9 - Multiport and differential S-parameter measurements

Summary

Introduction

The last ten years have witnessed an increasing interest in multiport S-parameter measurements, i.e. S-parameter measurements of devices with more than two ports, for two main reasons: the first one is the increasing complexity of modern microwave devices and circuits and the use of more complex MMICs.

But the main reason is definitely the shift toward microwave frequencies of the personal computer's processors speed, which implies that such digital applications must now face typical microwave challenges. These topics have recently been addressed in [1]. Preserving the signal integrity of a microwave signal through the packages, sockets, connectors, and PCB traces, commonly found in today's computer systems, is one of the main issues. System architectures with hundreds of parallel channels, operating at higher and higher data rates, involve microwave multiport measurements for the characterization, design, and analysis of the structures and their effects on the signals. Microwave designers and engineers are thus facing new challenges in multiport measurement hardware and calibrations.

The first challenge comes from the typical media of digital interconnections: the PCB. It can include both planar and three-dimensional (3-D) DUTs, as found, for example, in memory modules. So, on one hand many data lines must be connected and measured simultaneously, and they do not necessarily lie on a single plane. On the other hand, these connections from the boards to the typically coaxial test ports of the VNA must have good performances at microwaves, i.e. be “transparent” for the measurements.