at 10 GHz if we are trying to measure 1 degree of phase-shifting on a DUT S11 parameter. Not only the phase, but also the magnitude as well is affected, due to different attenuation paths in various system sections. Clearly without a proper correction the measurement quality would be unacceptable.

In the early development of VNA, hardware compensation with line stretchers and variable gain amplifiers was attempted, but it's only with the introduction of computer controlled digital VNAs, that specific signal processing techniques allow a real-time correction of the most important errors. During the last forty years several algorithms have been proposed especially for one- or two-port VNAs; some of them like TRL, SOLT, LRM, and SOLR, became a de-facto standard in all modern VNA firmware; however, many others have been proposed to solve particular problems [1–4]. This chapter presents a review of the error models and the main VNA calibrations, by focusing the attention on their commonalities and by pointing out their different fields of application. The development follows the system approach born in the early 1990s, rather than the traditional one based on the analysis of all possible sources of error [3–5].