Table of contents

The reasons for the changes in the values of the UK national reference standards of EMF and resistance are discussed. The agreed values for the Josephson constant and the von Klitzing constant are given, together with a brief discussion of the evidence upon which these values are based.

A technique for determining the impulse and step-response function of fast sampling oscilloscopes with rise times down to 20 ps is described. Electrical pulses are generated by a photodiode using a picosecond laser. The electrical pulses are measured using an electro-optic sampler and applied to the oscilloscope under test. The response of the oscilloscope is derived from its record of the photodiode pulse by deconvolution.

A new improved method of calibrating waveguide power standards is described. The main improvements are that the measurement time has been reduced from two hours to 15 minutes and a residual error has been eliminated. This has been achieved while maintaining a mechanically flexible design. The theory for using the isothermal process is developed and it is shown that all linear (i.e. conductive) imperfections of the microcalorimeter are corrected for, together with any effects due to incomplete settling of the exponential time constants. The uncertainty budget is presented giving a total uncertainty of +or-0.5% (1 sigma ). The isothermal method is compared with the conventional method at 75, 80, 85 and 94 GHz and is shown to agree to within the overall uncertainty.

Twenty-four samples of the so-called Ultra-Zener, an integrated circuit containing a buried Zener junction, type no LTZ 1000, have been tested since March 1988. To measure the promised low rate of change of voltage with time (aging), a voltage measuring system with a resolution of 0.1 mu V at the 7 V level has been developed and is described. Preliminary results are that half of the samples have an aging rate of less than1.5 PPM/year from about two months after being assembled onto their circuit boards. Both intermittent operation and setting a lower chip temperature have been found to reduce the aging rate. The standard deviation of a set of 24 readings of the Zener voltage over half a minute is typically 0.05 PPM (0.3 mu V on a 7 V level). For a 'serious' voltage standard, a 1 PPM/year aging rate has been stated to be a desirable maximum level. This is attainable with the Ultra-Zener. In addition, its low noise, excellent portability and good electrical robustness make it a replacement for a standard cell bank as a voltage transfer standard.

Knowledge of the in vivo dielectric properties of human tissues is essential to the development of biomedical techniques which make use of radiofrequency and microwave radiation. The sensors used for measurement of tissue properties can be conveniently calibrated using dielectric reference liquids, but this in turn requires that the dielectric properties of such liquids be accurately known. Many complex permittivity measurements on suitable liquids, such as the primary alcohols and saline solutions, have been reported in the literature, but unfortunately there are in some cases significant discrepancies between reported values. The authors describe a program of work undertaken to resolve some of these anomalies. A number of techniques which make use of 14 mm coaxial cells and sensors coupled to an automatic network analyser are under investigation. Comparison of the different techniques should facilitate evaluation of systematic uncertainties. The liquid measurement geometries and the associated computational methods are described. Details are given of the uncertainty analysis, and examples of mappings from the complex permittivity plane on to the scattering coefficient plane are shown. Some preliminary results are presented, which highlight the value of the comparative approach that has been adopted.

The modulated scatterer technique allows one to measure fine structure in electromagnetic fields with the minimum of perturbation. The authors present a general theory for the monostatic use of a modulated scatterer and show that in certain circumstances, for example in the reactive near-field of an antenna, the modulated scatterer may be used monostatically where conventional techniques would fail. They have verified the technique experimentally by constructing a broadbanded homodyne detector, working over the frequency range 2-18 GHz, and using it to measure scattering from an optically modulated electric dipole in the near-field of an open-ended waveguide. The modulating waveform is conveyed to the scatterer via a flexible plastic optical fibre, this provides a constant level of modulation as the scatterer is moved around, without introducing any conducting material into the neighbourhood of the scatterer.

The author outlines briefly the material properties of ceramic superconductors, the techniques for making and processing them and specific problems which prevent them being used more widely. Most attention is paid to YBa₂Cu₃O₇ material and its deposition as thin films.

The problems of thermal inertia and self-heating encountered when using platinum resistance thermometers for the precise measurement of gas temperatures are discussed and a technique for remote measurement of the response function of these devices is described. Results of sensor response times measured in different gas flow velocities are given together with theoretically calculated corrections for thermal response errors.

The construction of a new high-resolution multichannel low-cost pulse generator is reported. It is fully computer controlled through a simple RS232 serial interface. Its features are 1 ns resolution within any time, pulse delays up to 16 ms and pulse lengths up to 65 mu s, and pulse sequence repetition rate from 66 Hz up to 250 kHz. It has fully programmable sequencing, including step increments for any pulse delay or length. It governs a pulsed ESR spectrometer, which is also described, but it could be used in a very wide range of experimental set-ups. A few examples of spin-echo detected ESR and ESEEM of some paramagnetic centres are shown.

An advantageous method of determining the uranium and the thorium series content of natural samples is described and assessed. This novel application of coincidence counting to natural dosimetry could be use in a range of studies of geological and archaeological materials, including thermoluminescence and related dating processes.

Expressions for the mechanical deflection and the piezoelectric current of a piezoelectric beam located in a general electric field are derived. It is shown that the relationships between the interesting macroscopic electric and mechanical quantities are determined by only three integrals. Two of these are proportional to each other for a beam and the third is normally negligible. The remaining integral depends only on the dielectric field and can be determined by using the well known Laplace equation. The value of this integral can be estimated accurately without the need of a computer. It is shown by eigenmode decomposition that it is enough to study the static case in order to find the dynamic behaviour. The integrals are used in order to study some electrode configurations. It is indicated how the most efficient electrode configuration, for a given direction of vibration, can be determined.

The authors have used matrix methods to calculate the electrode potentials required to maintain a given conjugate focus condition for two five-element electrostatic cylinder lenses of slightly different proportions. They show that there is a simple parametrisation which allows one to calculate the focusing potentials given the overall voltage ratio and the magnification required. By using an extension of a straightforward method for calculating the potential within two closely spaced coaxial cylinders they have determined the matrix elements for a two-cylinder lens for voltage ratios from 0.01 to 100 in 200 equal logarithmic steps. These small steps simplify accurate interpolation especially for ratios close to unity.

For pt.I see ibid., vol.1, no.8, p.738-46 (1990). The authors have constructed a cylindrical electrostatic lens using 31 discs, spaced at 0.1 diameter, mounted between two cylinders and have studied its focusing characteristics by both calculation and measurement. They apply potentials V₁ and V₃ to the cylinders, V₂ to a group of contiguous discs, and potentials intermediate between V₁ and V₂ and between V₂ and V₃, to the discs between the cylinders and the contiguous group. This potential distribution differs from that used in calculations by Read and the authors make a detailed comparison of the available ranges of magnification and the spherical aberration coefficients of these two lens systems. The problems of the practical operation of multi-element lenses of this type suggest that they have limited application.

In order to improve both the reliability and stability of the conventional colliding pulse mode-locked (CPM) laser the authors have added an electronic feedback to the system. By linking the integrated output of the CPM laser to the output of the Ar⁺ laser the average energy in the CPM cavity is kept constant. In this way, the stability of the CPM laser can be maintained for several days on an optical table of moderate quality. In addition, the alignment is substantially easier and the pulse height distribution of the autocorrelation function is improved by a factor of two due to the feedback loop.

Extension of the tuning range of a pulsed dye laser to the mid infrared in a novel low-loss waveguide-type Raman cell is described. The guide has an adjustable cross section and length and is provided with a thin dielectric coating resulting in low propagation losses over a wide wavelength band. By employing high-pressure hydrogen as a Raman medium, quasi-continuous tuneability from 5.5 to 17 mu m was obtained from a single red dye.

It has been shown that it is possible to to generate very small lenses by melting 'islands' of photoresist on a glass substrate. The authors have made lenses with diameters ranging from 5 mu m to 750 mu m in the form of spheres, cylinders and crossed cylinders and have studied their optical properties. Lenses with numerical apertures between 0.5 and 0.2 may be made close to diffraction limited but those of lower numerical aperture can only be achieved using more complex techniques.

A theoretical evaluation is reported of the burst digital correlator (BDC) as a signal processor of a laser Doppler velocimeter (LDV). The results obtained by a statistical method are arranged and examined in connection with the ratio of sampling frequency to Doppler frequency. The influences of SNR, fringe number, trigger level and correlated sampling number are investigated on mean value, standard deviation and validated rate. The results indicate that the BDC has a wide dynamic range even when the SNR of the Doppler signal is low.

An experiment was conducted in a grid generated, nominally homogeneous, isotropic turbulent flow, in a wind tunnel of 0.33 m*0.33 m cross-sectional area, in order to evaluate the performance of an explicit non-real time data analysis algorithm for a crossed wire anemometer in highly turbulent flow. The 7000 data values acquired at various downstream locations with different turbulence intensities were analysed by the explicit non-real time method which assumes a constant yaw factor K_T in the fixed angle calibration procedure, and by the 'look-up table' method which requires calibration at several yaw angles. It was found that the simple explicit non-real time method is adequate in computing the Reynolds stresses up a turbulence intensity of 40% when compared with the tedious look-up table method. The discrepancies are less than +or-6% for Reynolds normal stresses and less than +or-15% for Reynolds shear stress. A theoretical model based on the Gaussian probability density function was also developed to predict the probability distribution of the yaw angle of flow at various turbulence intensities in order to refine the explicit non-real time and look-up table methods of data analysis.

The authors deal with the use of X hot-wire probes for measurements of two velocity components. Using the effective velocity concept, four different analytical expressions for the yaw response of the hot wire signals have been considered. Using a set of reference calibration data the curve-fit accuracy of the four methods are first investigated. Then the related sum and difference signal analysis methods are compared and finally the velocity component errors caused by uncertainty in the value of the mean yaw angle are established.

A description is given of three near-field methods of measuring the amplitude attenuation and velocity of longitudinal ultrasound in fluids. The methods, all using simple commercial apparatus, are based on interferometric (continuous wave), tone-burst (a short harmonic wave) and pulse techniques. They are all compared by making measurements on a reference silicone fluid, with excellent agreement within the experimental uncertainties of approximately +or-0.5% in velocity and approximately +or-3% in attenuation. Furthermore, agreement is found within these uncertainties with far-field ultrasonic data obtained from a far-field technique for the same fluid. The particular advantages of each method are discussed and applications are demonstrated to measurements in colloidal suspensions of rigid and swelling colloidal particles. The swelling of bentonite clays is monitored by time-dependent changes in the attenuation and velocity and a comparison is made with similar changes which are commonly measured rheologically at much lower frequencies.

A technique based on high resolution X-ray diffractometry employing multicrystal X-ray diffractometers is described. The wafer is mounted against the flat surface of a device and is aligned for diffraction of X-rays from lattice planes that are nearly parallel to its surface. The device is rotated azimuthally around an axis perpendicular to the wafer surface in a stepwise manner. The specimen has to be reoriented after each step to be on the peak of the diffraction curve due to finite angle alpha between the visible surface and the lattice planes. A plot of the reorientation angle as a function of the azimuthal position gives a sinusoidal curve. From three such plots at different known azimuthal orientations of the wafer with respect to the device, the value of angle alpha can be determined. The spatial orientation of the normal to the lattice planes is obtained. A general mathematical formulation has been given for the first time. The finite angle between the axis of rotation of the device and its flat surface against which the specimen is held, is determined. Once a device has been characterised only two azimuthal positions of the wafer are sufficient to determine its crystallographic orientation. Illustrative results of characterisation of a device and determination of orientation of a (111) silicon wafer are described. An overall uncertainty of +or-6 arc sec in the determination of alpha is achieved.

A quadrupole system is proposed for measuring in situ the dielectric and resistive properties of grounds and seas, with possible applications not only to prospecting on Earth but also to planetary exploration with penetrators and landers. The theoretical approach is outlined for simple geometries, a few practical arrangements are proposed for specific planetary missions and the feasibility of the measurements is demonstrated in the laboratory.

Instrumentation for monitoring the thermal history of individual plasma sprayed particles as they impact on a substrate is described. A double-wavelength fibre optic temperature sensor is focused on a small spot on the substrate surface to record the cooling rate of particles impacting on this region. Discrimination against in-flight particles intersecting the pyrometer field of view is obtained by a second fibre optic sensor viewing the same spot at an angle and working in coincidence with the first sensor. Typical recorded thermograms are presented and interpreted with reference to a numerical thermal propagation model.

A stirrer for microautoclaves is introduced which is an elegant simple device for mixing under severe conditions. The performance has been tested for a broad range of conditions. Two applications in laboratory research are given.

An optoelectronic system is described for making quasi-simultaneous recordings of muscle transparency at orthogonal beam polarisations. A Pockels cell is used to oscillate the E-vector of a laser beam ( lambda =632.8, 514.5, 488 and 457.9 nm) through pi /2 radians (at 7.4 kHz), between 0 degrees and 90 degrees relative to the muscle long axis. The intensity of light transmitted by the muscle is detected using a photomultiplier tube. The photomultiplier output is fed to a sample-hold device which generates two analogue signals, each of which records the transmitted intensity at one of the two incident E-vector orientations. The data are captured using a dual-channel transient recorder and are stored on floppy discs. The ability of the system to make time-resolved (millisecond scale) measurements of muscle conservative dichroism is illustrated with recordings made during 'ramp' stretches applied to fully activated frog muscles.

The authors have designed a simple, low-cost HV (0-400 V) DC power supply whose output can be varied continuously down to zero volts, with very high ripple rejection and with low noise ( Delta V/V<or=25*10^-6).

A high-resolution timing system is described for processing the data from an optical time domain reflectometer using single photon detection. Standard CMOS devices are used to product a low-cost system for timing the arrival of photons with a resolution of 5 ns.

The decrease in the normalised frequency of a vibrating slender elastic cantilever with an absorbing polymer attached at its free end is proportional to the combined sorbed gas mass and entrained mass. To a good approximation the entrained mass contribution arises from two factors: the ambient gas density and the increase in the specimen compression area. The latter factor may increase if the polymer swells during sorption and this process can significantly increase the entrained mass contribution. An appropriate correction is required. The authors describe how this may be achieved and provide sample results for a silicone elastomer sorbing CO₂ at high gas pressures.

A method for analysing local variations of colour centre concentrations in inhomogeneous layers is described. For measurements a microdensitometer and a spectroradiometer are combined with some microcomputer facilities. As an example a NaCl crystal coloured with 200 keV electrons is analysed.