Probe Circuit Research Articles

An in Vivo NMR probe circuit for improved sensitivity

An in Vivo NMR probe circuit for improved sensitivity

Measurements of Fast Time Evolutions of Plasma Potential Using Emissive Probe

We present a new method of measuring the floating potential of an emissive probe when the floating potential is very close to the plasma potential. The probe potential at which the probe current becomes zero in the current-voltage curve of the probe is automatically biased by a new probe circuit connected to a bipolar voltage source. This method enables direct measurement of rapid changes in the floating potential with a time resolution of ≃1 µs, giving a continuous data trace. The method was verified experimentally using a DP machine.

Carbon-13 NMR triple-resonance measurements using simultaneous fluorine and proton decoupling

A triple-irradiation 13C NMR method which permits simultaneous broadband decoupling of protons and fluorine nuclei is desribed. The technique allows for gating of the fluorine decoupler in order to reduce the average power in the probe circuits and to permit measurement of the nuclear Overhauser enhancements. A bandwidth of 1.5 kHz at 7 W of rf power for protons and a bandwidth of 3.0 kHz at 30 W of rf power for flourine nuclei are achieved. 13C-{ 1H, 19F} spectra for 2,3,5,6-tetrafluorophenol and poly(vinyl fluoride) are reported and discussed.

Optically-Coupled Floating Double Probe for Local Electric Field Measurements in Pinch Discharge Plasmas

Local electric field signals from a floating double probe immersed in a pinch discharge plasma are transmitted via a pair of fiber optic cables in order to achieve electrical insulation between the probe circuit and an oscilloscope. This optical coupling scheme has a higher electrical insulation voltage and a wider frequency response than the commonly-used isolation transformer methods and commercially-available photocoupler devices. The measuring system consists of a transmitter with light-emitting diodes and a receiver with PIN diodes and responds in the frequency range up to 3 MHz. Transient noise, which disturbs the probe measurements in the implosion phases of pinch discharges, has been eliminated.

Simple RF matching for NMR coils in ENDOR spectrometers

In order to avoid reflections of RF power from the probe circuit to the power amplifier, the authors have used a circuit which preserves a constant resistive impedance during the frequency sweep.

Double probe circuit used in pulsed plasma diagnostics

The construction of a new double probe circuit using wide band optically isolated electronic elements is described. The circuit is adaptable to measuring plasma characteristics generted by pulsed discharges. The entire system is powered by a high performance dc/dc converter. The probe and ramp generator circuits are at floating potential and possess an over-all capacity relative to ground <50 pF. The bandwidth of the probe circuit is better than 250 kHz.

Improved circuit for potential fluctuation compensation in the plasma during the electron energy distribution function measurement by a probe

An improved circuit consisting of a simple voltage follower for the measurement of electron energy distribution function in a plasma has been proposed. This circuit makes it possible to eliminate the fluctuation of plasma space potential, the deviation of dc probe voltage caused by the probe current drawing across the plasma resistance, and the attenuation of the second derivative signal during the measurement, simultaneously. Both the probe circuit considerations and the experimental results are presented.

Water-Manometer Tensiometers Installed and Read from the Land Surface

Abstract Soil-moisture tension has been measured beneath an artificial-recharge basin in Suffolk County, Long Island, N.Y., with water-manometer tensiometers that can be installed and read from the land surface. The manometer-tensiometer unit is buried at the desired depth, and soil-moisture tension is measured from the land surface with a simple wire probe and electrical circuit that detect the level of water in the manometer. The water manometer is more sensitive than other common methods for determining unsaturated pressure head or soil-moisture tension over small ranges and is most helpful in situations where small changes in soil-moisture tension can reflect significant changes in hydraulic conductivity. Although the configuration may not be appropriate for installation at great depth (> 6 m) or in extremely coarse or fine soils, it provides rapid, reliable measurement of soil-moisture tension.

XM-712 COPPERHEAD (CLGP) projectile storage reliability verification test program : Thomas R. Gagnier. Proc. IEEE Reliab. Maintainab. Symp., Philadelphia, Jan. 18–20, 1977. p. 353

Laser-based electro-optic probing is a proven noninvasive technique for testing high-speed microwave circuits on substrates such as gallium arsenide (GaAs) and indium phosphide (InP). We have extended this technique to probe circuit structures on doped and poled polyimide substrates that are useful in high-density packaging applications such as multichip modules (MCMs). Our results demonstrate the potential to improve the testability of MCMs by probing circuit structures that are buried in the central layers of an MCM. In addition to direct, point-by-point electric field measurements in representative MCM structures, we have investigated the poled polyimide dielectric efficacy both as a dielectric circuit layer and as a suitable electro-optic material.

Single coil probe with transmission-line tuning for nuclear magnetic double resonance

First we discuss a method of tuning NMR probe circuits and matching impedances which employs a variable length transmission line as one of the tuning elements. This method is useful for high voltage circuits where, due to space considerations, suitable tuning elements may not be placed adjacent to the rf field-producing coil. Second, we describe the design of a single coil double resonance probe which uses λ/4 transmission lines as filters as well as tuning elements. This double resonance probe is useful when the two frequencies of interest are widely separated.

Versatile plasma probe

We have developed a plasma probe circuit which couples standard Langmuir probe diagnostic procedures with pulse mode operation. Circuit versatility provides the means for studying time-dependent phenomena and establishes an improved capability for measuring plasma density and temperature. As an application of our circuit, we show that pulse mode operation substantially reduces the time variance of electrode surface conditions which often causes hysteresis in the Langmuir probe current–voltage characteristic.

Low-frequency probe noise and its relationship to space potential

Measurements of the low-frequency noise flowing in a probe circuit and the first derivative of the probe characteristic are described. Almost complete correlation is found between the two, and this reinforces the view that the low-frequency noise current is determined primarily by the first derivative. It is concluded that the noise current is not an independent means of locating space potential.

The use of capacitance probes in gas fluidised beds

There has been a trend recently towards using capacitance probe circuits to investigate events occurring in freely bubbling fluidised beds. The development of a capacitance probe circuit and the geometry of a mobile probe head to give minimum disturbance to passing bubbles is described. The calibration of events on the probe trace with respect to cine film records of bubbles passing the probe head is shown to be essential if reliance is to be placed on the probe results. Results of such a calibration using a two-dimensional fluidised bed are described together with the application of this calibration to three-dimensional situations. The limitations of this procedure are emphasised together with the necessity for always calibrating the probe in circumstances closely similar to those in which it is to be used. It is suggested that interpretation of capacitance probe results in terms of bubble sizes, bubble voidages or cumulative numbers of bubbles passing a given level should be treated with some reserve unless it is known that careful calibration has been employed.

Phase Transients in Pulsed NMR Spectrometers

When a square wave modulated rf voltage is applied to a tuned probe in an NMR spectrometer, the effective transverse magnetic field in the rotating reference frame changes direction as it grows and decays. The resulting deviations in the nutational behavior of the spins from naive expectations are typically small, but may lead to important cumulative errors in experiments employing long pulse trains. These errors can be made to vanish by very accurate tuning of the probe circuit to its free ringing frequency ωr [not to ω0=(1/LC)½] and by providing suitable coherence between the rf carriers and the timing of pulses.

Flow Angle and Shear Stress Measurements Using Heated Films and Wires

The heat transfer characteristics of yawed slender filaments have been generalized to include pairs of heated-film skin friction gages, as well as conventional hot wire probes, New sensor designs have been combined with a novel double-bridge electronic feedback system with accurate differential power capabilities for improved measurements in complex environments. Results of detailed calibration experiments illustrate the accuracy, simplicity, and utility of the probe designs and circuit techniques.

A high frequency eddy-current, non-destructive testing apparatus with automatic probe positioning suitable for scanning applications

By considering the equivalent circuit of an inductive probe in the vicinity of a metal object it is shown that at high frequencies the reactance reflected into the probe circuit is independent of the resistivity of the metal object. This reflected reactance can be used to control the degree of coupling between the probe and the object with the result that the reflected resistance is determined solely by the resistivity of the metal. A prototype instrument is described which makes use of these principles. When the instrument is placed near a metal object the test probe is automatically adjusted to be at the correct distance from the object. The instrument can be scanned over a surface with an irregular topography without making mechanical contact, and the information from the instrument will distinguish between changes in topography, variations in resistivity, and mechanical defects such as cracks.

Direct Display of Electron Temperature in Magnetized and Time Varying Plasmas

A method of direct display of electron temperature, which uses three closely spaced Langmuir probes at constant biases, is presented. Ion components of probe currents, which are relatively large in magnetized plasmas, are subtracted in the probe circuit. Direct display of the local value of the electron temperature is possible without any sweeping to the probe voltage. The sources of error are known and their effect can be readily evaluated during the experiment. Measurements of electron temperature in a P. I. G. discharge in hydrogen is presented as an example of application of this method.

High-Frequency Measurement of Electric Fields Using a Telemetering Probe

When image plane construction is impossible, accuracy of field strength measurements is greatly reduced by the presence of connecting cables and apparatus. This problem may be circumvented by telemetering the field information to a distant control station. This paper describes a system designed to measure relative electric field strengths at 26 MHz. The sensing probe employs a voltage comparison technique, and a telemeter whose output drops to a sharp null when a specific electric field is present at the probe. If calibration is desired, the specific field may be measured, or deduced through probe circuit analysis. The system gives better than 0.1 dB accuracy if care is taken in positioning the probe. If a large scattering field obstacle is at least 10 meters from the probe, errors in relative field measurements will be less than 1 percent due to the presence of the probe. Various suggestions are given to decrease the effects of some sources of inaccuracy. The probe system may be applied in almost any medium, providing that temperature excursions are not too drastic.

Effect of a.c. amplitude on the measurement of electron energy distribution functions

The energy distribution function of the electrons in the positive column of a glow discharge containing spectroscopically pure nitrogen at a pressure of 0.1 torr has been determined. This involved the measurement of the second-harmonic component of the current in a Langmuir probe circuit when an a.c. signal is superimposed on the d.c. bias applied to the probe.

Elektrische Sondenmessungen in schnellen, stromstarken Stoßentladungen

In hot dense plasmas—as in the case of shock discharges—LANGMUIR probe measurements are difficult because complete probe characteristics are mostly unobtainable. Therefore, this work shows the possibility of getting information about the plasma at low probe voltages. It is shown that for a sufficiently small probe voltage the current drawn by a symmetrical plane double probe is proportional to ni/√Te (ni=ion density, Te=electron temperature). Knowing the approximate value of the electron temperature, the probe current gives the ion density immediately. Also the directed motion of ions can be determined by changing the orientation of the probe. A good time resolution (10-7 sec) was achieved by matching the impedance between the probe and the plasma to the coaxial probe circuit. The usefulness of this probe technique was investigated in a linear shock discharge with a coaxially, circular, plane double probe. Ion densities up to 1015 cm-3 and directed ion velocities of some 105 cm/sec were observed. Simultaneously the electrode voltage, current derivative, and the local azimuthal magnetic field in the plasma were measured.