TDR Single Ended and Differential Measurement Methodology

Abstract

As the frequency of the high speed interfaces in computer platforms and systems continue to ramp due to architectural advancement and strong market demands, electronic packages have been stretched to its limit to support these interfaces. Furthermore, due to increase in the complexities in the package, signal integrity, manufacturing concern and package size reduced, designers are facing a lot of signal quality issue caused by impedance mismatch. To ensure signal integrity, it is necessary to understand and control impedance in the transmission environment through which the signals travel. Mismatches and variations can cause reflections that decrease signal quality as a whole. TDR (Time Domain Reflectometry) is the most common tools for verification and analysis of the transmission properties of high-speed systems and components. It is able to locate signal path discontinuities that cause reflections, verify traces characteristic impedance and estimate traces length. Hence, this paper is to discuss about TDR set up and type of TDR measurement procedure and methodology for single ended and differential pair interface in the package. There are several key elements that determine the precision of TDR measurement such as the edge speed of the stimulus pulse produced by the TDR, bandwidth of the channel used to received the pulse from the DUT, calibration and deskew process. Correlation between measurement data vs simulation data is performed in order to ensure the accuracy of the data collected. The simulation method is done by using Ansoft 3D and 2D simulation tools to characterized package interconnect such as traces, wirebond, VIA/PTH and ball structures. The output file (Spice File, .sp) will be important to Ansoft Designer software for TDR simulations.