Technical Paper: High Resolution TDR Measurements Using the PSPL Model 4020 9 ps TDR Source Enhancement Module
By Picosecond Pulse Labs, Inc.
Abstract
This paper describes how the PSPL model 4020 is applied to significantly improve Time Domain Reflectometry (TDR) resolution relative to other commercially available solutions. Effective TDR analysis of modern multi-gigahertz package interconnects, circuits, and devices requires the high bandwidth characterization of very small features. High resolution TDR increases the accuracy of a TDR measurement system for characterizing the location and magnitude of small features. TDR resolution is directly proportional to the risetime of the incident TDR pulse. Standard commercial TDR sources provide typical risetimes ranging from 25 ps to 40 ps. The model 4020 provides incident TDR pulses with a risetime of 9 ps and a resolution improvement ranging from 2.5X to over 4X.
Introduction
Higher clock frequencies and faster rise/fall times of modern serial communications systems have made the characterization of high-speed interconnections even more critical. The parasitic elements associated with some circuit features can cause significant impedance discontinuities that effect circuit operation at higher speeds. As the bandwidth requirement of these interconnections extends further into the multi-gigahertz range, the size and distance between features of interest become smaller. Along with these smaller features comes the need for high-resolution measurement systems to obtain accurate characterization of these wideband interconnects and devices.
Time domain reflectometry (TDR) measurements are a popular method used in characterizing high speed interconnects, packaging, and devices. These methods model the interconnect discontinuities resulting from bends, steps, vias, etc. through developing lumped element models from the resulting TDR waveform. The models are then incorporated into simulations to assure the circuit's functionality. The accuracy of the parasitic models is critical in the proper design of a system. A high-resolution TDR measurement system provides the accuracy
needed of these high-speed interconnect models.
The resolution of a TDR measurement system is directly related to the rise/fall time of the incident pulse. The edge speed of the incident pulse will set the minimum decipherable feature size that can be extracted from the measurement. As the resolution of the measurement system increases, the model of the interconnections becomes more detailed, leading to more accurate simulations and more functional designs. There are currently several TDR measurement solutions commercially available. Two of the more widely used systems are the Agilent and Tektronix TDR systems. Both of these systems use a digital sampling oscilloscope with a TDR plug-in module. The TDR plug-in module incorporates a TDR pulse source and a sampler.
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