IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-32, NO. 1, MARCH 1983

Keynote Address:Conference on Precision Electromagnetic

Measurements

ERNEST AMBLER

IT IS A PLEASURE to welcome the Conference on Preci-sion Electromagnetic Measurements once again to the

United States and specifically here to Boulder.It was in 1958, almost 25 years ago, that this Conference

was first held in Boulder. It was not the first conference of theseries, but it was the first one covering the whole range offrequencies, from dc to the highest frequencies that could begenerated. That 1958 Conference set a pattern that the suc-ceeding biennial conferences have all followed. I notice thatone or two of the speakers from the 1958 Conference are alsogiving papers at this Conference. Although some of the peoplehere are the same, there are many new faces, and certainly thetrend to higher frequencies and newer techniques continues.

In 1958 the meter was defined by the meter bar kept atBIPM, and the second was defined as a fraction of the tropicalyear 1900. Since then, as everyone knows, we have adoptedatomic definitions for both the meter and the second. Thesecond is defined in terms of the frequency of a spin flip tran-sition in the Cesium 133 atom. The meter is defined by thewavelength of a line in the spectrum of krypton 86. But, we arenow able to make measurements well beyond the limits set bythis definition. This is because the development of lasers, andthe means to stabilize them, have brought about a tremendousimprovement in measurement techniques. There is a long storyof technical accomplishments in this area. The progress hasbeen reported at many sessions of these conferences. This week,BIPM Director Giacomo will tell us of the climax of the story:The proposal for a unified definition of length and time madepossible by the measurement of both frequency and wavelengthof a stabilized laser. The decision of the Comite Consultatifpour la Definition du Metre (CCDM) to recommend a newdefinition for the meter is an important milestone for lengthmetrology. Incoherent radiation from the krypton lamp canbe replaced by coherent radiation lasers that are frequencystabilized to atomic or molecular spectral lines. The meter canbe realized in practice by using the wavelength of any laserwhose frequency has been measured. The unification of thebasic units of length and time is an exciting accomplishment.It is a step closer to the ideal of the founders of the metricsystem, that is to root our measurement system in the constants

This Keynote Address was presented at the 1982 CPEM, Boulder, CO.The Speaker is with the National Bureau of Standards, Washington,

DC.

of nature. It also offers the user new measurement opportu-nities by providing a convenient, reliable technique to makeaccurate measurements of length of all magnitudes, fromatomic dimensions to astronomical dimensions.

Besides the development of lasers and their application asmeasuring instruments, there have been other vigorous de-velopments in optics that have greatly influenced measurementtechniques, e.g., optical fibers. Telecommunication throughoptical fibers has developed very rapidly into a commercialsuccess. On the way, it has defeated at least one other rivaltechnology overmoded circular metal waveguides. This isthe first of these conferences to devote a full session to themeasurement problems associated with optical fibers.

There have also been significant advances in techniques forthe measurement of optical radiation. Black-body radiatorshave been for a long time the principal basis for optical radi-ometry. These are now being replaced in many applicationsby absolute detectors. The detectors are often better suited tocertain applications and can be tied more tightly to SI units,at least in the visible and near ultraviolet. Significant progresshas been made in the last few years especially in the area ofhigh-accuracy measurements based upon photodiodes whosequantum efficiencies can be determined from first princi-ples.

Black-body radiators have also been the principal basis forphotometry. Turning back to the 1958 Conference again, recallthat the candela was then defined in terms of a black-bodycavity operating at the freezing points of platinum. This def-inition had prevented modern photometrists from taking fulladvantage of the developments in electrooptics, spectroradi-ometry, and detector physics. These photometrists have newneeds. They are dealing increasingly with high-efficiencyfluorescent lamps, high-intensity discharge lamps, and othertypes of specialized lamps with complex spectral distributions.As a result, the candela had been redefined. The new definitionties the photometric unit of luminous flux directly to the watt,and effectively unifies the fields of photometry and radi-ometry.

Thus in many ways this Conference is a landmark, onewhere the optical frequency range has begun to play a more-or-less equal role with other frequency regimes.

Cryoelectronics is another field that has appeared and de-veloped during this Conference series. Its most successfulapplication in metrology so far has been the Josephson voltage

0018-9456/83/0300-0001$01.00 © 1983 IEEE

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-32, NO. 1, MARCH 1983

standard. Many refinements of this application will be dis-cussed this week.Two years ago, at the 1980 Conference, we experienced the

excitement of hearing the first announcement by von Klitzingof his discovery of the usefulness for metrology of anotherlow-temperature phenomenon, the quantized Hall effect.There has been much work on this effect in the intervening twoyears. Several speakers will discuss the significance of this workin the general context of fundamental constants and stan-dards.

Recent work in cryoelectronics has also demonstrated theusefulness of Josephson-effect devices for very fast samplingand digitization of electrical waveforms. Work is in progressto develop complete waveform measurement systems usingJosephson technology at the front end. The combination, ona single chip, or a fast optical detector with sampling and di-gitizing devices, using Josephson technology, is also a veryappealing idea. It will probably be several years before we seethe results of this work, but it is exciting research.

Meanwhile, the whole field of waveform metrology is beingchallenged by the increasing speed with which data can beprocessed and transmitted. There is an increasing recognitionthat much scientific information can be retrieved by analyzingtransient events such as photochemical reactions. Progress withthe problems of sampling and analyzing waveforms in the timedomain, and providing proper waveform standards, will bediscussed at two sessions of this Conference.

Telecommunications via satellite is another technology thathas appeared since 1958. This brings with it new metrologicalchallenges. For example, greater precision of measurementof the characteristics of antennas and other components ofsatellite telecommunication systems are needed. Satellitetelecommunication is also an excellent way to disseminatestandard time signals. Some accomplishments in this area, aswell as some of the measurement challenges, will be discussedduring the Conference.

Perhaps the most striking change in measurement tech-niques in recent years has been the introduction of automation.The fact that three sessions in this Conference are devoted toautomated measurements attests to their importance. Im-perfections of a measurement system no longer need to becompensated for by tuning and trimming. Rather, they can bemeasured and corrections then applied by the computer. Te-dious and extremely fine mechanical adjustments are givingway to the act of punching instructions on a keyboard. Elec-tronic servo loops are replacing manual adjustments. The resultis higher accuracy and more consistent performance. Mea-

surements can be repeated often enough to create a soundstatistical basis for uncertainty statements. A good exampleof the radical change that automation can bring to a mea-surement system can be seen in the six-port system for mi-crowave measurements, which will be discussed at thismeeting.The first requirement for automation was the availability

of small, inexpensive computers. This has been satisfied. Thenext challenge was to have the computer control and monitorthe variety of electronic measuring instruments that comprisea particular measurement system. This step is complicated bythe fact that the instruments are usually made by severalmanufacturers and are not necessarily compatible. A solutionto this problem was the development of the IEEE standardinterface bus. Nearly all present-day electronic instrumentsand minicomputers are compatible with this standard inter-face. This development is most commendable. It will be rec-ognized during the Conference with the presentation of theIEEE Instrumentation and Measurement Award to the prin-cipal authors.

Finally, automation has enabled the development and uti-lization of large, complex, automatic testing systems. Theseare indispensible in modern, automated factories for themaintenance of complex electronic equipment. Calibration ofautomatic test equipment has imposed new challenges on users,manufacturers, and the standards laboratories. There is a needfor broad measurement ranges, high operating speeds, andremote calibration capabilities, since it is usually not feasibleto transport automatic test equipment to a standards labora-tory. Strategies and physical standards need to be developedto evaluate the equipment in its operating environment. Thiscan be done through built-in standards, self-checking opera-tions, and periodic system-based calibrations with transport-able standards. NBS has been active in this area. We have beendeveloping automated calibration methods and facilities suchas those for D/A and A/D converters and precision pulsegenerators. These efforts provide support to the developmentof viable approaches to the calibration of automatic testequipment. Several papers will report progress in this area.

All the topics addressed at this Conference are very closeto the basic mission of NBS. The Conference provides avaluable opportunity for NBS staff to meet their colleagueswho are working on similar problems in other organizationsand in other countries. I am proud that the National Bureauof Standards is hosting this Conference. I welcome you, andhope that the Conference series will continue to fluorish in thefuture.

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