A NIST Traceable PCB Kit for Evaluating the Accuracy of De-Embedding Algorithms and Corresponding Metrics

摘要

The constant increase in high speed digital data rates creates a corresponding decrease in design margins and the need for engineering innovations. Specifically, exploring design trade-offs to achieve reduced design margins requires accurate electrical measurements of the physical layer components that make the end-to-end digital link. The measurement accuracy provided by traditional coaxial connectors is non-existent in modern day high-density PCB interconnect designs. This has created the opportunity for innovations in measurement technology to remove custom fixturing with calibration, and the need for validation and comparison to ensure measurement accuracy. The challenge is that there is no one solution for fixture removal, and fixture removal often requires significant engineering experience to understand the trade-offs of accuracy and calibration standard choice. In addition, most test engineers are faced with using commercial software tools that de-embed the test fixture using unknown proprietary algorithms based on a 2x-thru or 1x-reflect methodology, and traditional calibration verification standards are not readily available for these custom fixture removal techniques. Since the de-embedded measurements cannot be validated, engineers are concerned about the accuracy of the measurement of a device under test (DUT) after fixture removal. This concern is exacerbated when the DUT measurement plays a major role to determine the system margin of multi-gigabit serial link systems. The IEEE P370 standard is being developed to address these concerns and this paper focuses on the working area defining methodologies to evaluate the accuracy of a de-embedding algorithm. The IEEE P370 standard uses both simulated and measurement data. A simulation library contains modeled fixtures, DUTs, and verification standards. Thus, the DUT after removal can be compared directly to its modeled counterpart. Conversely, a plug-and-play PCB kit connects fixtures and DUTs together using NIST traceable connections. The result is a DUT that is accessible both directly with coaxial connection and indirectly with de-embedding algorithms. In this paper, we will present the plug and play kit for single-ended and coupled differential applications and show how it can be used to verify de-embedding algorithms with open-source 2x-thru and 1x-reflect de-embedding methodologies. Because the kit is composed of real PCB test structures, it contains all the problems associated with real PCBs, and its design is such that it allows the results of a deembedding algorithm to be compared with the real DUT results in a NIST traceable measurement setup.