Contains reports on two research projects.Joint Services Electronics Program (Contract DAAG29-78-C-0020)National Science Foundation (Grant AST77-26896

Allen, Barry R.

Bennett, Charles L.

Burke, Bernard F.

Garcia-Barreto, J. Antonio

Greenfield, Perry E.

Gupta, Madhu S.

Lawrence, Charles R.

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XI. MICROWAVE AND MILLIMETER WAVE TECHNIQUESAcademic and Research StaffProf. Bernard F. Burke Prof. Robert L. Kyhl Dr. Alan ParrishProf. Madhu S. Gupta Dr. David H. RobertsGraduate StudentsBarry R. Allen Perry E. Greenfield Charles R. LawrenceCharles L. Bennett Aubrey D. Haschick Daniel D. StancilJ. Antonio Garcia-Barreto Peter V. Wright1. FLUCTUATIONS IN NONLINEAR SYSTEMSJoint Services Electronics Program (Contract DAAG29-78-C-0020)Madhu S. GuptaDuring the year 1978, the study of thermal fluctuations in physical systems led tothe following four principal achievements:a. Thermal Noise in Driven Nonlinear SystemsA new theorem expressing the spectrum of thermal fluctuations in terms of the phe-nomenological response of the system has been proved. It is more general than the so-called fluctuation-dissipation theorem, in that it applies to nonlinear systems, subjectedto a stationary driving force.l In an attempt to understand the scope of the theorem, aconsiderable amount of effort was devoted to explicitly stating the assumptions inherentin the method of proof, and to exploring the necessity of those assumptions. Applicationsof the theorem to electron devices are presently being studied and will be published later.b. Hot-Electron Diffusivity in Two-Valley SemiconductorsThermal velocity fluctuations in a two-valley semiconductor were studied by an ana-lytical random-walk model of carrier scattering. This study resulted in the calculationof the electric-field dependent longitudinal diffusivity of charge-carriers, a transportparameter of considerable importance in high-frequency semiconductor devices. Therandom-walk model has the novel feature that it models carrier motion in terms of twosimultaneous random walks, which are coupled. The calculated results for GaAs agreewell with the available experimental data on diffusivity. 2c. Ultimate Limits to Nonlinearity of Lossless Energy-Storage SystemsA study of the equilibrium thermal fluctuations of an extensive variable in an ideal,lossless, energy-storage system led to the hitherto unknown fact that the nonlinearityof such a physical system is thermodynamically limited. An upper and a lower boundPR No. 121(XI. MICROWAVE AND MILLIMETER WAVE TECHNIQUES)were established for a measure of nonlinearity of the system. The bounds were illus-trated through the example of varactor nonlinear capacitors. Such bounds are of theo-retical interest in studies of the fundamental limitations of physical devices. 3d. Fluctuations and Dissipation in Information ProcessingA one-bit information storage system is modeled as a bistable equilibrium thermo-dynamic system, with one nonthermal, conserved extensive parameter serving as theinformation-bearing degree of freedom. The system is assumed to be in contact witha thermal reservoir which serves to induce randomizing fluctuations within a state butdoes not cause jumps between the two stable states. Such a system is shown to be nec-essarily nonlinear, energy-storing, and dissipative during switching. 4 Energy dissipa-tion during a state change of the system, caused by a flow of the extensive variable, iscalculated by adding the components due to the irreversibility of flow and due to thenecessity of determining the state of the system prior to state change. The minimumdissipation required is approximately 2 kT per switching operation and occurs whenapproximately 2 bits of information is obtained about the state of the system. It is em-phasized that this dissipation is not confined to the information storage system itself,and therefore the heat to be removed from the system may be lower.e. Microwave DevicesTwo other papers, describing the work of the previous year, were prepared for5, 6publication during this year.Referencesi. M. S. Gupta, "Thermal Fluctuations in Driven Nonlinear Systems, " Phys. Rev. A 18,2725-2731 (1978).2. M. S. Gupta, "Random Walk Calculation of Diffusion Coefficient of Hot Electrons inTwo-Valley Semiconductors," J. Appl. Phys. 49, 2837-2844 (1978).3. M. S. Gupta, "Thermodynamic Limits to Nonlinearity: Lossless Energy StorageSystems," J. Appl. Phys. 50, 569-573 (1979).4. M. S. Gupta, "Thermodynamics of Information Processing: Minimum Energy Dissi-pation Requirements, " to be published.5. M. S. Gupta, "Stability Measure for Negative-Resistance Amplifiers, " Int. J. Elec-tronics 45, 241-245 (1978).6. G. K. Montress and M. S. Gupta, "Microwave Characteristics of Silicon BARITTDiodes under Large-Signal Conditions, " to appear in IEEE Transactions on Micro-wave Theory and Techniques, Vol. MTT-27, No. 5, May 1979.PR No. 121(XI. MICROWAVE AND MILLIMETER WAVE TECHNIQUES)2. THE M.I. T. THREE-ELEMENT MICROWAVE INTERFEROMETERNational Science Foundation (Grant AST77-26896)Bernard F. Burke, Barry R. Allen, J. Antonio Garcia-Barreto,Perry E. Greenfield, Charles L. Bennett, Charles R. LawrenceThe M. I. T. aperture-synthesis microwave interferometer consists of two fixed andone movable 5.5-meter paraboloids suitable for wavelengths as short as 7 mm. Theantenna separations are east-west, and range between 9 and 300 meters. At the currentoperating frequency of 22 GHz, these correspond to spacings between 680 and 23, 000wavelengths.The 22-GHz radiometers are conventional regulated ambient-temperature diodemixers fed by circular polarizers. The bandwidth is 75 MIHz, and the double-sidebandnoise temperature is approximately 600 K. The IFs are sampled at 150 MHz by ECLcomparators, and are correlated and summed in eight parallel streams at 18.75 MHzin a three-level scheme. Digital delays between 6.7 and 853. 3 ns in 6.7-ns steps areaccomplished by multiplexers and latches at 150 MHz and by shift registers clocked at18.75 MHz. Additional cable delays of 0.83, 1.67, and 3. 33 ns are used to keep thesystem response within 1 percent of optimal. A Nova 820 minicomputer controls thepointing of the antennas, steps the delays, interrogates the correlators, performs areal-time fit to the fringes, and writes the results on magnetic tape.The interferometer has recently been brought into operating condition, and we arebeginning to take data toward aperture synthesis. Panel adjustments in two of the disheshave resulted in substantial increases in antenna efficiency, which we now measure tobe approximately 0. 50. The antenna pointing scheme has been revised. Correctionsfor azimuth zero-point, gear-ratio, and collimation error and for zenith gear-ratio andcollimation error are now made by real-time computer evaluation of empirically deter-mined correction formulae. This replaces a table lookup procedure which resulted inunacceptably large pointing discontinuities at some positions in the sky. Further im-provements in the operating system which are under way include extension of its un-manned operating capability to permit an arbitrarily long sequence of sources to beobserved automatically.The processing of visibility data from the interferometer will be accomplished bya combination of M. I. T. and NRAO programs on the M. I. T. IBM/370 computer. TheM. I. T. software performs minor corrections related to the real-time fringe fitting.The NRAO-VLA package (kindly provided by Dr. Eric Greisen) has been modified forour site, antenna configuration, and computer. This package of programs is especiallywell suited to our system since, like the VLA, we have alt-az mounted dishes and em-ploy circular polarizers. Adaptation of these programs is being completed, and debug-ging is beginning. Preliminary working versions of the M. I. T. programs exist inPR No. 121(XI. MICROWAVE AND MILLIMETER WAVE TECHNIQUES)Fortran, but these will be translated into PL/I for compatibility with the VLA package.This development has value for other programs as well, since we are making extensiveVLA studies, and will be doing much of the processing in-home eventually.Our first goal is to produce a high-quality map of the supernova remnant Cassio-peia A. One year ago, as we began this project, a number of subtle flaws in the sys-tem were uncovered, and we are confident that they are now all fixed. During the summerof 1978, a study of Saturn and its rings was made, and we derived an upper limit of 250for the apparent brightness temperature of the map. Amplitude stability of the systemwas clearly a problem in these measurements, which would otherwise have produced amore interesting measurement. During the fall and winter of 1978 the problems weregrappled with, and the system is ready to go. We built circularly polarized feeds forthe system, which makes the separation of total-power distribution and linear-polariza-tion distribution far more tractable for strongly polarized sources such as Cas A.Calibration data are currently being taken with the system. The phase stability ofthe system has always been good (30 repeatability day-to-day), and the synthesis ofCas A should proceed during the summer and fall of 1979. A total-power map will firstbe derived, followed by a polarization map. The resolution of the maps will be com-parable to that of the best studies made at longer wavelengths. By studying the degreeof polarization at the shorter wavelength, compared to the more depolarized and Faraday-rotated longer wavelength studies, we should be able to make a detailed study of mag-netic-field strength and geometry in the remnant. We also plan to map the compact H IIregion DR21, which requires relatively few interferometer spacings for a complete map.The next step in the interferometer work will be to change the frequency to 43 GHz.While we could continue working at 22 GHz, we have developed a high-quality cooledmixer for 43 GHz that is described in our new NSF research proposal. Millimeter-waveinterferometry is a largely unexplored field, and while some interesting work will re-main at 22 GHz, many of the more attractive possibilities will be better studied by theVLA. (Cas A is not a practical VLA target, and is much better suited to our smallsystem.)In view of our greatly improved system noise temperature at 43 GHz, a change inobserving frequency at the close of the Cas A synthesis work is indicated. At the sametime, we are developing a 100-channel crosscorrelator that should also be ready for usein early 1980. Studies of SiO radiation will be made with the system, but continuum andother line sources will also be studied.There is a philosophical point that should be made concerning the interferometer.If we chose to make it the major focus of our observing program, concentrating our fivegraduate students, three engineers, and one or two postdoctoral graduates on the proj-ect, it would proceed at a far more rapid pace. It would not then be a small project,however, and the major thrust of our graduate student training and research programPR No. 121(XI. MICROWAVE AND MILLIMETER WAVE TECHNIQUES)would be drastically changed. As it now stands, the interferometer project representsan incremental cost to the program of about $70,000 per year, since the engineeringforce is also used for other developmental programs, such as building new front endsand observing hardware for VLBI experiments. Most of the hardware in recent yearshas come from M. I. T. private funds, and the result has been to provide a unique train-ing ground for students. Our graduate students have always been in high demand, sincethey are familiar with hardware and software, and are experienced in the use of majornational facilities. The interferometer gives a truly effective introduction to the realworld of instruments, without which there would be no observing. It thus justifies itsexistence both as a teaching tool and as a driver for new research. We have developedthe fastest correlator presently in use in radio astronomy, and future state-of-the-artdevelopments will continue.PR No. 121

Microwave and Millimeter Wave Techniques

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Microwave and Millimeter Wave Techniques

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