Measurement Of Transient Signals Research Articles

High bandwidth data recording systems for pulsed power and laser produced plasma experiments

We present two high bandwidth data transmission and recording systems for the measurement of transient signals during pulsed power and laser produced plasmas. These systems use fiber optic cables to transmit analog data over long distances to high bandwidth digitizing oscilloscopes. One system is based on the direct modulation of a laser diode and has a bandwidth of 1.5GHz. The other system is based upon a Mach-Zehnder modulator and has a bandwidth of 12GHz, and is limited by the photoreceiver. The signals are recorded on commercial digitizing oscilloscopes that have approximately six effective bits. The transmission systems use many off-the-shelf components from the telecommunications industry and thus have a high reliability and a moderate cost. Investigation of the reduction in optical transmission of the fibers during exposure to high dose radiation will also be discussed.

Flow injection potentiometric determination of total antioxidant activity of plant extracts

A new flow injection potentiometric (FIP) method, rapid, reproducible and simple to apply, has been developed for the in vitro evaluation of antioxidative capacity of aqueous plant extracts. This method is based on the transient negative signal measurements with a flow-type platinum electrode detector due to the composition change of a [Fe(CN) 6] 3−/[Fe(CN) 6] 4− redox-reagent solution. The variables affecting the signal height such as composition and concentration of redox-reagent, injected sample volume, flow rates of carrier and redox-reagent solution streams were studied in details and the conditions were optimized. For the compounds under study, a linear relationship was stated between the potentiometric signal height and the logarithm of antioxidant concentration. It was stated that a wide antioxidant activity range from 1 μM to 10 mM could be determined by the changing concentrations of the hexacyanoferrate(III) from 5 to 0.01 × 10 −4 . The present FIP method was applied to quantify relative antioxidant activity (RAA index) of the representative water-soluble antioxidants (ascorbic acid, pyrocatechol, pyrogallol, caffeic acid, chlorogenic acid, gallic acid, tannic acid, uric acid, l-cysteine, trolox). The high sampling rate (100 h −1) and a satisfactory reproducibility (R.S.D. = 0.7–1.8%, n = 5, 0.1 mM each compound) were obtained. The method was also applied to estimate total antioxidant activity (TAA) of real samples (green and black tea infusions, herbal infusions and fresh fruit extracts) and the results were compared with those achieved using well-known in vitro testing methods.

Isotope ratios on transient signals with GC–MC–ICP-MS

ICP-MS instruments with multiple ion collectors (MC–ICP-MS) provide highly precise and accurate isotope ratio measurements. These instruments are easily coupled to chromatography, laser ablation or flow injection devices because of the ICP source, and thus attract general analytical interest for measurement of highly precise element isotope ratios on transient signals. Some few papers have been published today where MC–ICP-MS was employed for isotope ratios on transient signals, and in general, results are encouraging. Isotope ratio precision obtained is usually at least 10-fold superior compared to measurements with ICP-QMS. Nevertheless, most authors describe a drift of isotope ratios during the transient peak. This behaviour was either interpreted as a problem in the sample introduction system, or was related to instrumental mass bias drift, but has to date not been completely evaluated or explained. In this paper, we systematically investigate the drift observed on lead and mercury isotopes during short transient signals obtained from GC coupled to different MC–ICP-MS systems. We can clearly show that neither changes in instrumental mass bias, nor chromatographic fractionation effects are the source of the observed isotope ratio drift. An influence of analyte concentration on the observed drift was as well excluded as a source for this drift, because the slope on isotope ratios show the same values for different concentrations evaluated. In contrast, the peak width was found to influence the extent to which the isotope ratio drifts during peak elution. Thus, the relative intensity change per time seems to be an important factor for the measurements of transient signals with MC–ICP-MS.

Multichannel single-shot transient signal measurements with a fiber delay line loop

An instrument has been developed to measure single-shot electrical transient signals, in two channels, with frequency responses above 10 GHz ( http://www.YYLabs.com). This instrument, which is named Fiber Loop Single-Shot Scope, utilizes an optical fiber recirculating delay line loop to regenerate captured single-shot signals, and then recovers the original single-shot signals with a sampling scope.

Calibration errors due to variations in peak characteristics in the measurement of transient signals by inductively coupled plasma-scanning mass spectrometry

The impact of variations in peak characteristics on the fidelity of transient signal measurement by inductively coupled plasma-quadrupole mass spectrometry (ICP-QMS) was investigated. Specifically, the question as to whether the multi-element capabilities or the accuracy in determined analyte amounts were deteriorated compared to what has been reported previously when not considering peak variations was addressed. The peak characteristics considered were the time of the signal maximum (tpeak), the standard deviation of the assumed Gaussian input function generated by the sample introduction system (σG), and the time constant for signal decay (τ). Investigations of simulated exponentially-modified Gaussian peaks revealed that, for variations of peak characteristics within reasonable ranges, measurement noise and variations in tpeak, σG and τ all contributed to calibration uncertainty. Electrothermal vaporisation (ETV) and flow injection (FI) systems were used to experimentally generate transient signals of varying peak characteristics. Removing data points from the raw signals simulated the monitoring of up to 100 mass-to-charge ratios, allowing calibration data and analyte amounts to be determined from the processed signals. To obtain calibration graph slopes with relative standard deviations below 1% for the ETV-ICP-QMS system, it was found necessary to acquire 7–24 data points per peak for 50–5 ms dwell times. On this basis, the maximum number of mass-to-charge ratios that could be monitored in a typical ETV-ICP-QMS analysis was 4–10 using dwell times of 50–5 ms. With the FI-ICP-QMS system, variations in the peak characteristics between calibration standards and samples meant that, to obtain less than 3% error in determined analyte amounts, at least 7 or 10 points per peak were required for external and internal standardisation, respectively. It was found that variations in peak characteristics contributed more than measurement noise to the error in determined analyte amounts. In recent studies it has been reported that 3–4 data points per peak are sufficient to accurately monitor a transient if peak variations are not considered, which, for typical ETV signals, would allow the monitoring of 20 mass-to-charge ratios during a single measurement cycle. Thus the results obtained here show that the multi-element capability of ICP-QMS when monitoring transient signals can be severely compromised by such variations.

Time-of-flight mass spectrometry as a tool for speciation analysis

Time-of-flight mass spectrometry (TOFMS) has recently been introduced as an alternative to scanning-based mass analyzers for use in elemental analysis. Coupled with an inductively coupled plasma or alternative ion source, TOFMS can produce a complete atomic mass spectrum in less than 50 μs. Because of this high spectral-generation rate, even very brief transient signals can be recorded with high fidelity. Furthermore, each mass spectrum is derived from the same sub-microsecond pulse of ions, so high precision can be achieved by using either isotope-ratioing or internal standardization techniques. All these features make TOFMS attractive for the measurement of transient signals, such as those commonly encountered in speciation analysis. In this paper, the capabilities of TOFMS in speciation will be demonstrated through the coupling of gas chromatography and capillary electrophoresis with an inductively coupled plasma-TOFMS. Additionally, the development of novel switched gas sampling glow discharge (GSGD) ionization sources will be described and their role in chemical speciation will be evaluated. The switched GSGD has the ability to collect both atomic and molecular mass spectra in rapid succession, to provide additional information about chemical species. The coupling of various sample introduction systems (flow-cell, exponential dilutor, capillary gas chromatography and electrothermal vaporization) to the GSGD is outlined.

Isotope ratio monitoring gas chromatography/Mass spectrometry of D/H by high temperature conversion isotope ratio mass spectrometry.

Of all the elements, hydrogen has the largest naturally occurring variations in the ratio of its stable isotopes (D/H). It is for this reason that there has been a strong desire to add hydrogen to the list of elements amenable to isotope ratio monitoring gas chromatography/mass spectrometry (irm-GC/MS). In irm-GC/MS the sample is entrained in helium as the carrier gas, which is also ionized and separated in the isotope ratio mass spectrometer (IRMS). Because of the low abundance of deuterium in nature, precise and accurate on-line monitoring of D/H ratios with an IRMS requires that low energy helium ions be kept out of the m/z 3 collector, which requires the use of an energy filter. A clean mass 3 (HD(+.)) signal which is independent of a large helium load in the electron impact ion source is essential in order to reach the sensitivity required for D/H analysis of capillary GC peaks. A new IRMS system, the DELTA(plus)XL(trade mark), has been designed for high precision, high accuracy measurements of transient signals of hydrogen gas. It incorporates a retardation lens integrated into the m/z 3 Faraday cup collector. Following GC separation, the hydrogen bound in organic compounds must be quantitatively converted into H(2) gas prior to analysis in the IRMS. Quantitative conversion is achieved by high temperature conversion (TC) at temperatures >1400 degrees C. Measurements of D/H ratios of individual organic compounds in complicated natural mixtures can now be made to a precision of 2 per thousand (delta notation) or, better, with typical sample amounts of approximately 200 ng per compound. Initial applications have focused on compounds of interest to petroleum research (biomarkers and natural gas components), food and flavor control (vanillin and ethanol), and metabolic studies (fatty acids and steroids). Copyright 1999 John Wiley & Sons, Ltd.

Electrical characterization of the SiO 2Si system

The classical capacitance-voltage measurement methods and the conductance technique which are frequently used to analyze the SiO 2Si system, are briefly introduced and critically evaluated. These measurement techniques are based on macroscopic device structures, average structure parameters are determined. Transient and random telegraph signal measurements, and charge pumping are now available as new methods to study the SiO 2Si interface in sub-μm device structures. These techniques permit to analyze single, individual traps in the interface. Traps in small devices are isolated. The capture rates are thermally activated by the Coulomb energy.

Study of thermal effects in GaAs micromachined power sensor microsystems by an optical interferometer technique

A contactless and non-invasive optical interferometric method is used to study the temperature distribution and thermal time response in a GaAs micromachined power sensor. Temperature variations in the sensor active area-a cantilever beam—are sensed by an infrared laser beam. The temperature increase due to pulsed power dissipation in the cantilever induces an increase both in the GaAs refractive index and in the cantilever thickness. This results in a change in the phase and intensity of the reflected laser beam which is interferometrically detected. The spatial temperature distribution along the cantilever beam is studied using measurements of the optical phase and intensity as a function of the dissipated power. The optical signal is analysed taking into account Fabry-Perot interference. The thermal time constant of the sensor of about 5 msec is obtained from transient optical signal measurements. Results of the optical analysis are consistent with those of electrical characterization of the sensor and with the simulation of the temperature distribution.

A Spectrophotometric Apparatus with Laser Sources for Measurements of Fast Transient Signals

Abstract A double beam spectrophotometric apparatus, based on laser light sources and on photodiode detectors, was built in order to perform accurate measurements of small concentration changes as those produced in chemical relaxation experiments. Relaxation techniques, where small and fastly decaying signals have to be monitored, call for high analytical sensitivity. To improve this figure of merit, the choice of a laser source proved to be particularly advantageous, since the very high light intensity, providing a remarkable reduction of the shot noise, allows signals of good quality to be obtained without amplification problems. The measured noise was found to be close to the calculated noise, which represents the theoretical lower limiting value. Under the best conditions the signal-to-noise ratio was improved by a factor 40 with respect to a commercial apparatus equipped with conventional light source, monochromator and photomultipliers. It is shown that the circumstances where the use of the present apparatus is specially suitable are those where the dynamic techniques become superior to the static methods in the determination of thermodynamic parameters of reacting systems.

Time-resolved scanning probe microscopy: Investigations and applications of dynamic magnetostriction (invited)

We have measured the magnetostrictive response of small nickel and Tb0.3Dy0.7Fe1.95 cylinders to sudden step changes in applied magnetic field with a time-resolved scanning fiber interferometer. Spatially resolved measurements reveal the nonuniform distortion of the face of the rod as the induced magnetization propagates in toward the center. A crossover is observed, as a function of decreasing cylinder diameter, from eddy current- to longitudinal phonon-limited response times. The ability to characterize dynamic magnetostriction in small samples has also enabled us to perform time-resolved measurements of fast transient signals with a scanning tunneling microscope. Modulation of the tip-sample separation on short time scales is provided by the use of a nickel magnetostrictive tip driven by a local magnetic field coil. The technique offers a broadly applicable means of obtaining fast dynamical information in tunneling microscopy, and is demonstrated here on nanosecond time scales. These experiments illustrate, in the context of magnetics, the general promise of time-resolved scanning probe methods.

Time-resolved scanning tunneling microscopy through tunnel distance modulation

The ability to perform time-resolved measurements of fast transient signals with a scanning tunneling microscope has been achieved through direct control of the tunneling distance on short time scales. Modulation of the tip-sample separation is provided by the use of a magnetostrictive tip driven by a local magnetic field coil. The technique offers a broadly applicable means of obtaining fast dynamical information in tunneling microscopy, and is demonstrated here on nanosecond time scales.

Gated photomultiplier response characterization for DIAL measurements

The characteristics of various detector responses are studied to understand the cause of various systematic biases and to minimize these undesirable effects in measurements of transient signals with large dynamic range. We quantitatively evaluated signal induced bias, gain variation, and the linearity of commonly used gated photomultipliers in the current integrating mode. Analysis of the results indicates that impurity ions inside the photomultiplier tube are the source of the signal induced bias and gain variation. Two different photomultiplier tubes used in this study show significant differences in the magnitude and decay behavior of signal induced bias. We found it can be minimized by using an external amplifier to reduce PMT gain, and by applying a low potential between the cathode and first dynode. The linearity of a photomultiplier tube is also studied over a large dynamic range of input intensities employing a new technique which does not require an absolute calibration. The result of this study shows that the photomultiplier response is linear only for a limited input intensity range below a certain anode current.

Baseline compensation circuit for measurement of transient signals

An automatic compensation circuit (back-off circuit) is described which incorporates a sample-and-hold amplifier with a digital memory in order to avoid droop problems at long durations of measurement. This device is helpful in pulse radiolysis and laser-flash photolysis experiments on the nanosecond to second timescale. The range of current compensation is ±5 mA. The resolution is ±1.27 μA and the accuracy is better than ±1% for a range of compensation of 0.15 to 5 mA. A voltage output is provided as a measure (1 V/mA) of the compensated current.

Pulse and time-domain measurements

A review of the state of the art and science of pulse parameter measurements is given including recent advances in the use of real-time oscilloscopes, waveform recorders, equivalent time sampling oscilloscopes, and counter timers in the measurement of repetitive and single transient signals. Recent advances in the use of artifact waveform standards and modern signal analysis techniques to compensate for measurement distortion are highlighted. The formation and progress of an IEEE committee which is developing a performance standard for waveform recorders is also described.

Effect of Tubing on Measurements of Small Transient Pressure Signals in Liquids

It is often necessary to connect the pressure transducer used for measurements of static and dynamic pressure signals to the pressure source by means of a connecting tube. The effects of the tube length and diameter on a relatively small step pressure signal in a liquid system have been studied experimentally and analyzed theoretically. Theoretical equations to predict the natural frequency and the damping coefficient of the system are obtained so that the proper length and diameter of tubing can be selected for minimum distortion of the step input signal. Agreement between theoretical predictions and experimental measurements was found to be reasonably good.

Picosecond applications of Josephson junctions

The behavior of simple superconducting circuits in the picosecond regime is described in a comprehensive way, with primary emphasis being given to the step function and pulse responses of these circuits. Topics receiving detailed discussion include Josephson-junction modeling with both the microscopic and shunted-junction models. Limitations of the shunted-junction model are explored by comparing it with experimental results and with the microscopic model. An approximate evaluation is given of the important dynamical properties of junctions made with the dominant fabrication technology (Pb-alloy systems), as a function of tunneling barrier thickness. Rounding out the device aspects of the discussion, we describe in detail the properties of superconducting microstrip transmission lines, with an emphasis on their high-speed behavior. Turning to simple circuits we review experimental results on the measurement of picosecond regime transient signals. The concept of turn-on delay is analyzed anew, providing simplified and extended results. Details of concepts for pulse height and pulsewidth measurements are explored, leading to the conclusion that the time resolution of superconducting circuits is limited to approximately the period of one plasma oscillation. With present Pb-alloy fabrication technology this limit is 2 ps.

Background current subtractor for the detection of transient signals in kinetic spectrophotometry

In real-time kinetic spectrophotometry the background current signal due to the analyzing light beam must be backed off to enable the sensitive detection of the superimposed transient signal. Methods employing filters or amplifiers in the signal path have restrictions in their frequency response so that not all signals of practical duration from a few nanoseconds to a millisecond can be measured with one system. This note describes a current subtraction technique which isolates the subtraction circuit from a direct, low-impedance path between the photomultiplier and a wide-band oscilloscope, thereby imposing no significant frequence limitation on the measurement of transient signals.

Drift-compensating integrator for measurement of transient atomic absorption signals

Drift-compensating integrator for measurement of transient atomic absorption signals

A correlation method for semiconductor transient signal measurements

A wide variety of experiments in semiconductor physics involve the observation of a particular class of time-dependent transient signals; namely, decaying exponentials. Such signals are characteristic of systems relaxing back to equilibrium following an abrupt change in the populations of carriers occupying the available states. The magnitudes of such signals indicate the density of states involved, while the decay time constants (as a function of temperature) provide information on the energy level of the states in question. As such, both the amplitudes and the time constants of such signals are of physical interest. The problem of processing such signals has been examined using the theory of linear filtering and as a result a special purpose signal processor has been developed. This processor performs a continuous real-time cross correlation between the experimental signals and an appropriately synchronized locally generated exponential waveform. The resulting system has been used to process signals from transient space-charge measurements on reverse-biased gold-doped silicon junction diodes, providing both correlation spectra that yield energy levels and also trap profiles that expose the spatial distribution of the gold inside the junctions.