Robust Algorithm for Eye-Diagram Analysis
NIST researchers Jeffrey Jargon Paul Hale, and Jack Wang have developed a new method for analyzing eye diagrams. Eye-diagrams are multivalued displays used for assessing the quality of high-speed digital signals [1]. They are usually constructed by applying a data waveform to the input of a sampling oscilloscope, and then overlapping all possible one-zero combinations on the instrument’s display. Eye diagram measurements have a huge economic impact on the optical and electrical communications industries. With cost pressures driving manufacturers to create products that just meet specifications, the ability to make accurate and repeatable measurements for equipment testing is becoming more important. Conflicts may arise between component manufacturers and their customers when different test equipment leads to measurement inconsistencies. These discrepancies can be attributed to both software and hardware differences. The focus of the current work is on an algorithmic method that can be implemented in software The NIST algorithm makes use of a robust, least-median-of-squares (LMS) location estimator. In contrast to commonly used histogram techniques, the LMS algorithm provides a repeatable solution that is insensitive to outliers and data distributions. The motivation for developing this algorithm was to create an independent, benchmark method that is both amenable to a thorough uncertainty analysis and can function as a comparison tool since no standardized industry algorithms currently exist. Utilizing this technique, the researchers can calculate the fundamental parameters of an eye diagram, namely the one and zero levels, as well as the time and amplitude crossings. With these parameters determined, eye-mask alignment can be performed and various performance metrics can be derived, such as extinction ratio and root-mean-square jitter. The algorithm in its entirety was published in IEEE J. Lightwave Technol. vol 26, no. 21, pp. 3592-3600, Nov. 1, 2008.
[1] D. Derickson, Fiber Optic Test and Measurement, Prentice-Hall, 1998.

Traceable Transfer Standard for High-speed Measurements
NIST researchers Paul Hale and Dylan Williams have developed a novel transfer for calibrating high-speed instruments used to measure optical and electrical waveforms. Waveform measurements are required throughout the optical communications, computer, wireless communications, radar, and remote sensing industries. Waveform measurements verify signal fidelity and standard compliance for the design and qualification of components and systems.

The new transfer standard is a fast photoreceiver that generates electrical pulses with calibrated magnitude and phase to 110 GHz. Using this photoreceiver, engineers can calibrate their high-speed test equipment, such as oscilloscopes and lightwave component analyzers, in such a way that the equipment is traceable to fundamental SI units. When combined with the NIST Timebase Correction Software, engineers can overcome some of the bandwidth limitations of their test equipment. Finally, these new transfer standards can greatly improve confidence in high-speed measurements by enabling direct calibration of both magnitude and phase of a variety of high-speed instrumentation. With this technology, chip designers can define product specifications with known uncertainties rather than the current practice of placing upper limits on signal accuracy, leading to needlessly large tolerances in high-speed components and systems. These transfer standards are available for purchase by contacting either Paul Hale or Dylan Williams.

New Technique Addresses Calibration Issues for Both Time- and Frequency-Domain Test Equipment
NIST researchers have developed and validated a covariance-based uncertainty analysis for NIST's electro-optic sampling (EOS) system, which provides calibrated results for both time- and frequency-domain electronic instruments. With this EOS system, NIST presently offers photodiode calibrations to 110 GHz. The electrical response of the photodiodes is determined in the frequency domain, and the measured spectrum and phase of the photodiode's electrical output makes the NIST-calibrated photodiodes ideal for calibrating a variety of other electrical frequency-domain instruments, such as lightwave component analyzers and large-signal analyzers. This approach creates a framework for maintaining the correlations in the uncertainties of the frequency-domain impedance measurements and mismatch corrections used in the electro-optic sampling system so that uncertainties in the time-domain can also be determined. Now NIST photodiodes can be used to calibrate both temporal and frequency-domain electronic instruments, making the calibrated photodiodes useful to a wider range of customers and applications than ever before. Perhaps the most important new application of the photodiodes is for the calibration of oscilloscopes and other high-speed temporal waveform measurement systems used in the digital electronics, wireless communications, and fiber optic communications industries. The work is described in D. F. Williams, A. Lewandowski, T. S. Clement, C. M. Wang, P. D. Hale, J. M. Morgan, D. Keenan, and A. Dienstfrey, "Covariance-Based Uncertainty Analysis of the NIST Electro-optic Sampling System," IEEE Trans. Microwave Theory Tech., vol. 54, no. 1, pp. 481-491, Jan. 2006.

Free Software for Oscilloscope Timebase Corrections
NIST has demonstrated a new method of correcting both random and systematic timebase errors in sampling oscilloscopes. High-speed sampling oscilloscopes suffer from systematic timebase distortion (TBD) and random jitter that cause errors in the time in a waveform at which samples are acquired. By implementing the NIST correction method, end users can achieve the best aspects of these oscilloscopes by correcting for both random jitter and systematic timebase distortion, thus providing the end user with an estimate of the residual timing error after the correction process has been applied. This method is nonproprietary. Details of the procedure are described in P. D. Hale, C. M. Wang, D. F. Williams, K. A. Remley, and J. Wepman, "Compensation of random and systematic timing errors in sampling oscilloscopes," IEEE Trans. Instrum. Meas., vol. 55, no. 6, pp. 2146- 2154, Dec. 2006. Both the reprint and software package are available on the High-Speed Measurements Software web site.

EEEL Demonstrates Traceable Waveform Measurement up to 200 GHz
NIST has demonstrated temporal waveform measurements, in a coplanar waveguide, that are calibrated and traceable to fundamental physical units using electro-optic sampling techniques. The calibrations, from 500 MHz to 200 GHz, enable us to correct for the complex characteristic impedance and dispersion of the coplanar waveguide, impedance mismatches, and multiple reflections in the measurement system. Because they are calibrated, these measurements make it possible to calculate such quantities as the Thevenin and Norton equivalent circuits describing the electrical source, and can be used to calibrate future generations of temporal on-wafer measurement systems. This work will support future generations of high-speed components and test equipment that are necessary for 40 Gbit/s and faster fiber-optic communications systems that cannot be supported by present-day state-of-the-art oscilloscopes, whose bandwidths are limited by their coaxial connectors.

Although electro-optic sampling has been used to measure high-speed electrical waveforms for many years, the NIST measurements are the first mismatch-corrected waveform measurements (using any method) that include a full time-domain uncertainty analysis and are calibrated up to 200 GHz, a factor of four improvement over previous achievements elsewhere. The team quantified the uncertainty in the temporal measurement using a Monte-Carlo simulation that included both systematic and random sources of uncertainty from a measurement of a 5.96 ps pulse with a 95% confidence interval of only 0.21 ps. This time-domain uncertainty analysis is the first of its kind. Details of the procedure are described in D.F. Williams, P.D. Hale, T.S. Clement, and J.M. Morgan, "Calibrated 200 GHz Waveform Measurement," IEEE Trans. Microwave Theory and Tech., Vol 53, no. 4, pp. 1384-1389,April 2005.

Page updated: 9/24/2009