Microwave Measurement Services

Project Goals

Setting up experimental vector network analyzer
calibration using 1.5 millimeter airline standard.

The goal of the Microwave Measurement Service Project is to ensure the availability for the U.S. scientific and industrial base of a measurement system for radio frequency (RF) and microwave quantities that is reliable, accurate, reproducible, traceable to the International System of Units (SI), and internationally consistent. We do this by developing and maintaining the U.S. national measurement standards for RF and microwave quantities, providing a wide range of state-of-the-art calibration services, and developing new measurement methods and verification techniques to improve the quantitative measurement of microwave quantities.

Customer Needs

The customers who use our services span a large part of the electronics industry. They include aeronautics and communication companies, instrument manufacturers, and other government agencies. Additionally we make measurements for many internal programs in areas such as antennas, optoelectronics, and electromagnetic properties of materials. Our services provide the fundamental microwave properties that customers rely on to establish the critical factors in design and performance of RF and microwave equipment. Our customers also establish traceability to the SI through our measurements. Economic gains are realized through improvements in accuracy. The verification of calibrations and measurement processes on commonly used microwave measurement systems is of paramount importance to our customers. We support this through our measurements and the techniques that we make available.

Microwave metrology is expanding in many different directions. There is a constant push to use higher frequencies. Signals are becoming much more complex and include modulation effects, multiport/differential signals, complex waveforms, and other unusual signal schemes. On-wafer measurements are in greater demand. Improved means for the dissemination of our services are also necessary. These new requirements are dictated by the needs of the telecommunication and computing communities; 100 gigabit per second data rates will require 400 gigahertz support as well as modulated power, waveform analysis and other signal scheme support. Optoelectronic applications need scattering parameter (sparameter) and power measurement support above 50 gigahertz in 1.85 millimeter connectors. Molecular resonance measurements for chemical identification will need precision measurement support in the 500 to 700 gigahertz range. Remote sensing will require measurements of unprecedented accuracy. New imaging systems will require support in many different microwave parameter areas.

A 2.4 millimeter primary power transfer standard.

A 2.4 millimeter thin-film primary power transfer standard.

Technical Strategy

We provide a large range of state-of-the-art microwave measurements and standards. We will continue to maintain the primary national standards in thermal noise, s-parameters, power, and waveform metrology, and to offer measurement services that enable customers to achieve traceability to those standards and to verify their own measurements.

Maintaining and delivering these services is a major task. The systems and standards for the services are generally designed and built at NIST and are not commercially available. There is not a large market for primary standards and, therefore, companies have no economic incentive to develop them. To cover all parameters requires many different systems and standards. The systems are aging, need more maintenance time, and are very costly to replace. The primary standards that support the services are also aging. In many cases the present standards are either nonreplaceable, or the technical expertise to recreate them has been lost, or the parts of the standards obtained from commercial sources are no longer available (for example, the WR15 and WR10 thermistor standards that have been used are no longer made by any manufacturer).

In part, our strategy for moving the measurement services into the future is to develop improved methods for delivering our services. This will include alleviating the stress on measurement systems and standards, new methods of supporting our customer needs, and more interlaboratory comparisons.

S-parameter measurement comparison kits for
Type-N 7 millimeter, 3.5 millimeter, 2.92 millimeter
and 2.4 millimeter coaxial connectors.

Scattering Parameters — S-parameter measurements are required for accurate measurements in all of the other microwave disciplines. An example is the mismatch correction for power measurements that is calculated from reflection coefficients of various parts of the system and devices. The accuracy of s-parameter measurements is directly related to the calibration of the vector network analyzer (VNA) used for the measurements. The calibration method and standards can be chosen to match the end use. It is important to educate the users of the modern network analyzers about the choices of standards and calibration methods and to develop tools that will enable them to make reliable, accurate measurements.
There is an increasing demand for s-parameter measurements. This is particularly evident for measurements above 50 gigahertz. Higher frequencies and smaller waveguide and connector sizes are starting to be used routinely. We are going to address these needs by adding 1.85 millimeter and 1.0 millimeter connector size capabilities to our measurement services in the near term and look at supporting small waveguide sizes (frequency coverage up to at least 500 gigahertz) in the longer term.
There is general agreement among the principal users and makers of VNA systems that much still remains to be understood about VNA calibrations and measurements. This is true for both traditional measurement areas and emerging areas, which include electronic calibration units, multiport, and differential measurements. We plan to take a very active role in developing VNA calibration and measurement theory and techniques for these areas. We intend to pursue these VNA techniques to higher frequency ranges, up to approximately 500 gigahertz.
Verification of VNA calibrations is very important, and the current verification process is not sufficient. We will try to improve the verification process through several different methods. The Verify and Calkit software we have developed, which compares the contents on verification and calibration disks to measurements made based on LRL calibrations, will be made available to the public. Our measurement comparison kit program will be expanded. Finally services will be developed to support ongoing interlaboratory comparisons. These will aid not only in system verification, but also in proficiency testing for accreditation.

Noise — We have recently added the capability to perform noise-parameter measurements on low-noise amplifiers, and we plan to develop mechanisms, such as verification methods, to support such measurements in industry. In conjunction with our on-wafer noise efforts (see the Micro/Nano Electronics Project), we are working to improve on-wafer measurement methods for noise parameters of transistors, and we will also develop methods of supporting those measurements in industry.
New microwave remote-sensing radiometers are designed for unprecedented accuracy. In order to verify (and possibly to even achieve) that accuracy, a stable, accurate reference standard is required. We have proposed development of national standards for microwave brightness temperature in frequency bands of interest. The standards would be traceable to the NIST primary noise standards. We are also working on methods to characterize calibration targets commonly used in microwave radiometry. Additionally we are automating our noise temperature systems to greatly increase their efficiency.

Direct Comparison power measurement system

Calibration measurements on the direct
comparison power system.

Power — For a number of years, power measurements above 50 gigahertz were based on calorimetric and sixport reflectometer measurements using rectangular waveguides. NIST’s internal power standards were characterized in the calorimeters and the measurements transferred to customer devices by use of the six-port systems. The NIST primary transfer standards were modified commercial power sensors, and the calorimeters were designed specifically for these standards. Measurement services were available in 1 gigahertz steps from 50 to 75 gigahertz and from 92 to 96 gigahertz.
Improvements in our standards are needed for a number of reasons. Frequency coverage in 1 gigahertz steps is not adequate for characterizing broadband digital devices such as optoelectronics components that operate at 40 gigabits per second. There is increasing use of frequencies above 75 gigahertz that were not previously measured. The modified commercial power sensors that NIST used as transfer standards are no longer reliable and cannot be replaced.
In order to address these problems, new calorimeters are being developed so that a wider set of transfer standards can be used. Faster transfer measurements are being developed so that denser frequency coverage can be readily obtained. We have obtained a new synthesizer (up to 67 gigahertz) and a backward-wave oscillator (50 to 110 gigahertz) source that have replaced the manually tuned Gunn diode oscillators for most measurements. A direct comparison system that can evaluate a customer device at about 50 frequencies per day has been developed for WR-15 (50 to 75 gigahertz) and is being used for customer calibrations. A direct comparison system for WR-10 (75 to 110 gigahertz) has been constructed but not yet evaluated. New calorimeters are being designed for both WR-10 and WR-15. They will accommodate a wider variety of internal standards than the present calorimeters. Future plans include the extension of the direct comparison measurements to 1.85 millimeter coaxial connectors that will allow measurements from DC to 65 gigahertz with a single connector. These measurements will be traceable to the WR-15 and 2.4 millimeter calorimetric primary standards.
RF power measurements have traditionally been traceable to DC power measurements through RF/DC substitution techniques. An alternative approach is to measure the field strength of microwaves through their effect on the quantum state of atoms. In this measurement, a group of atoms is created in a single quantum state. They are then exposed to microwaves at a frequency that corresponds to the energy difference between this state and a second quantum state. The atoms will oscillate between the two states at a frequency that is proportional to the field strength. This process is known as a Rabi oscillation. By measuring the number of atoms in each state, the field strength can be determined. A proof-of-concept experiment was conducted in collaboration with the Physics Laboratory. The next stage in this work will be an experiment that accurately compares a traditional measurement with the quantum measurement.

Waveform — We are developing calibration procedures for today’s high-performance electrical probes for on-wafer measurement. We are also laying the foundations for 200 gigahertz to 400 gigahertz calibrations for tomorrow’s on-wafer probes. We are also developing techniques for performing noninvasive high-impedance on-wafer waveform measurements for characterization of signal integrity in digital silicon integrated circuits (ICs) and other small circuits. This effort is particularly important for the development of electrical metrology for nanoscale devices, which, due to their small sizes, have extremely high electrical impedances.
Our plan is to electrically characterize an active high-impedance probe with our existing VNA calibration methods. We will then characterize the same probe on our 200 gigahertz bandwidth electro-optic sampling (EOS) system. This will lay the groundwork for very high-speed, on-wafer calibrations for digital IC and nanoelectronics. We will develop joint time-domain/frequency-domain uncertainty analysis for coaxial photodiode pulse sources. The calibration and uncertainty representation will include imperfections in the electrooptic sampling system and electrical mismatch corrections, and will be suitable for calibrating oscilloscopes with coaxial ports in either the time or frequency domains to 110 gigahertz. We will develop pulse sources with 400 gigahertz bandwidth and, based on these sources, develop on-wafer waveform characterization ability to 400 gigahertz. We will apply high-speed waveform metrology to microwave problems, including the characterization of electrical phase standards, microwave sources, and microwave mixers.

Accomplishments

Low Noise Amplifiers — Low-noise amplifiers (LNAs) are used in a variety of applications involving detection and processing of low-level signals; noise parameters are the critical characteristics of LNAs. We performed measurements of the noise parameters of an LNA for an informal comparison with another National Measurement Institute (NMI) in order to verify international agreement on such measurements.
Microwave Brightness-Temperature Standards — New microwave remote-sensing radiometers for weather, climate monitoring, and other applications are designed for unprecedented accuracy. In order to verify (and possibly to even achieve) that accuracy, a stable, accurate reference standard is required. We proposed and documented a plan for development of national microwave brightness-temperature standards, traceable to fundamental noise standards.
Calibration Targets — Heated calibration targets are commonly used as standards for microwave remote sensing radiometers, but important characteristics of such targets are not well measured. We performed preliminary measurements of the electromagnetic properties of materials commonly used in calibration targets with the Electromagnetic Properties of Materials Project
and also performed infrared imaging of thermal gradients in a target borrowed from NASA with the Physics Laboratory. With the Radio-Frequency Fields Group, we performed a preliminary study of near-field effects on the calibration of microwave radiometers when the calibration target is in the near field of the radiometer antenna. It is current practice to ignore such effects; however, this study indicated that they could be significant.
International Comparisons — Under the Mutual Recognition Agreement (MRA) of NMIs worldwide, measurement comparisons among NMIs are performed on key quantities to assure
international harmonization of standards and of measurements for commercial regulatory purposes. We participated in two completed Consultative Committee for Electricity and Magnetism
(CCEM) Key Comparisons (KCs) in noise, one KC in s-parameters, one KC in power, and one supplementary comparison in power, for which we were the pilot lab.
WR-15 and WR-10 Waveguide Systems — Waveguide services above 50 gigahertz are being revitalized to compensate for difficulties with existing transfer standards and to improve the traceability for high frequency signals used in digital communications. A direct-comparison system for WR-15 waveguide has been implemented and is being used for customer calibrations. A
WR-10 waveguide system has been constructed. These systems greatly reduce the time required for a measurement. New WR-15 and WR-10 calorimeters were designed and tested. After initial tests, it was decided to modify the new design to improve their performance.
Microwave Field Strength — Basic research testing a fundamentally new method for measuring microwave signal strength was conducted in collaboration with the Physics Laboratory. The RF
magnetic field strength in a cavity was measured by observing the oscillation of cesium atoms between Electromagnetics Division 9 two quantum states. The proof-of-principle experiment demonstrated a rough agreement between the new technique and traditional microwave power measurements.
Power Standards at Low Frequency — A new coaxial direct comparison system has been built to cover the frequency range of 100 kilohertz to 18 gigahertz. This system extends the lower
range of our direct comparison systems from 50 megahertz down to 100 kilohertz. This new system gives us much greater efficiency in measuring power standards. Before, two independent systems had to be used. There was no significant change in uncertainties.
Software — The Verify and Calkit software that supports the Agilent 8753 VNA has been completed and delivered to the U.S. Air Force. The Air Force will be using this to verify in-house
their inventory of Agilent 8753 verification and calibration kits at an estimated cost savings of approximately $100,000 per year.

WR 15 six-port reflectometer head.

Calibrations

We provide a wide range of state-of-the-art calibration services for fundamental microwave quantities, including scattering parameter, power, thermal noise, and waveform.
For scattering parameters, we provide calibrations of one- and two-port devices, in a variety of waveguides and coaxial connector sizes. For coax, we provide measurements from 10 megahertz to 50 gigahertz. In waveguide, we cover the range from 8.2 to 110 gigahertz and 92 to 96 gigahertz.
For microwave power, we provide effective efficiency calibrations of thermistor and thermoelectric detectors in coax from 100 kilohertz to 50 gigahertz. We provide calibration in waveguide for
thermistor detectors from 8.2 to 75 gigahertz.

For thermal noise, we provide calibration of coaxial noise standards at 30 megahertz, 60 megahertz, and from 1.0 to 50 gigahertz. Waveguide standards are calibrated from 8.2 to 65 gigahertz.

For waveform, in collaboration with the Optoelectronics Division, we provide calibration of photodetectors and oscilloscopes for microwave signal characterization. We provide measurements
in coaxial media at frequencies up to 110 gigahertz and on-wafer up to 200 gigahertz.

Short Courses

We organized the annual Automatic RF Techniques Group (ARFTG)/NIST Short Course on Microwave Measurements in Washington, DC, in November 2005. The course is a great way to
bring measurement theory and techniques to many people (attendance is usually around 40) at one time. During the course we are able to teach the attendees the latest methods for making the most accurate microwave measurements. Presentations were given by Jim Randa on “Thermal Noise Measurements,” Ron Ginley on “RF Connectors and Transmission Lines” and “VNA Uncertainties,” and Tom Crowley on “Microwave Power Measurements.”
Dave Walker gave lectures at the Asia-Pacific Microwave Measurements Training Course in Christchurch, New Zealand, in April 2006, and in Auckland, New Zealand, in May 2006: “Introduction to Microwave Power Measurements,” “Microwave Power Measurements in Digital Communications Systems,” and “Thermal Noise Measurements.”