Frequency‐domain description of a lock‐in amplifier
01 Feb 1994-American Journal of Physics (American Association of Physics Teachers)-Vol. 62, Iss: 2, pp 129-133
TL;DR: In this paper, the basic principles behind the operation of a lock-in amplifier are described, with particular emphasis on looking at the frequency components of the signal present at the various stages of the lockin during a typical measurement.
Abstract: The basic principles behind the operation of a lock‐in amplifier are described. Particular emphasis is placed on looking at the frequency components of the signal present at the various stages of the lock‐in during a typical measurement. The description presented here has been used successfully to explain lock‐in operation to upper‐level laboratory students at Oberlin College.
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TL;DR: In this paper, a calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments is presented.
Abstract: We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.
446 citations
01 Jan 2009
TL;DR: Comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models regarding temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements.
Abstract: We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.
329 citations
TL;DR: Molecular dynamics simulations revealed that back-and-forth motion of DNA strands through a 1 nm diameter pore exhibits sequence-specific hysteresis that arises from the reorientation of the DNA bases in the nanopore constriction.
Abstract: Molecular dynamics simulations revealed that back-and-forth motion of DNA strands through a 1 nm diameter pore exhibits sequence-specific hysteresis that arises from the reorientation of the DNA bases in the nanopore constriction. Such hysteresis of the DNA motion results in detectable changes of the electrostatic potential at the electrodes of the nanopore capacitor and in a sequence-specific drift of the DNA strand under an oscillating transmembrane bias. A strategy is suggested for sequencing DNA in a nanopore using the electric field that alternates periodically in time.
201 citations
TL;DR: A low-cost miniaturized fluorescence detection system for lab-on-a-chip applications with a sensitivity in the low nanomolar range; a built-in lock-in amplifier enables measurements under ambient light.
Abstract: We present a low-cost miniaturized fluorescence detection system for lab-on-a-chip applications with a sensitivity in the low nanomolar range; a built-in lock-in amplifier enables measurements under ambient light.
185 citations
TL;DR: The field of spin chemistry has been studied extensively in liquids, solids, and constrained media such as micelles as discussed by the authors, and the role played by spin chemistry in the remarkable magnetic compass sense of birds and other animals.
Abstract: Chemical reactions that involve radical intermediates can be influenced by mag- netic fields, which act to alter their rate, yield, or product distribution. These effects have been studied extensively in liquids, solids, and constrained media such as micelles. They may be interpreted using the radical pair mechanism (RPM). Such effects are central to the field of spin chemistry of which there have been several detailed and extensive reviews. This re- view instead presents an introductory account of the field of spin chemistry, suitable for use by graduate students or researchers who are new to the area. It proceeds by giving a brief his- torical overview of the development of spin chemistry, before introducing the essential the- ory. This is then illustrated by application to a series of recent developments in solution-phase magnetic field effects (MFEs). The closing pages of this review describe the role played by spin chemistry in the remarkable magnetic compass sense of birds and other animals.
177 citations
Cites background from "Frequency‐domain description of a l..."
...Using a lock-in amplifier [123, 124], it is possible to extract the oscillatory part of the measured signal (e....
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