C. M. Wang

Calibration of sampling oscilloscopes with high-speed photodiodes

We calibrate the magnitude and phase response of equivalent-time sampling oscilloscopes to 110 GHz. We use a photodiode that has been calibrated with our electrooptic sampling system as a reference input pulse source to the sampling oscilloscope. We account for the impedance of the oscilloscope and the reference photodiode and correct for electrical reflections and distortions due to impedance mismatch. We also correct for time-base imperfections such as drift, time-base distortion, and jitter. We have performed a rigorous uncertainty analysis, which includes a Monte Carlo simulation of time-domain error sources combined with error sources from the deconvolution of the photodiode pulse, from the mismatch correction, and from the jitter correction

An optimal multiline TRL calibration algorithm

We examine the performance of two on-wafer multiline Thru-Reflect-Line (TRL) calibration algorithms: the popular multiline TRL algorithm implemented in the MultiCal/spl reg/ software package, and a newly implemented iterative algorithm designed to give optimal results in the presence of measurement noise. We show that the iterative algorithm outperforms the MultiCal software in the presence of measurement noise, and verify its uncertainty estimates.

Compensation of Random and Systematic Timing Errors in Sampling Oscilloscopes

In this paper, a method of correcting both random and systematic timebase errors using measurements of only two quadrature sinusoids made simultaneously with a waveform of interest is described. The authors estimate the fundamental limits to the procedure due to additive noise and sampler jitter and demonstrate the procedure with some actual measurements

Estimating the Magnitude and Phase Response of a 50 GHz Sampling Oscilloscope Using the "Nose-to-Nose" Method

We describe estimation of the magnitude and phase response of a sampling oscilloscope with 50 GHz bandwidth using the nose-to-nose method. The measurements are corrected for the non-ideal properties of the oscilloscope and calibration apparatus, including mismatch and time-base distortion, drift, and jitter. The mean and standard deviation of repeated measurements of an ensemble of three oscilloscope samplers are reported, along with attempts to verify the magnitude calibration using a swept sine-wave method.

Identifying RF Identification Cards From Measurements of Resonance and Carrier Harmonics

We show that careful measurements of the unloaded resonance frequency and quality factor of RF identification proximity cards allow identification of different card models and, for the set of cards we studied, identification with minimal error of individual cards of the same model. Furthermore, we show that card identification performance is improved by considering an electromagnetic signature that combines measurements of the energy at carrier harmonics during a reader/card transaction together with measurements of unloaded resonance frequency and quality factor.

A transfer standard for measuring photoreceiver frequency response

We have developed a photoreceiver frequency response transfer standard which can be used to measure the optical modulation transfer function of a modulated optical source. It combines a photodiode with an RF power sensor or an amplified receiver with an RF power sensor. It is calibrated with an expanded uncertainty of 0.06 dB (coverage factor=2) using a heterodyne technique at 1.319 /spl mu/m. We present a theory which allows use of the transfer standard with arbitrary source modulation depth. The calibration is transferred to a SDH/SONET test equipment manufacturer giving a final uncertainty well below the 0.3 dB uncertainty specified by ITU-TS (formerly CCITT) recommendation G.957. The transfer standard may have other applications including calibration of CATV test equipment, light-wave component analyzers, and light-wave spectrum analyzers.

Use of electronic calibration units for vector-network-analyzer verification

We present measurements demonstrating that electronic calibration units are stable enough to be used in place of mechanical vector-network-analyzer verification artifacts. This enables a new fully automated approach to verifying microwave vector-network-analyzer calibrations with a single computer-controlled electronic verification artifact. The new system presents verification results in easy-to-understand performance metrics that, unlike those derived from measurements of mechanical verification artifacts, are independent of the actual artifacts employed.

Uncertainty in Reference Values for the Charpy V-notch Verification Program

We present a method for computing the combined standard uncertainty for reference values used in the Charpy machine verification program administered by the National Institute of Standards and Technology. The technique is compliant with the ISO Guide to the Expression of Uncertainty in Measurement and models the between-machine bias using a Type B distribution. We demonstrate the method using actual data from the Charpy machine verification program.

Effects of noise level in fitting in situ optical reflectance spectroscopy data

In an effort to increase the accuracy of the parameters extracted from in situ optical reflectance spectroscopy (ORS) data, the effects of noise and curve-fitting methodology are investigated. Parameters such as the growth rate and the index of refraction of growing films are acquired by fitting ORS data, and the accuracy of those parameters is determined by factors such as noise level and uncertainty in the absolute calibration of the magnitude of the reflectance. This uncertainty is represented by an overall scaling factor in modeling the reflectance with the virtual interface model. Experimentally, the scaling factor is included to adjust for drift in photodetector temperatures, the effects of ORS light source stability, and transparency changes of the window of the growth chamber. Our goal is to determine index of refraction n to an accuracy of 0.0015, or about 0.044 % for Al/sub 0.5/Ga/sub 0.5/As at growth temperature, so that the Al mole fraction x can be extracted to x = /spl plusmn/ 0.002 for Al/sub x/Ga/sub 1-x/As. Curves of reflectance as a function of time were generated using parameters so that the simulated data resemble actual ORS data. Gaussian noise was added to the generated curve. The simulated data curves were fit by means of a Levenberg-Marquardt nonlinear algorithm. The fitting results of different noise levels show that the accuracy of curve fitting depends on noise level and the uncertainty range of the scaling factor.

Accuracy of AlGaAs growth rates and composition determination using RHEED oscillations

Reflection high-energy electron diffraction (RHEED) oscillations are widely used in molecular beam epitaxy (MBE) as a technique to calibrate material growth rates. The growth rates are used to predict the composition of the following growth run. For many applications, the predicted composition uncertainties of a few percent are adequate, but some applications, like vertical cavity lasers (VCSELs) and distributed Bragg reflectors, demand greater accuracy. To improve the accuracy in determining the composition of MBE-grown films, it is obvious we need to understand the uncertainties and limitations associated with RHEED as an MBE tool. In this study, we investigate several aspects of RHEED intensity oscillations of the specular spot during growth of AlAs, GaAs, and AlGaAs on GaAs substrates, including the effects of beam positioning, substrate size, different growth rates, and incident beam along the [0(-1)1] (corresponding to the 4x reconstruction) and [011] (corresponding to the 2x reconstruction) direction. Additionally, we examine beat phenomena in the RHEED oscillations (due to nonuniformity in growth rate across the sample) and beam flux transients and their implications on composition. For the two largest factors, electron beam positioning and flux transients, the overall uncertainty can be reduced with careful experimental technique. The lower end of the range corresponds to technique that minimizes the error, while the upper number corresponds to allowing these factors to be essentially uncontrolled. We also present a procedure that uses the measured variance in the growth rates to calculate the composition with the smallest mean square error.