Showing papers on "Noise (radio) published in 1994"

Noise-induced nonequilibrium phase transition.

Abstract: We report on a simple model of a spatially distributed system which, subject to multiplicative noise, white in space and time, can undergo a nonequilibrium phase transition to a symmetry-breaking state, while no such transition exists in the absence of the noise term. The transition possesses features similar to those observed at second order equilibrium phase transitions: divergence of the correlation length and of the susceptibility, critical slowing down, and scaling properties. Furthermore, the transition is found to be reentrant: The ordered state appears at a critical value of the noise intensity but disappears again at a higher value of the noise strength.

Turbulent Mixing Noise from Supersonic Jets

Abstract: There is now a substantial body of theoretical and experimental evidence that the dominant part of the turbulent noise of supersonic jets is generated directly by the large turbulence structures/instability waves of the jet flow. Earlier, Tam and Burton provided a description of the physical mechanism by which supersonically traveling instability waves can generate sound efficiently. They used the method of matched asymptotic expansions to construct an instability wave solution which is valid in the far field. The present work is an extension of the theory of Tam and Burton. It is argued that the instability wave spectrum of the jet may be regarded as generated by stochastic white noise excitation at the nozzle lip region. The reason why the excitation has white noise characteristics is that near the nozzle lip region the flow in the jet mixing layer has no intrinsic length and time scales. The present stochastic wave model theory of supersonic jet noise contains a single unknown multiplicative constant. Comparisons between the calculated noise directivities at selected Strouhal numbers and experimental measurements of a Mach 2 jet at different jet temperatures have been carried out. Favorable agreements are found.

Noise in an ac biased junction: Nonstationary Aharonov-Bohm effect.

Abstract: We study excess noise in a quantum conductor in the presence of constant voltage and alternating external field. Becasue of a two-particle interference effect caused by Fermi correlations the noise is sensitive to the phase of the time-dependent transmission amplitude. We compute spectral density and show that at T=0 the noise has singular dependence on the dc voltage V and the ac frequency \ensuremath{\Omega} with cusplike singularities at integer eV/\ensuremath{\Elzxh}\ensuremath{\Omega}. For a metallic loop with an alternating flux the phase sensitivity leads to an oscillating dependence of the strength of the cusps on the flux amplitude.

Residual noise shaping technique for active noise control systems

Abstract: An active noise control system attenuates the overall sound field. However, in some applications, it is desirable to change the spectral contents of the residual noise. In this Letter, a residual noise spectrum shaping technique based on the filtered‐E least‐mean‐square algorithm has been developed. This technique can be applied to active noise control of one‐dimensional ducts and three‐dimensional enclosures, for both narrow‐band and broadband noises. Computer simulations demonstrate that this method not only attenuates the noise level, but also effectively reshapes the spectrum of the residual noise.

Statistical noise due to tunneling in superconducting tunnel junction detectors

Abstract: We discuss the statistical noise associated with tunneling which is intrinsic to superconducting tunnel junctions. We deduce a simple expression for the noise in the general case where the tunneling probabilities may be dissimilar. The statistical tunneling noise exceeds the Fano‐factor limited statistical noise of the quasiparticle creation for most cases.

The potential of a noise‐reducing antenna for surface NMR groundwater surveys in the earth's magnetic field1

Abstract: A method of non-invasive NMR in the earth's field has been developed and is now used for groundwater surveys to depths of investigation of 100 m or more. A circular wire loop of diameter 100 m, laid out on the ground, is employed to excite and receive the NMR signal in the earth's field. However, in areas with high electromagnetic noise, the NMR measurements may be inaccurate. To overcome this problem, a noise-reducing figure-of-eight-shaped antenna, consisting of two touching coils each of diameter 50 m, has been utilized. Using this antenna, the NMR signal has been calculated for different depths of water-saturated layers with various inclinations of the geomagnetic field. The model calculations and experimental data have been compared and found to be mutually consistent. The two-coil antenna is shown to be suitable for studies at depths of up to 30–40 m, which is of practical importance for engineering geology.

Temperature effects in a nonlinear model of monolayer Scheibe aggregates

Abstract: A nonlinear dynamical model of molecular monolayers arranged in Scheibe aggregates is derived from a proper Hamiltonian. Thermal fluctuations of the phonons are included. The resulting equation for the excitons is the two dimensional nonlinear Schr\"odinger equation with noise. Two limits of the complicated spectrum of the noise are considered: time independent, spatially white noise, simply corresponding to disorder in the arrangement of the molecules, and pure white noise. Parameter values are found by comparison with experiments by M\"obius and Kuhn [Isr. J. Chem. 18, 375 (1979)] and order of magnitude estimates given where experiments are not available. The temperature dependent coherence time is found from numerical simulations. Experiments show that the excitons stay coherent during their lifetime. This is in correspondence with the model at temperatures lower than 3 K. To increase this limiting temperature it is found that the dipole-dipole coupling and the exciton-phonon coupling must be decreased significantly.

Magnetic flux noise in copper oxide superconductors

Abstract: We report on the magnetic flux noise in thin films of YBa2Cu3O7-x (YBCO), Tl2Ca2Ba2Cu3Ox, and TlCa2Ba2Cu3Ox and in crystals of YBCO and Bi2Sr2CaCu2O8+x, measured with a Superconducting QUantum Interference Device (SQUID). We ascribe the noise to the motion of flux vortices. In the low magnetic fields in which the experiments are performed the average vortex spacing always exceeds the superconducting penetration depth. The spectral density of the noise usually scales as 1/f (f is frequency) from 1 Hz to 1 kHz and increases with temperature to a peak which is of the same magnitude in all samples, at the transition temperature. Furthermore, the noise power increases with the magnitude of the magnetic field in which the sample is cooled, with a power-law dependence over several decades, whereas a supercurrent well below the critical current density applied to YBCO films suppresses the noise power by an order of magnitude. Most of the measurements were made on YBCO films, and for this set of samples the noise decreases dramatically as the crystalline quality is improved. A model of thermally activated vortex motion is developed which explains the dependence of the noise on frequency, temperature, magnetic field, and current. The pinning potential is idealized as an ensemble of symmetrical double wells, each with a different activation energy separating the two states. From the noise measurements, this model yields the distribution of pinning energies, the vortex hopping distance, the number density of mobile vortices, and the restoring force on a vortex at a typical pinning site. The distribution of pinning energies in YBa2Cu3O7-x shows a broad peak below 0.1 eV. Over narrow temperature intervals, most samples exhibit random telegraph signals in which the flux switches between two discrete levels, with activation energies and hopping distances much greater than those deduced from the 1/f noise measurements.

Investigation of Telstar 4 spacecraft Ku-band and C-band antenna components for multipactor breakdown

Abstract: Multipactor is an electron resonance phenomenon which occurs at radio frequencies in components and transmission lines operating in vacuum. Multipactor represents a possible payload failure mechanism for communications satellites since it can destroy RF components or transmission lines, or it can significantly raise noise levels. The Telstar 4 series of spacecraft presently being built for AT&T by Martin Marietta will carry 24 high-power transponders for Ku-band (/spl sim/60 W/transponder) and 24 medium-power transponders for C-band (/spl sim/25 W/transponder). The outputs of a number of transponders are frequency multiplexed prior to being input to the Ku-band or C-band antenna feed networks (AFN). Computation shows that instantaneous peak powers due to the combined signals can reach 4500 W at Ku-band and 3900 W at C-band. For the purpose of ensuring that the AFN would not suffer multipactor breakdown in orbit, each of the individual components in the C-band and Ku-band AFN were subjected to instantaneous peak powers well in excess of that which will be experienced in space. The high instantaneous peak powers required for this pre-flight testing were realized by phasor addition of the high-VSWR standing waves of two diplexed carriers of slightly different frequencies. This technique effectively provided an instantaneous peak power 32 times the average power output of each of the two input carriers. Test sets to accomplish this were constructed at both C-band and Ku-band. The ability to detect multipactor (by two independent means) was assured by measuring test cells which were specifically designed to break down for the test levels previously mentioned. Investigations of the separate AFN components for the Telstar 4 spacecraft showed that all components passed with substantial margin. >

Road surface condition detector for automotive vehicle

Abstract: A road surface condition detector for detecting a dry/wet condition of a road surface includes an ultrasonic wave receiver 2 for picking up the ultrasonic noise generated by a road wheel tire 1, which increases conspicuously when the R is wet. The output of the ultrasonic wave receiver 2 is amplified by a amplifier 21, and the absolute level thereof is detected by an absolute level detector 22, and the absolute level is smoothed by a received-wave intensity detector 20. The smoothed signal is compared with a reference voltage Vt to determine the road surface condition. Preferably, the reference voltage Vt is varied in accordance with the vehicle speed. When the automotive vehicle is provided with ultrasonic wave transmitter/receiver units 3a and 3b, the ultrasonic noise component is extracted from the received signal by an ultrasonic noise intensity detector circuit 40, and the extracted component is compared with a reference voltage level to determine the road surface condition.

Low-frequency noise in modern bipolar transistors: impact of intrinsic transistor and parasitic series resistances

Abstract: In modern submicrometer transistors, the influence of the internal base and emitter series resistances, on both the I-V characteristics and the LF noise at higher bias currents, becomes important. In this paper expressions are presented for the LF noise in transistors, where the influence of the series resistances has been taken into account. The expressions have been compared with recent experimental results from the literature obtained from modern submicrometer (heterojunction) bipolar transistors. At low forward currents the LF noise in such transistors is determined by spontaneous fluctuations in the base and collector currents. In most transistors at higher forward currents, the parasitic series resistances and their noise become important. >

Apparatus for cooling NMR coils

Abstract: In Nuclear Magnetic Resonance (NMR) spectroscopy and microscopy, noise from the receiver coil of the probe limits sensitivity. This noise may be reduced by cooling the receiver coil. Noise may be even further reduced by use of a superconducting receiver coil. However, high temperature superconductors must be maintained at temperatures significantly below the critical temperature, typically in the range of 10-60 K for proper performance. The invention provides an apparatus for cooling an NMR receiver coil to a desired temperature using a closed circuit refrigeration system. A cold fluid is circulated to a heat exchanger which is in thermal contact with a thermally conductive substrate having low magnetic susceptibility. The receiver coil is deposited on a portion of the substrate located distally from the heat exchanger. In the preferred embodiment, the substrate is sapphire and the receiver coil is a superconductive oxide. In one embodiment of the invention, the cold fluid is delivered through a cold-finger device, thus permitting easy access to the receiver coil while maintaining the closed circuit cooling.

Reduction of blade-vortex interaction noise through porous leading edge

Abstract: The effect of the porous leading edge of an airfoil on the blade-vortex interaction noise, which dominates the far-field acoustic spectrum of the helicopter, is investigated The thin-layer Navier-Stokes equations are solved with a high-order upwind-biased scheme and a multizonal grid system The Baldwin-Lomax turbulence model is modified for considering transpiration on the surface The amplitudes of the propagating acoustic wave in the near field are calculated directly from the computation The porosity effect on the surface is modeled in two ways: (1) imposition of prescribed transpiration velocity distribution and (2) calculation of transpiration velocity distribution by Darcy's law Results show leading-edge transpiration can suppress pressure fluctuations at the leading edge during blade-vortex interaction and consequently reduce the amplitude of propagating noise by 30% at a maximum in the near field

Motion-contrast sensitivity: visibility of motion gradients of various spatial frequencies

Abstract: The purpose of our experiments was to estimate basic sensitivity to motion gradients and to evaluate the evidence for second-order integration and differentiation of motion signals. We measured sensitivity to spatially sinusoidal contrast modulation between two oppositely moving bandpass-filtered noise images. The motion-contrast sensitivity function, defined as the inverse of threshold modulation amplitude as a function of modulation spatial frequency, was bandpass in shape with declines at both highest and lowest frequencies. The functions for three noise spatial frequencies had approximately the same shape when modulation frequency was expressed as a fraction of noise frequency. We compared the data with a model in which linear motion filters, whose outputs are squared or rectified, are followed by a second stage of excitatory or inhibitory pooling. The data are consistent with a model in which (1) all excitatory pooling occurs at the linear stage and (2) the second stage contains a large inhibitory pooling area, with a radius approximately eight times that of the linear receptive field.

The Radiated Noise from Isotropic Turbulence

Abstract: The noise radiated from isotropic turbulence at low Mach numbers and high Reynolds numbers, as derived by Proudman (1952), was the first application of Lighthill's “Theory of Aerodynamic Noise” to a complete flow field. The theory presented by Proudman involves the assumption of the neglect of retarded-time differences and so replaces the second-order retarded-time and space covariance of Lighthill's stress tensor, T ij , and in particular its second time derivative, by the equivalent simultaneous covariance. This assumption is a valid approximation in the derivation of the ∂2T ij /∂t2 covariance at low Mach numbers, but is not justified when that covariance is reduced to the sum of products of the time derivatives of equivalent second-order velocity covariances as required when Gaussian statistics are assumed. When these assumptions are removed the changes to the analysis are substantial, but the change in the numerical result for the total acoustic power is small.

Emission linewidth measurements of two‐dimensional array Josephson oscillators

Abstract: We have coupled emission from 10×10 arrays of Josephson junctions at 4 K to a room‐temperature mixer through a fin‐line antenna and a WR‐12 waveguide. A single voltage‐tunable peak was detected in the frequency range from 53 to 230 GHz. A stripline resonance in the antenna reduced the array’s dynamic resistance and thereby the emission linewidth to as low as 10 kHz. We extract an effective noise temperature of 14 K from the linewidth data.

Noise in high T/sub c/ superconductors

Abstract: Very recently, significant progress has been achieved in the understanding of the excessive strength and behavior of the conductance noise in the conductor-superconductor transition region of high T/sub c/ superconductors. For the high temperature part of the conductor-superconductor transition region, the model incorporates number and mobility noises of charge carriers: while in the low temperature part of the transition, classical and novel percolation noise effects (including possible effects due to flux motion) determine the behavior of the measured noise. In present high-quality (in situ annealed) films, the novel percolation noise effect ("p-noise," Phys. Rev. Lett., vol. 71, p. 2817, 1993) seems to be the most important. Some other important topics will also be briefly examined in this review: magnetic noise, noise in devices, and practical problems of measurements (e.g., comparison of the noise of different materials, temperature fluctuations). >

Low‐frequency excess noise in YBa2Cu3O7−x dc superconducting quantum interference devices cooled in static magnetic fields

Abstract: We have investigated the performance of YBa2Cu3O7−x dc superconducting quantum interference devices (SQUIDs) cooled in static magnetic fields, B0, of 0.01–1 mT. For fields less than the earth’s ambient field, about 0.05 mT, the white noise of the devices at 77 K is not materially affected. However, at a frequency f of 1 Hz the spectral density of the 1/f noise, SΦ (1 Hz), at 0.05 mT increases by an order of magnitude over that for zero field. Furthermore, SΦ (1 Hz) scales approximately linearly with B0, suggesting strongly that the noise originates in the motion of vortices in the YBCO film. This increase in noise is likely to be an issue for SQUIDs operated in the earth’s field.

Raman noise and soliton squeezing

Abstract: A model is constructed for the Raman effect as responsible for the self-frequency shift of solitons. The effect is related to and compared with the measured Raman gain in silica fibers. The model is quantized and includes the thermal and the quantum noise of the optical phonons generating the Raman effect. It is shown that the Raman effect causes excess noise for ultrashort pulses and limits the squeezing of subpicosecond solitons.

Mixing enhancement by and noise characteristics of streamwise vortices in air jet

Abstract: Streamwise vortices are generated in the exit of a Mach-0.6 rectangular air jet by half-delta wings at various angles of attack. They entrain close to 50% more mass by 3.7 jet diameters and reduce the downstream jet noise at 23 diameters away from the jet nozzle by up to 3 dB. The mixing improvement and jet noise characteristics are both strong functions of the strength and relative position of the vortices. Mass entrainment measurements were measured quantitatively with local pitot-static pressure measurements and through image analysis. The image analysis also gave average spatial information about the mass entrainment through scalar transport

Relationships between ocean bottom noise and the environment

Abstract: Observations of ocean bottom low-frequency noise and surface environmental data over a period of 27 days in the northern Atlantic during the SAMSON and SWADE experiments reveal how closely related the noise is to meteorological conditions. Double-frequency microseisms produced by nonlinear interactions of storm-induced surface gravity waves are especially evident in the frequency band 0.16 to 0.3 Hz and show a high variability in both amplitude and peak frequencies. Bifurcated at times, the peak that characterizes the microseism band contains local and distant or “teleseismic” components, which are generated at different locations. Weather and storm fetch appear to be the major contributions to the size and shape of microseism spectra. Storm development on the sea surface is associated with progressively lower microseism frequencies along with a concurrent increase in amplitude. The single-frequency microseism peak is a continuous feature and is observed to portray the same time-dependent spectral characteristics as the portion of the double-frequency peak associated with distant storms. Coherence studies confirm that both peaks (single and teleseismic double) originate at a distant source. These peaks are generated at roughly the same location with some storm component over the coastline.

High-frequency photodiode characterization using a filtered intensity noise technique

Abstract: Optical filtering of amplified spontaneous emission improves measurement dynamic range for frequency response measurements of optoelectronic receivers. For high bandwidth receivers, a novel periodically filtered intensity noise technique is proposed. Response measurements using these techniques on a 1 GHz and 30 GHz receiver are demonstrated. >

Adaptive synchrophaser for reducing aircraft cabin noise and vibration

Abstract: An "adaptive synchrophaser" is disclosed for modifying the phase angle relationship between aircraft propellers to reduce cabin noise and/or vibration. Rather than use a constant pre-selected angle for a specific passenger cabin configuration during a particular flight mode (e.g., during liftoff or cruise), the synchrophaser periodically monitors actual operating conditions and modifies the phase angle accordingly. In the preferred embodiment, a plurality of transducers (microphones) are installed at several cabin locations to sample noise periodically. The signals are then transmitted via a signal conditioner, a multiplexer and an analog-to-digital converter to a signal processor. The processor calculates the maximum acoustic noise at each microphone location for all possible phase angles. It then identifies the optimum phase angle that resulted in the lowest maximum noise anywhere in the cabin, and signals a synchrophaser to set that angle. This process not only reduces noise based on actual on-going conditions, but also balances the cabin noise so that there are not any "hot" seats with high noise levels.

Sectored receivers for indoor wireless optical communication systems

Abstract: This document proposes and studies a novel receiver structure for indoor optical wireless communication systems that exploits the directional nature in both signal and noise propagation through the use diversity techniques. In this document we concentrate on the ability of the proposed sectored receiver in combating the ambient noise. Significant optical power gains are demonstrated and are seen to increase with the relative weight of the directional noise within the cell, with the sharpness of the directional noise source beam width and in environments where there are noise sources positioned outside the cell. Also the SNR of a sectored receiver is seen to be much less sensitive to the position and beamwidth of the noise sources than the SNR of a non-sectored receiver allowing for more universal transceiver designs.

Animal control system

Abstract: There is described a system for controlling the movement of an animal relative to a controlled area delineated by a wire loop antenna. The controlled area may be inclusive or exclusionary. A frequency modulated signal is transmitted by the FM transmitter connected to the loop. An FM receiver attached to the animal detects the transmitted signal and provides an alarm signal to the animal. The loop antenna is untuned. Low radio frequencies are used and the carrier frequency deviation is large to reduce the systems susceptability to noise. The modulating frequency is limited, also to reduce susceptibility to noise. Integrated circuits are especially designed to permit operation with very low power.

Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser.

Abstract: The noise in single-shot coherent anti-Stokes Raman (CARS) spectroscopy that employs a broadband modeless dye laser (MDL) is examined and the results are compared with those of a conventional dye laser. The noise of the dye-laser, the nonresonant CARS, and the resonant N(2) CARS signals are determined. The use of a MDL is shown to result in substantially reduced CARS noise when the CARS signal is generated with a single-mode pump laser, but only a marginal reduction of noise is observed with a multimode pump source The noise measurements are compared with theoretical predictions that are based on models that assume modes of random amplitudes and phases in the multimode laser sources. The combination of a MDL and a single-mode pump laser is shown to increase the precision of single-shot N(2) CARS temperature measurements.

Heterodyne mixing with Nb tunnel junctions above the gap frequency

Abstract: The noise and gain of a heterodyne waveguide mixer employing Nb/Al2O3/Nb superconducting tunnel junctions with an on‐chip integrated tuning element are measured and analyzed at 680–750 GHz and at 840 GHz The lowest receiver noise temperatures are 400 K (double side band) at 720 GHz and 1500 K (3000 K including the beam splitter loss) at 840 GHz We compare data of the pumped I–V curves with the quantum theory of mixing and demonstrate good agreement at frequencies well above the gap frequency