Search

This article is a revision of the previous edition article by Stata Norton, volume 2, pp. 621–628, © 1997, Elsevier Inc.

There are a number of factors that give rise to the selective vulnerability of the nervous system to toxic insult. This chapter attempts to give an overview to allow the reader to better understand what about the nervous system produces vulnerability to toxic insult as well as give some examples of neurotoxins and how they affect the brain. This chapter presents the events during development such as migration and differentiation as well as gene expression and neurotrophic guidance that if disrupted can result in permanent dysfunction of the nervous system. This chapter also presents the functional aspects of the mature brain such as metabolic demand, high lipid content, and lack of functional reserve that present targets for toxic insult of the mature brain. Finally, this chapter concludes with an overview of region specific neurotoxins such as methylmercury (MeHg) and the cerebellum, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and dopaminergic neurons, and trimethyltin (TMT) and the hippocampus, and how the unique cellular physiologies produce specificity for these toxins.

A new approach to the design of a wide-band feed-forward amplifier (FFAMP) is presented in this paper. Phase equalizers are employed in an FFAMP to match the nonlinear delay/phase characteristics of the main and error amplifiers, improving phase balances within the cancellation loops and providing improvement in signal cancellations over a wide bandwidth. The proposed 1.7-1.9-GHz FFAMP was fabricated and characterized. The conventional FFAMP obtains an average of 15-dB third-order intermodulation (IM3) distortion cancellation over the whole bandwidth. With the phase equalizers, the proposed FFAMP achieves a further 6-dB reduction on IM3 level.

The authors describe an efficient and robust approach to the computation of the steady-state response of periodic nonlinear microwave circuits. The problem of solving a set of differential equations is converted into that of solving a system of nonlinear algebraic equations using a technique called convolution-based sample balance. Although exact in all cases for which harmonic-balance techniques are exact, this technique does not require the use of discrete Fourier transforms, and calculating the Jacobian is straightforward. For the solution of the resulting system of nonlinear equations, an efficient and yet robust algorithm has been developed. In the examples given, savings in computational effort of over 85% are reported when this algorithm is compared with Newton's method. >

The Wolfenstein parameters Dt, Rt and At have been measured in free np elastic scattering with an accuracy of ±0.05 at 220, 325, 425 and 495 MeV in the centre of mass angular range 60° to 160°. The polarisation P has been measured with an accuracy of ±0.015. The I=0 phase shifts differ significantly from theoretical predictions, particularly in the central and spin-orbit combinations of D waves.

High momentum parts of the momentum distribution of annihilation photons are examined both theoretically and experimentally for vacancies and vacancy-oxygen complexes in Si, Atomic relaxations in the neighbourhood of the defects are taken into account, Theoretical calculations are compared with measurements performed on undefected and electron irradiated Czochralski grown and float-zone Si samples.

A physical model has been used to characterise the large-signal behaviour of a GaAs MESFET terminated with a wide range of load impedances and a new, automated, load-pull system developed to make measurements in order to verify the performance of the model. The heart of the measurement system is a motorised slide-screw tuner which operates over 2-18 GHz with reflection coefficients greater than 0.93 above 3 GHz.

A quasi-state, large-signal metal-semiconductor field-effect transistor (MESFET) circuit model was developed. It is based on a comprehensive quasi-two-dimensional semi classical device physical simulation where its unique formulation and efficiency makes it suitable for the CAD (computer-aided design) of nonlinear MESFET subsystems. A single/two-tone harmonic balance analysis procedure which uses the describing frequency concept was also developed and combined with the MESFET model. Application of the describing frequency concept drastically reduced the complexity of the two-tone excitation problem by transforming it to an equivalent single-tone case. Numerical load-pull contours and intermodulation distortion contours were simulated, and excellent agreement with experimental results was demonstrated. >

A numerical simulation to characterize both the DC and RF behaviour of GaAs MESFETs is described. It is based on coupling the four semiclassical semiconductor equations with analytical expressions for the channel and solving these using a forward-difference scheme. In the approach taken, the semiconductor equations are reduced to a one-dimensional approximation using the expressions for the channel, requiring substantially less computing resources than a full two-dimensional simulation. CPU time is typically reduced by a factor of 1000. DC and RF results for a 0.5- mu m MESFET are presented. >

An experimentally derived frequency-dependent residual error budget to 60 GHz in well-calibrated state-of-the-art measurement systems for millimeter-wave devices is presented. The budget identifies errors resulting from the imperfections in the calibration standards, both on- and off-wafer, probe-wafer interface, and instrumentation. Root-mean-square errors are assumed uncorrelated and are added to arrive at a worst-case scenario. The impact in terms of level of significance and sensitivity of measurement uncertainty on small-signal modeling is shown. The effects of measurement uncertainty on extracted device parameters can be minimized by selecting a suitable frequency range and appropriate calibration structures. >

The authors present a CAD (computer-aided design) tool which combines a novel quasi-two-dimensional MESFET physical model with an efficient nonlinear circuit analysis technique. Device technological parameters (dimensions, doping profile, etc.) can be directly related to electrical performance with this tool, which can therefore be used in nonlinear circuit yield study and optimization. An excellent agreement between simulated and measured results was obtained. >

The measurement of an active device produces results which depend on the transmission medium in which the device is embedded. These differences, insofar as a particular device is concerned, are related solely to its extrinsic elements, the intrinsic device being the same in all cases. Results are presented for on-wafer measurement and 2-D simulation of a specially designed active device and its associated transmission structure in both microstrip and coplanar waveguide. The measured and simulated results are obtained for pseudomorphic HEMTs (high electron mobility transistors) with a 0.2- mu m gate length arranged in a 6- mu m*15- mu m configuration with typical extrinsic f/sub t/'s of 70 GHz. The dual approach considered here has the advantage of enabling a model for the intrinsic device to be validated and the extrinsic model elements for both structures to be obtained. >

An efficient physical model capable of predicting the effects of self-heating as well as ambient temperature on the DC and microwave characteristics of MESFETs is outlined. Excellent agreement has been obtained with measured characteristics for a range of gate length devices for both DC and microwave characteristics. The model provides great insight into device behavior and is capable of relating physical structure to device performance while still being efficient enough to run on a personal computer. >

The coplanar-to-microstrip transition part of on-wafer microstrip line-reflect-line (LRL) calibration structures has been modeled and measured to determine calibration repeatability due to variations in this structure. On-wafer small-signal device characterization is dependent on the quality and repeatability of the calibration, which depends on the calibration structures. When these are fabricated on the same wafer as the devices to be characterized they are also affected by process variations, the most important of which, when using LRL calibration structures in microstrip, is the variation in the thickness of the wafers. The LRL calibration solution is modified to include the process variation in the microstrip calibration structures in terms of propagation constant and effective line length. Both probe contact and probe position repeatability are identified in the experimental results as they are inevitably measured with process variations. >

The sensitivity of distortion cancellation in feedforward amplifiers to amplitude and phase imbalance is investigated. Some workers have suggested that the amplifier is more sensitive to imbalance in signal cancellation loop. It is shown here that this is not true in general. It can become more sensitive to imbalance in either cancellation loop depending on their level of cancellation. Results of a feedforward amplifier simulation are presented to confirm this finding.

High-resolution spatial data is being used to re-evaluate the hydrology and hydraulics of Jordan Creek in Springfield, Missouri. This analysis and evaluation was conducted to demonstrate the applicability and efficiency of using digital satellite data for flood and floodplain evaluation.

Publisher Summary This chapter presents a series of results on the photo degradation of polymer-based coatings induced by UVA, UVB, Xe-light and natural light irradiation as studied by slow positron annihilation and electron spin resonance methods. The S-parameter and o-Ps intensity from the positron method are found to decrease with exposure time. Current results show that the slow positron technique is a sensitive tool for detecting coating degradation in a time much earlier than any existing testing methods. The electron spin resonance results show promise for identifying the specific chemical bonds or locations that are responsible for the deterioration of coatings. Combining these two atomic and molecular testing methods will be pursued next in a systematic way for both commercial products and model compounds. The next major research activity is to perform engineering tests and to corelate change in macroscopic engineering properties with observed nanoscale changes detected by PAS and ESR.

Computer Aided Design method has been developed to synthesise E-plane band-pass filters with improved stopband performance. The theoretical investigation of a novel type of E-plane bandpass filters with improved stopband performance composed of a metal insert which when mounted in the rectangular waveguide housing introduce ridges in the resonators is presented. The solution not only maintaining the simplicity of the structure, but also providing good stopband performance and reducing the overall length of the filter. Design data for a five resonator X-band (8.2 - 12.4 GHz) E-plane bandpass filter with ridges at 9.5 GHz midband frequency are given.

The purpose of this paper is to identify the differences between millimetre-wave small-signal scattering parameter device measurements on-wafer in coplanar and microstrip test formats. Calibration in coplanar and microstrip is examined to determine the measurement reference planes and possible calibration problems in the millimetre-wave frequency range. Different device model reference planes are considered.

The sensitivity of FET parameters to variations in geometry and manufacturing tolerances is predicted and qualified using an accurate physical model. The simulated results are compared to a specialily designed wafer of MESFETs containiing a range of devices, with differing geometrical parameters, chosen to give a wide range of both DC and small-singal performance.

A technique for finding the steady-state response of nonlinear microwave circuits is presented. Although exact in all cases for which Harmonic-Balance Techniques (HBTs) are exact, its formulation permits a somewhat easier implementation and does not require the use of Discrete Fourier Transforms (DFTs). The technique takes an interesting form when the nonlinear block-SOR method is used to solve the resulting nonlinear system of equations. Stability conditions for this situation are given together with examples of its application.

For the original article see ibid., vol.37, no.11, p.1675-80, 1989. In the above paper by R.R. Pantoja, M.J. Howes, J.R. Richardson, and R.D. Pollard generalized TRL method was proposed as an alternative to the TRL and LRL calibration methods. The contributions of the work, according to its authors, are the reformulation in terms of S-parameters and the removal of the requirement to specify a line length. The commenter argues that only the formulation, not the method itself, is novel. A detailed reply from the original authors follows in which the four specific points raised by the comments are dealt with in order to ensure proper understanding not only of what is described in their paper but also of the whole family of calibration procedures under the increasingly common TRL classification. >

Feedforward linearisation offers a viable way to reduce intermodulation distortion generated in power amplification, the actual reduction depending on the degree of cancellation in the feedforward cancellation loops. Commonly, cancellation is calculated from design formulae developed by assuming perfect impedance matches between subcircuits and perfect isolation in the power splitting/combining networks, but these are difficult to achieve in practice. The effects that mismatches and imperfect isolation have on cancellation are studied and investigated and it is shown that the transmission characteristics of the cancellation loops can deviate significantly from those calculated using the above assumptions, resulting in poorer cancellation. The use of tuning circuits to compensate for these effects is also examined and simulation shows that mismatch and imperfect isolation can effectively be compensated over a very narrow frequency band by this technique and good cancellation achieved, though it becomes less effective as bandwidth increases. Experimental results are presented to verify the simulation results.

Feedforward amplifiers with very good distortion cancellation over narrow bandwidth have been reported. However, if an amplifier in a cancellation loop has a nonlinear phase response, the 180/spl deg/ phase difference between halves of the loop required for perfect cancellation cannot in general be maintained over a wideband if only simple delay line is used. The use of a phase equaliser to improve wideband feedforward cancellation is proposed. Simulated and measured results show a significant improvement in cancellation over the frequency range 1.7-1.9 GHz.

Measurements over an extensive angular range of the D/sub t/ and P parameters in free neutron-proton elastic scattering at laboratory energies of 220, 325, 425, and 495 MeV are reported. Experimental and analytical details are given.

A phase-shift analysis is reported incorporating TRIUMF data on np total and differential cross sections and values of Δσ L (pp) and Δσ T (pp), together with all other np data from 120 to 550 MeV and pp data from 120 to 831 MeV. The solutions are unique over these energy ranges. They are compared critically with the predictions of the Paris group.

Data on D/sub t/, R/sub t/, A/sub t/, and P lead to unique and accurate np phase shift solutions at 210, 325, 425, and 515 MeV. The I=O, D, and G waves show large systematic deviations from current theoretical models.

A novel procedure for the calibration of microwave integrated circuit test fixtures, based on a generalization of the through-reflect-line (TRL) algorithm, is presented. Its advantages compared with previous methods, namely bandwidth of validity and standards availability, are discussed. The approach is verified through the characterization of a particular microstrip verification standard using both the generalized TRL and precision 7-mm calibration techniques. Comparison of the results obtained from these schemes indicates that both the effective directivity and the source/load match are better than 30 dB. >

This paper is concerned with a policy issue associated with the specification of insurance policies for medical care expenditures, viz. whether health insurance schemes should incorporate provisions (deductibles and/or co-insurance rates) such that patients bear some part of the cost of any medical service. The we 1 fare economic literature on this issue is shown to be can fused as a result of non-recognition of the significance of risk aversion. When the issue is viewed as a problem of port folio selection it is clear that no simple policy conclusion is forthcoming. With respect to the pragmatic argument that co-payments will reduce health expenditures it is shown that such an effect is "once-off", is quantitatively small and has the effect of increasing administrative coats. It is argued that the distributional effects may be adverse for persons with low incomes.

A quasi-static, large-signal MESFET circuit model is presented. It is based on a comprehensive quasi-two-dimensional, semiclassical, physical device simulation, and its unique formulation and efficiency make it suitable for the computer-aided design of nonlinear MESFET subsystems. Using this approach the semiconductor equations are reduced to a consistent one-dimensional approximation requiring substantially less computing resources than a full two-dimensional simulation. CPU time is typically reduced by a factor of 1000. A single/two-tone harmonic balance analysis procedure which uses the describing frequency concept is also developed and combined with the MESFET model. Numerical load-pull contours as well as intermodulation distortion contours have been simulated; their comparison with measured results validates the approach taken. >

The beta /sup +/ decay of Ga/sup 54/ and the subsequent gamma spectrum from the de-excitation of Zn/sup 64/ were studied. The beta /sup +/ transition to the ground state of Zn/sup 64/ has an end point energy of 6.05 plus or minus 0.03 Mev with log ft = 6.6 and another beta /sup +/ group or groups has an end point energy of 2.79 plus or minus 0.08 Mev with a log ft = 4.6. No beta / sup +/ transition to the first excited state of Zn(2+) was observed and it was shown that the log ft corresponding to this transition is greater than 7.7. The gamma rays that were observed (0.80, 0.99, 1.25, 1.38, excited states at 0.99(2+), 1.78(2+), and 3.32(1+) Mev. The positron results indicate that the ground state of Ga/sup 64/ has spin and parity 0+. (auta)

The dependence of the switching current of a continuous sheet memory cell on drive line width has been self-consistently calculated. It is shown that there exists a region of reduced width dependence for such cells if the drive-line width is comparable with a fundamental length defined in terms of the film thickness and penetration depth.

Accurate measurements of the P and DT parameters in np elastic scattering at 325 MeV resolve a major ambiguity in the phase shift analysis. The mixing parameters e 1 = 8.4 ± 0.6° and e 3 = 7.2 ± 0.4° are particularly well determined, and in good agreement with theoretical expectations.

The existence of a 11 isomeric state is reported in 24Al, with a half-life of 129±5 ms. The energies and intensities of the isomeric gamma transition and of three beta transitions have been measured.

The final state p-p interaction has been observed by means of a 3 He(p, pd)p angular correlation experiment. It was found that the probability of formation of the singlet state is most pronounced for an internal energy of the two-proton system of ≈ 0.4 MeV.

Correlation spectra from the p + d breakup reaction have been measured at 46 MeV. The data, which show pronounced effects due to final state interactions and quasi-free processes, have analyzed in terms of three different spectator model calculations and the Chew-Low extrapolation procedure.

The variation of the input impedance of microstrip antennas on PTFE and GaAs substrates with substrate size is presented. This is of particular relevance to the design of monolithic active sub-arrays. The results show significant changes of resonant frequency and resistance with substrate size. The effect on the impedance can be neglected for substrates greater than ?/4 in width.

An analysis technique for MESFET microwave oscillators based on a large-signal equivalent circuit model is presented. The operation of the circuit may be simulated, generating long periods of time-domain data that may be used to accurately predict the oscillator's frequency of operation, power output and harmonic content. By incorporating low-frequency, time-domain noise sources derived from physically representative frequency spectra the technique has the potential for modelling close-to-carrier noise, and could be used for the analysis of noise up-conversion and the effect of the device/circuit interaction on output noise. The model, and its use as both an analysis technique and a design aid is described. Difficulties encountered with the demodulation of the noise spectrum are discussed.

Perturbation theory is applied to a rectangular resonant cavity to measure the complex permittivity of small rectangular samples of microwave circuit substrates at X-band frequencies. The technique employed is non-contacting and requires no metallization of the sample. An automatic network analyser is used to determine the resonant frequency and Q-factor of the cavity both with and without the sample and this enables fast and highly accurate measurements to be performed; the real and imaginary parts of the permittivity may be measured to less than 1% and 10% respectively. The accuracy of the technique, however, depends greatly on the sensitivity of the cavity response to errors in the location of the sample and the importance of choosing a suitable position for the sample is illustrated.

This paper reviews the most recent automated tuning techniques and comments on their respective advantages. The development of a simple model, capable of predicting the response of a slide-screw tuner from 45 MHz to 18 GHz, is described and compared with measurements of an existing tuner. The parameters affecting slide-screw tuner performance are identified with particular attention being paid to the slug. A tuning slug design is presented achieving optimum tuner performance over the 2 to 18 GHz frequency range.

The 3-Port Scattering and Noise parameters of a GaAsFET are measured. The noise measurement method is novel and is verified by good agreement between the measured 3-Port Noise parameters of a passive network and predicted noise parameters calculated from its 3-Port S-Parameters. Circuit theory is presented for calculating the Scattering and Noise parameters of a 3-Port transistor embedded in a passive network. The circuit is represented by a signal flow graph containing partially-correlated stationary random noise sources. An algorithm is presented which uses the measured transistor data and the circuit analysis technique to design amplifiers with simultaneous input power match and optimum noise figure. Such amplifiers do not use isolators or a balanced configuration and hence are more amenable to economic MMIC implementation. The procedure is illustrated using the example of a circuit consisting of a transistor, a lossless series feedback inductor and a lossless parallel feedback capacitor.

Publisher Summary This chapter discusses the role of simple mechanisms in the proton induced break up of 3He and 3H. One of the approaches related to the understanding of the few nucleon problems lies in the experimental study of the contributions from various open channels to the total reaction cross section of the light nucleus. All experiments have been performed in a coplanar geometry at angular settings that were either symmetrical for both the arms or chosen so as to demonstrate the enhancement in the cross -sections because of a particular reaction mechanism. Among the three body processes, both the (p,2p) and (p,pd) reactions show large enhancement in the reaction cross sections because of quasi-free scattering. Enhancement because of p-p final state interactions is observed at appropriate pairs of angles. Within the four breakup region of 3He, the 1S0 neutron–proton correlation can be isolated. Contributions from quasi-elastic scattering in the spectra can be reasonably well reproduced by plane wave impulse approximation calculations, except in the case of (p,pd) reaction, where possibly neutron pickup becomes important. The cross sections for final state interactions can be obtained reasonably well by using Watson–Migdal formalism.