Square-wave Frequency Research Articles

Peak current—frequency relationship in adsorptive stripping square-wave voltammetry

A reversible redox reaction between the adsorbed reactant and the dissolved product or the amalgam is analysed. The relationship between the square-wave (SW) stripping peak current and the SW frequency is a curve defined by Δφ max = 0, for f sw = 0, and an asymptote Δφ max = kf SW + z. The intercept z depends on the duration t 0 of the adsorptive accumulation and apparently vanishes when t 0 > 30 s. Consequently, the ratio Δφ max / f SW may not be constant for all frequencies. This effect is caused by the additional adsorption of the reactant after the end of the accumulation period. The SW peak potentials depend linearly on the logarithm of frequency, with a slope which approaches −2.3 RT/2 nF and depends on both the accumulation time and the step increment. If only the product is adsorbed, the SW peak currents depend linearly on the square root of frequency. The peak potentials are a linear function of the logarithm of frequency, with slope 2.3 RT/2 nF.

Production of Silicon Powder by Square-Wave Modulated Rf Silane Plasma

ABSTRACTSilicon powder has been produced by means of square-wave modulated rf silane plasma. The plasma conditions were: 30°C of reaction chamber temperature, 70 Pa of pressure, 80 W of rf power and 0. 1 Hz of square-wave modulation frequency. The gas flow rate was varied from 5 to 90 sccm.The distribution and size of the powder grains were determined by transmission electron microscopy. The compositional and microstructural analysis of the powder was performed by FTIR spectroscopy. This analysis has revealed the dominant presence of SiH2 groups (2100 cm−1), in a highly polymeric form (850 and 890 cm−1), as well as the existence of an absorption band (1000-1100 cm−1) associated with oxidation of the powder.These results have shown the great influence of the flow rate, through the gas residence time, on the composition and microstructure of silicon powder.

The effects of mercury film electrode morphology on square wave voltammetric peak current response as a function of frequency

The net peak-current response as a function of frequency, for both direct and stripping square-wave voltammetry at mercury film electrodes, can differ significantly from theory when either the surface morphology of the mercury film deviates from being a true flat film or the substrate interacts with the analyte in some manner. Mercury layers of various morphologies (film, patches or droplets) and thicknesses (1.0 to 24.0 μm) were prepared by depositing Hg(II) onto an iridium or glassy carbon substrate. Test solutions of Cd(II), Pb(II) and K 4Fe(CN) 6 in 0.1 M potassium nitrate were used. For square-wave frequencies of 10 to 2000 Hz, the iridium electrodes with true mercury films and the bare Ir electrodes gave a theoretical response for both direct SWV and SWASV with all analytes tested. For the Hg films on glassy carbon or the non-solid Hg-film on Ir, the peak-current response for direct SWV began to decrease at SW frequencies of 900 and 100 Hz respectively. The effect with SWASV was more pronounced, the decrease occurring at 100 and 50 Hz. This behavior is attributed to several factors but mainly to the increased exposure of the substrate as a result of the decreasing thickness of the diffusion layer, and the behavior of the mercury droplets as a collection of ultramicroelectrodes.

Effects of Servo Loop Modulation in Atomic Beam Frequency Standards Employing a Ramsey Cavity

Some kind of modulation needs to be imposed on the mi- crowave signal to acquire lock in all passive atomic frequency stan- dards. In this way a frequency-discriminating dc signal is obtained by synchronous detection, which allows control of a slave oscillator. Fre- quency stability and frequency biases of the latter under closed-loop operation depend on the type and characteristics of the used modula- tion. The case of atomic beam standards using a Ramsey cavity is stud- ied, and different modulation schemes are considered under the aspects of efficiency (obtainable stability) and insensitivity to systematic effects. Theoretical results for the dependence of these on modulation type and parameters and on microwave power are reported and discussed. schemes and their sensitivity to some systematic effects are given as a function of modulation type and parame- ters, and are calculated numerically for an existing atomic beam tube. In particular, the effects of sine-wave modu- lation and slow square-wave frequency modulation on short-term stability and on various biases are discussed. A perfect realization of these ideal schemes will be as- sumed, and system imperfections like second harmonic distortion will be overlooked in the scope of this paper. 11. SYSTEM ANALYSIS Whatever the particular coherent detection system may be, signal processing of the modulated output Pod from the atomic resonator can be ideally divided in two steps: a filter section with a time-dependent output IF synchron- ous with the modulation, and a __ phase-sensitive detector with a time-independent output Pd.

The electrochemical reduction and determination of reduced nicotinamide adenine dinucleotide in acidic media

The apparent reduction of reduced nicotinamide adenine dinucleotide (NADH) in acidic media at a static mercury drop electrode was investigated. A simple, quick pretreatment procedure was developed to convert the NADH to its acid-hydrated form. This adsorbs on the mercury surface during a film deposition time and the film is then reduced. The adsorption is diffusion-controlled and hence the peak currents for square-wave and linear-scan voltammetry are proportional to Ct 1/2 p Af and Ct 1/2 p Av, respectively, where t p is the effective film deposition time, C the concentration of NADH, A the electrode area, f the square-wave frequency, and ν the linear scan rate. Several electrochemical techniques were compared for the determination of NADH; the method of choice is square-wave voltammetry, although staircase or linear scan voltammetry can also be used. The detection limit is less than 7 nM, and the range of linear response covers 2–3 orders of magnitude of NADH concentration.

Vocal mimicry of computer-generated sounds and vocal labeling of objects by a bottlenosed dolphin, Tursiops truncatus..

A bottlenosed dolphin (Tursiops truncatus) was trained to mimic computer-generated "model" sounds, using a whistle mode of vocalization. Prior to training, the whistle sounds of this dolphin were limited to a few stereotyped forms, none of which resembled the model sounds. After training, high-fidelity imitations were obtained of model sounds having (a) moderately or widely swept, slow-rate frequency modulation (1-2 Hz), (b) narrowly or moderately swept frequency modulation at moderate to rapid rates (3-11 Hz), (c) square-wave frequency transitions, and (d) unmodulated (pure-tone) waveforms. New models, not heard previously, could be mimicked immediately, often with good fidelity, including mimicry of amplitude variation that had not been explicitly reinforced during training. Subsets of familiar models were mimicked with high reliability in repeated tests. In additional training, control of the mimic response was transferred from the acoustic model to objects shown the dolphin (e.g., a ball or a hoop) so that, in effect, the dolphin gave unique vocal labels to those objects. In a test of accuracy and reliability of labeling, correct vocal labels were given on 91% of 167 trials comprised of five different objects presented in random order. The dolphin's ability for vocal mimicry compared favorably with that of the more versatile mimic birds, and it contrasted sharply with the apparent lack of vocal mimicry ability in terrestrial mammals other than humans. The ability to label objects vocally was similar to abilities shown for some birds and similar, in principle, to abilities of great apes trained in visual languages to label objects through gestures or other visual symbols.

Microwave–microwave double resonance using a Fabry–Perot cavity spectrometer

The design and operation of a relatively simple, yet versatile spectrometer for microwave–microwave double resonance (MMDR) studies is described. The system consists of a high-Q, semiconfocal Fabry–Perot cavity resonator which is crossed at right angles by a second free-space microwave beam. We excite the cavity with a low-power unmodulated signal source, while the free-space irradiation source is square-wave frequency modulated and is of moderately high power (several hundred milliwatts). Both microwave sources are scannable; the signal source is locked to the cavity while the pumping klystron is free running. The signal (cavity) frequency has been operated in the 15–40-GHz range, and pump frequencies can be selected and easily implemented above or below the signal microwave frequency.

Nuclear magnetic double resonance with a doubly modulated radiofrequency field

Efficient broadband decoupling of heteronuclear spin systems is accomplished by a modulation method which successively phase-modulates the irradiating radiofrequency field with two square-wave frequencies that have a frequency ratio of four to one. Experimental observations show this efficient utilization of decoupler power can increase the signal heights by reducing residual splittings which broaden spectral lines. Compared to random-noise decoupling, line narrowing and signal-to-noise improvements of a factor of 3 are observed for proton-decoupled carbon-13 and phosphorus-31 NMR spectra. Efficient spin decoupling with only 1.5 W of decoupling power is demonstrated using this new modulation method.

Frequency-dependent response of ZrO 2 oxygen sensors to square-wave pulses of gas composition

The frequency response of ZrO 2 oxygen sensors to square-wave pulses of gas composition was studied on a laboratory flow reactor at 600°C. Square-wave frequencies between 0.001 and 5 Hz were applied to solenoid devices that switched alternate flow streams past the sensor. At frequencies <0.1 Hz, the sensor voltage is able to follow changes in bulk gas composition: during each half-cycle, the voltage attains values corresponding to the oxygen partial pressure in the individual flow streams. The sensor output cannot follow composition changes at 5 Hz. Here the voltage approaches a constant value characteristic of the average oxygen partial pressure of both gas streams. This behavior is viewed as a relaxation process involving bulk gas-boundary layer diffusive gas transport. Unusual features are seen when alternate gas streams contain reacting species. Voltage response to square-wave composition pulses resembles a pulse train whose “duty cycle” depends both on frequency and on the symmetry of composition excursions about a stoichiometric composition. A kinetic model is developed to account for this. Finally, it is shown that external coatings on the sensor surface lengthen the experimental relaxation time.

A Squarewave Frequency Modulation Servo System for Cesium Frequency Standard and its Influence on Frequency Stability

A Squarewave Frequency Modulation Servo System for Cesium Frequency Standard and its Influence on Frequency Stability

A microcomputer-controlled square-wave polarograph

A dedicated microcomputer has been developed for control and operation of a square-wave polarograph. Microcomputer control allows a high degree of variation in the experimental parameters, such as square-wave frequency, gate delay times, gate aperture, drop-time, the amplitude of the square wave, and the magnitude of the staircase rise, but still maintains the required coherency in the signals so that an appropriate separation between the faradaic and capacitance currents is achieved. The variation is achieved through software control. Measurement is accomplished by the application of a multiplicity of square waves to each “tread” of a staircase voltage, thus providing a true square-wave polarogram.

A new look at broadband square-wave frequency multipliers †

Abstract In a recent publication, Boutin et al. (1979) have presented some circuits ideas on a broadband square-wave frequency multiplier. These circuits need a comparator type A/D converter which is both rare and expensive. I n the present paper, some new circuits are presented in which the A/D converters have been replaced by D/A converters which are cheaper and more accessible.

Smoothing of Doppler spectra line shapes in electrophoretic analysis of biological cell suspensions

The study of cell and particle electrophoresis by laser Doppler techniques requires the application of an electric field to a conducting solution. Steady d.c. electric fields are usually not possible because of electrode surface reactions. Hence, it has been common practice to apply temporally varying electric fields, either pulses of alternating polarity or square-wave fields of constant amplitude. Oscillating particle motion leads to harmonic structure (multiples of the square-wave electric field frequency) in the scattered light spectra. This harmonic structure may be considerably smoothed by a suitable variation of the period of the applied square-wave field. The application of a pseudo-random period oscillating electric field yields line shapes that approach the line shapes to be expected from the application of a continuous d.c. electric field.

A synchronized broad-band square-wave frequency multiplier

A simple circuit is proposed to simultaneoulsy multiply by factors 2i-1 (1  i  N) the frequency of an incoming square wave. Each output square wave of 50% duty cycle is synchronized with the input signal whose frequency can be varied over many decades.

Digital frequency modulation in an NQR spectrometer

A digital system for a programmable frequency synthesiser is described. It performs a staircase frequency sweep with a step of 1 Hz and digitally programmed bidirectional square-wave frequency modulation in an NQR spectrometer with a frequency-locked super-regenerative oscillator. The system is particularly convenient when using digital averaging techniques. NQR spectra parameters are not influenced by frequency modulation even when extremely large frequency modulation deviations are used. Parameters such as the sweep rate and the amplitude of frequency modulation deviation are stable, accurately fixed and independent of the running frequency. NQR spectra of 35Cl in p-dichlorobenzene and 65Cu in Cu2O were plotted against frequency by an X-Y recorder.

Electrochemical dissolution of nickel in sulphuric acid by alternating current

The electrochemical dissolution of nickel in sulphuric acid by alternating current has been studied. Experiments with sine wave a.c. (50 cps) have shown that the dissolution efficiency increases with increase in concentration of sulphuric acid as well as current density. However, the dissolution efficiency increases with increase in concentration of sulphuric acid but decreases with increase in current density when square wave type a.c. of low frequency (0.5 cps) is used. Increase of frequency of square wave a.c. increases the dissolution efficiency.

A low-cost integrator for preparative ultracentrifuges

An integrator is described for the measurement of the time integral ∫ 0 t rpm 2 dt in preparative ultracentrifuge where linearity exists either (a) between tachometer generator ac voltage amplitude and rpm (e.g., Sorvall RC2-B) or dc voltage and rpm, or (b) between the square-wave frequency from the tachometer generator and rpm (e.g., Beckman L2-65B). The construction and the precision levels of an integrator for Sorvall RC2-B preparative ultracentrifuge in the range 0–10,000 rpm and for Beckman L2-65B preparative ultracentrifuge in the range 0–40,000 rpm are described.

Cesium-Beam Servo System Using Square-Wave Frequency Modulation

This paper describes a new modulation and servo system that is being tested in the NRC long-beam cesium-frequency standard. Square-wave frequency modulation is employed; it is introduced by switching between two separately generated frequencies. The synthesizer producing these frequencies and the problems caused by switching transients are described. Since in a laboratory standard systematic errors play a very important role these errors are discussed in the description of the servo system. The resulting systematic error as calculated from results of experiments on its components is less than 1 part in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> .

Frequency Shift Keying and Square-Wave Frequency Modulation

Some students have difficulty during initial discussions in understanding the differences between frequency shift keying and square-wave frequency modulation. This article describes a problem for student assignment which is designed to assist the student who lacks, to some degree, this understanding. The problem consists of a requirement for calculating the spectra for three different, but related, deterministic signals. The article describes the signals and gives a solution.

Electrocutaneous pain thresholds in humans to low frequency square-wave pulses.

Electrocutaneous pain thresholds in humans to low frequency square-wave pulses.