32,531 research outputs found
Signal processing for a laser-Doppler blood perfusion meter
Two signal processing methods for laser-Dopper perfusion velocimetry are presented. The methods are based on the calculation of the moments of the frequency power spectrum. The first uses Vω-filtering (ω is the frequency) with analogous electronics, the second uses signal autocorrelation with digital electronics. Comparison is made with a third instrument: a spectrum analyzer coupled to a computer, using Fourier transform tecniques. The performance of these setups (sensitivity, limit sensitivity and accuracy) are investigated. We propose a calibration standard for signal processors to be used for blood perfusion measurements. The analogous instrument proved to be the cheapest but the digital instrument had the best performance
The Measurement of AM noise of Oscillators
The close-in AM noise is often neglected, under the assumption that it is a
minor problem as compared to phase noise. With the progress of technology and
of experimental science, this assumption is no longer true. Yet, information in
the literature is scarce or absent. This report describes the measurement of
the AM noise of rf/microwave sources in terms of Salpha(f), i.e., the power
spectrum density of the fractional amplitude fluctuation alpha. The proposed
schemes make use of commercial power detectors based on Schottky and tunnel
diodes, in single-channel and correlation configuration. There follow the
analysis of the front-end amplifier at the detector output, the analysis of the
methods for the measurement of the power-detector noise, and a digression about
the calibration procedures. The measurement methods are extended to the
relative intensity noise (RIN) of optical beams, and to the AM noise of the
rf/microwave modulation in photonic systems. Some rf/microwave synthesizers and
oscillators have been measured, using correlation and moderate averaging. As an
example, the flicker noise of a low-noise quartz oscillator (Wenzel 501-04623E)
is Salpha = 1.15E-13/f, which is equivalent to an Allan deviation of
sigma_alpha = 4E-7. The measurement systems described exhibit the world-record
lowest background noise.Comment: 39 pages, 22 figures, 8 tables, 21 references, list of symbol
Feasibility study of the application of existing techniques to remotely monitor hydrochloric acid in the atmosphere
A critical evaluation of existing optical remote sensors for HCl vapor detection in solid propellant rocket plumes is presented. The P branch of the fundamental vibration-rotation band was selected as the most promising spectral feature to sense. A computation of transmittance for HCl vapor, an estimation of interferent spectra, the application of these spectra to computer modelled remote sensors, and a trade-off study for instrument recommendation are also included
A correlation noise spectrometer for flicker noise measurement in graphene samples
We present a high-resolution digital correlation spectrum analyzer for the measurement of low frequency resistance fluctuations in graphene samples. The system exploits the cross-correlation method to reject the amplifiers' noise. The graphene sample is excited with a low-noise DC current. The output voltage is fed to two two-stage low-noise amplifiers connected in parallel; the DC signal component is filtered by a high-pass filter with a cutoff frequency of 34 mHz. The amplified signals are digitized by a two-channel synchronous ADC board; the cross-periodogram, which rejects uncorrelated amplifiers' noise components, is computed in real time. As a practical example, we measured the noise cross-spectrum of graphene samples in the frequency range from 0.153 Hz to 10 kHz, both in two- and four-wire configurations, and for different bias currents. We report here the measurement setup, the data analysis and the error sources
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Strain and temperature sensors using multimode optical fiber Bragg gratings and correlation signal processing
Multimode fiber optic Bragg grating sensors for
strain and temperature measurements using correlation signal processing methods have been developed. Two multimode Bragg grating sensors were fabricated in 62/125 m graded-index silica
multimode fiber; the first sensor was produced by the holographic method and the second sensor by the phase mask technique. The sensors have signal reflectivity of approximately 35% at peak
wavelengths of 835 nm and 859 nm, respectively.
Strain testing of both sensors has been done from 0 to 1000 με and the temperature testing from 40 to 80°C. Strain and temperature sensitivity values are 0.55 pm/με and 6 pm/°C, respectively.
The sensors are being applied in a power-by-light hydraulic valve monitoring system
Nano-optomechanical measurement in the photon counting regime
Optically measuring in the photon counting regime is a recurrent challenge in
modern physics and a guarantee to develop weakly invasive probes. Here we
investigate this idea on a hybrid nano-optomechanical system composed of a
nanowire hybridized to a single Nitrogen-Vacancy (NV) defect. The vibrations of
the nanoresonator grant a spatial degree of freedom to the quantum emitter and
the photon emission event can now vary in space and time. We investigate how
the nanomotion is encoded on the detected photon statistics and explore their
spatio-temporal correlation properties. This allows a quantitative measurement
of the vibrations of the nanomechanical oscillator at unprecedentedly low light
intensities in the photon counting regime when less than one photon is detected
per oscillation period, where standard detectors are dark-noise-limited. These
results have implications for probing weakly interacting nanoresonators, for
low temperature experiments and for investigating single moving markers
Validation of a new spectrometer for noninvasive measurement of cardiac output
Acetylene is a blood-soluble gas and for many years its uptake rate during rebreathing tests has been used to calculate the flow rate of blood through the lungs (normally equal to cardiac output) as well as the volume of lung tissue. A new, portable, noninvasive instrument for cardiac output determination using the acetylene uptake method is described. The analyzer relies on nondispersive IR absorption spectroscopy as its principle of operation and is configured for extractive (side-stream) sampling. The instrument affords exceptionally fast (30 ms, 10%–90%, 90%–10%, at 500 mL min–1 flow rates), interference-free, simultaneous measurement of acetylene, sulfur hexafluoride (an insoluble reference gas used in the cardiac output calculation), and carbon dioxide (to determine alveolar ventilation), with good (typically ±2% full-scale) signal-to-noise ratios. Comparison tests with a mass spectrometer using serially diluted calibration gas samples gave excellent (R2>0.99) correlation for all three gases, validating the IR system's linearity and accuracy. A similar level of agreement between the devices also was observed during human subject C2H2 uptake tests (at rest and under incremental levels of exercise), with the instruments sampling a common extracted gas stream. Cardiac output measurements by both instruments were statistically equivalent from rest to 90% of maximal oxygen consumption; the physiological validity of the measurements was confirmed by the expected linear relationship between cardiac output and oxygen consumption, with both the slope and intercept in the published range. These results indicate that the portable, low-cost, rugged prototype analyzer discussed here is suitable for measuring cardiac output noninvasively in a point-of-care setting
Applications of the optical fiber to the generation and to the measurement of low-phase-noise microwave signals
The optical fiber used as a microwave delay line exhibits high stability and
low noise and makes accessible a long delay (>100 microseconds) in a wide
bandwidth (about 40 GHz, limited by the optronic components). Hence, it finds
applications as the frequency reference in microwave oscillators and as the
reference discriminator for the measurement of phase noise. The fiber is
suitable to measure the oscillator stability with a sensitivity of parts in
1E-12. Enhanced sensitivity is obtained with two independent delay lines, after
correlating and averaging. Short-term stability of parts in 1E-12 is achieved
inserting the delay line in an oscillator. The frequency can be set in steps
multiple of the inverse delay, which is in the 10-100 kHz region.
This article adds to the available references a considerable amount of
engineering and practical knowledge, the understanding of 1/f noise,
calibration, the analysis of the cross-spectrum technique to reduce the
instrument background, the phase-noise model of the oscillator, and the
experimental test of the oscillator model.Comment: 23 pages, 13 figures, 41 reference
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