The Waveform Metrology Project provides measurement and algorithmic support for waveform traceability to the optics and high-frequency electronics industries. While NIST was the first to develop the electrooptic sampling (EOS) phase calibration of photodiodes and map this calibration into waveform measurements with uncertainty, other NMI’s have made considerable progress into more advanced optical methods for real-time optical network analysis. To stay at the forefront, NIST must continue to invent, develop, and provide tools for fundamental traceability and statistical analysis of high-speed signal generation and measurement, while providing quality measurement services to NIST’s internal and external customers. This effort relies on 3 primary areas: optical measurement techniques, waveform processing and analysis, and development and characterization of physical device artifacts.
The Electro-optic Sampling technique (EOS) measures the vector response of photodiodes and provides phase traceability for commercial instrumentation, such as large-signal network analyzers, lightwave component analyzers, vector signal analyzers, oscilloscopes, pulsed laser radiometers, optical time-domain radiometers. NMIs in South Korea, China, Germany, and the U.K. are actively working on developing similar waveform measurement services, so it is imperative for NIST to continue its leadership role in this area by making continual improvements and verifying our results through various comparisons. Ongoing research on this traditional EOS technique includes extending the traceability beyond 100 GHz.
The electro-optic sampling system developed at NIST has revolutionized the ability to characterize the frequency response of high-speed devices from 0.25 GHz to beyond 100 GHz. However, mechanical issues coupled with synchronous sampling create a long-duration measurement that is susceptible to external noise. Asynchronous optical sampling (ASOPS) techniques can be employed using two frequency combs with slightly different repetition frequencies to overcome these issues. When both combs are mixed onto a photodetector, they produce an RF comb at a different frequency. The overall advantages of the dual-comb ASOPS are there are no moving components and a better frequency resolution. The use of fiber lasers permits the system to be more compact with no moving parts, user access, and an easily tuned repetition frequency. The coherence of the comb allows for improvement of the signal-to-noise by employing coherent averaging or real-time phase correction methods.
It has long been the desire to see the propagation of an electrical wave down an on-wafer transmission line. In conjunction with the Terahertz Synthesizer IMS, a new type of EOS is being developed to take line scans along the CPW. The ability to measure the standing wave ratio for CW mmWave signals is an ideal capability for a receiver for the Terahertz Synthesizer.
The characterization of communications systems requires the measurement and characterization of waveforms subject to correlated uncertainties and noise. These waveforms require careful analysis to properly account for and propagate uncertainties accurately. CTL researchers in this project have historically pushed the state of the art in the analysis and uncertainty quantification of noisy time-domain waveforms. Such algorithmic innovation has helped in providing waveform traceability to the optics and high-frequency electronics industries.
Full waveform metrology involves several steps of calibrations in a variety of instruments. Some operate in the time domain, such as oscilloscopes, while others operate in the frequency domain, such as vector network analyzers. Furthermore, time-base corrections and finite bandwidth effects must be taken into account. Thus, traceability and correlated uncertainty analysis that can be propagated through each portion of the measurement process is required to maintain consistency when mapping back and forth between the time- and frequency domains.
Many wireless require traceable multichannel modulated-signal measurements. In principle, these measurements can be performed by Large Signal Network Analyzers (LSNAs), vector signal analyzers (VSAs), and equivalent-time sampling oscilloscopes. Traceability for each of these instruments is achieved with different traceability paths; each uses different triggering methods, and each instrument has different measurement restrictions. For example, the LSNA can be configured to make completely traceable mismatch-corrected modulated-signal measurements in a single step while the oscilloscopes and VSAs must be used in conjunction with a vector network analyzer (VNA) to perform the mismatch corrections. VSAs and now even LSNAs can measure longer modulated signals than equivalent-time sampling oscilloscopes. In addition, triggering for the VSAs is more flexible than LSNAs and oscilloscopes. This project will compare these instruments to better understand their advantages and limitations and build on calibrations developed in the Waveform Metrology and Metrology for Wireless Systems Projects.
As electronic test equipment increases in frequency, even higher frequency calibrations are required to validate the measurements. Commercially available photodiodes that are currently used for waveform traceability (absolute phase and relative magnitude) are limited by both the bandwidth of the device and the 1.0 mm output connector. Using a photodiode with an adapter to 0.8 mm is under validation, but the calibrated magnitude response above 110 GHz is unknown and it is not an extensible approach to even higher frequencies. Through the NIST on a CHIP program, we have previously developed photodetectors up to 1 THz and strategies on how to package these devices. This opportunity allows us to develop these expensive devices in-house that could compete with commercially available devices at frequencies above 110 GHz.