Measurement and modeling of passive surface mount devices on FR4 substrates

Abstract

Passive components like resistors, capacitors and inductors are used in every electronic system. These are the very basic components which affect the system performance at higher frequencies and it is necessary to understand and model the behavior of these components in a very accurate manner. This work focuses on utilizing Printed Circuit Board (PCB) test boards, or fixtures, made of FR4 for characterizing Surface Mount Device (SMD) components. Agilent's Advanced Design System (ADS) microwave circuit simulation software was used for designing the microstrip transmission lines as well as for generating the layout for manufacturing of the PCB. SMD resistors, capacitors and inductors were soldered into the fixture and then measured using the Vector Network Analyzer (VNA). The calibration kit was developed in ADS. The measured data were calibrated using the TRL (Thru-Reflect-Line) calibration algorithm. A calibration kit consisting of through, three transmission lines of various lengths, open and short was designed and manufactured. Calibration procedures were performed using Cascade Microtech's WinCal XE software. Based on our experience, TRL calibration did not perform to its full potential due to errors in the value of the characteristic impedance of microstrip transmission line. This impedance is ideally assumed to be 50 Ohm, but our lines had characteristic impedance of around 49 Ohm. Simple models for the resistors and capacitors were developed by our collaborators at the University of Zagreb and we developed the model for the inductors. We used ADS for simulations and comparison with the measured data. Extensive optimization of these models was done so as to fit the measured and modeled data. As the frequency goes above 4 GHz models and measurements don't match due to the limitations of the PCB material, the increasing effects of the parasitics and calibration artifacts. This work shows how and when we can use inexpensive FR4 PCB for the characterization of the passive SMD components in the low GHz frequency range. It also examines the range of operating frequency of SMD components, verifies the parameters extracted from the simple model and tests the TRL calibration algorithm.