Showing papers on "Noise (radio) published in 1987"

Low‐frequency noise in dc superconducting quantum interference devices below 1 K

Abstract: At temperatures below about 1 K, a series of dc superconducting quantum interference devices (SQUID’s) exhibited an apparent flux noise with a spectral density scaling as 1/f α, where 0.58<α<0.80. Typically, the magnitude of the noise increased as the temperature was lowered below 1 K, tending to flatten out at low temperatures with a value of 7±3 μΦ0Hz−1/2 at 1 Hz that was nearly independent of the parameters and materials of the SQUID’s. Although a large number of hypothetical sources of the noise have been eliminated, the origin remains unidentified.

ac method for measuring low‐frequency resistance fluctuation spectra

Abstract: An ac technique is described for measuring low‐frequency resistance fluctuation spectra with improved sensitivity over dc methods achieved by avoiding preamplifier 1/f noise. The technique, easily implemented with decade resistors and a lock‐in amplifier, allows the current noise of low‐resistance (r<10 kΩ) specimens to be measured to frequencies below 1 mHz. Use of a center‐tapped, four‐probe specimen geometry allows discrimination between specimen and contact noise and eliminates noise due to bath temperature variations. The technique is demonstrated in use to determine the dependence of the 1/f noise of Cr films on film area. Measurements with simultaneous direct and alternating currents provide means to study the noise of nonlinear devices and frequency‐dependent conductors.

Spread-Spectrum Multiple-Access Performance of Orthogonal Codes: Linear Receivers

Abstract: This paper analyzes a direct-sequence, spread-spectrum, multiple-access (SSMA) communication system which assigns a set of M orthogonal sequences to each user. With all direct sequence SSMA systems, K users share a channel by phase modulating their transmissions with signature sequences. However, the users of our system transmit log_{2}M bits of information/sequence. This contrasts classical SSMA schemes which use a pair of antipodal sequences and transmit 1 bit/sequence. In this paper, we assume that the channel noise is a combination of additive white Gaussian noise (AWGN) and multiple-access interference. We employ the optimum (single-user) demodulator for orthogonal signals in Gaussian noise. The multiple-user performance of this receiver is analyzed. We obtain approximations for the multiuser probability of error by using a Gaussian approximation for the multiple-access interference. We also obtain an upper bound on the exact probability by using characteristic functions. Our SSMA system is Well suited for application at the lower radio frequencies. Therefore, a companion paper describes a realistic model for low-frequency radio noise, modifies the receiver to include a zero-memory nonlinearity, and studies the performance of the nonlinear receiver.

Squeezed-state generation by the normal modes of a coupled system.

Abstract: A new regime for the generation of squeezed states of the electromagnetic field is described for a collection of atoms within a high-finesse cavity. The process responsible for squeezing is a coupling-induced splitting in the normal-mode structure of the atom-field system. A theoretical analysis is presented that predicts large degrees of squeezing for modest operating conditions. An experimental investigation of this regime has produced noise reductions of 30% relative to the vacuum noise level.

Observation of squeezed noise produced by forward four-wave mixing in sodium vapor

Abstract: We have generated squeezed-state light through forward four-wave mixing in sodium vapor as verified by homodyne detection. Optical phase-sensitive noise with a minimum falling 4% below the shot-noise limit was observed.

Generation of high-frequency broadband electrostatic noise: The role of cold electrons

Abstract: Broadband electrostatic noise (BEN) is commonly observed in the plasma sheet boundary layer in association with ion beams. The generation of these waves in a plasma consisting of an ion beam and a background of hot ions, hot electrons, and cold electrons is investigated. The cold electrons are of ionospheric origin. A complete, systematic study of electrostatic ion beam instabilities, including cold electrons, has been done, and it is shown that for the plasma configuration described, four instabilities can be excited: (1) ion acoustic, (2) Buneman, (3) beam resonant, and (4) electron acoustic instabilities. A low and high beam temperature division is shown to exist that separates when different instabilities can be excited. For typically observed parameters in the plasma sheet boundary layer, the ion beams lie in the high-temperature regime. In this regime, the beam resonant and electron acoustic instabilities are excited, and these instabilities can account for the high-frequency (higher than 500 Hz), low-power portion of the BEN spectrum. In the absence of cold electrons, no such wave growth occurs.

An Analysis of Whistler-Mode Radiation from the Spacelab-2 Electron Beam. Revision.

Abstract: : During the shuttle's Spacelab-2 mission, the Plasma Diagnostics Package (PDP) was released from the shuttle to freefly. At times during this freeflight when the PDP was magnetically connected to the shuttle, the Fast Pulsed Electron Generator (FPEG), located in the shuttle cargo bay, ejected a 1 keV - 50 mA electron beam. The plasma wave instrument on board the PDP detected intense whistler mode radiation during these beam ejections. This paper presents a study of a whistler mode emission detected during one particular continuous electron beam firing. Calculations indicate that the beam radiated approximately 1.6 mW in the whistler mode as the beam transversed the 200 meters from the shuttle to the PDP. The emissivity also decreased by about a factor of 10 over this same distance. The measured wave powers are 10 to the 7th power greater than wave powers expected from incoherent Cerenkov radiation, verifying that the radiation is generated by a coherent process. Estimates of the emissivity based on measured electric field intensities in the beam indicate that the whistler mode noise is produced by radiation from electron bunches created by an electrostatic beam plasma instability.

Weather data transmitting system

Abstract: A system is disclosed for transmitting a sensed temperature from a transmitter to a remote receiver over a modulated carrier through the atmosphere. The transmitter and the receiver are battery powered and include timing circuitry having low power consumption. The transmitter and the receiver are both cyclically turned on for a relatively short period and off for a relative long period with the receiver's on cycle being synchronized with the occurrence of the transmission of the modulated carrier. The receiver further employs techniques to minimize the effect of noise on the reception of the transmitted data which can occur from extraneous signal sources broadcasting in the same frequency band or from environmental noise.

A new shallow‐ocean technique for determining the critical angle of the seabed from the vertical directionality of the ambient noise in the water column

Abstract: In the absence of interference from local shipping, the vertical directionality of ambient noise in the shallow ocean overlying a fast, fluid sediment is a stable feature of the noise field. It is controlled primarily by the bottom reflectivity, which is a time invariant property of the channel. The noise field shows a peak symmetrically placed around the horizontal whose angular width is 2αc, where αc is the critical grazing angle of the bottom. By measuring the noise power distribution in the vertical, αc can be determined and hence the compressional sound speed in the bottom sediment deduced. Measurements of the vertical directionality of ambient noise in shallow water have been made at six sites around the coast of the United Kingdom, using specially designed vertical‐line‐array sonobuoys. Estimates for the critical grazing angle and the sound speed in the bottom have been derived from the noise data for all six areas. Similar data from independent seismic surveys are available for only one of these s...

Recording characteristics of submicron discrete magnetic tracks

Abstract: Discrete magnetic tracks with widths as narrow as 0.5 μm have been formed by etching the surface of Co-alloy film disks. Transitions can be written and read back from widely spaced discrete tracks using conventional heads. A linear variation of pulse area with track width is observed for isolated pulses. Noise measurements for tracks that are either dc erased or written with transition densities up to 3000 flux reversals/mm show a linear variation of media noise with (track width)1/2, in agreement with the behavior expected for media noise uncorrelated across the width of the track.

Outer magnetospheric fluctuations and pulsar timing noise

Abstract: The Cheng, Ho, and Ruderman (1986) outer-magnetosphere gap model was used to investigate the stability of Crab-type outer magnetosphere gaps for pulsars having the parameter (Omega-square B) similar to that of the Crab pulsar. The Lamb, Pines, and Shaham (1978) fluctuating magnetosphere noise model was applied to the Crab pulsar to examine the type of the equation of state that best describes the structure of the neutron star. The noise model was also applied to other pulsars, and the theoretical results were compared with observational data. The results of the comparison are consistent with the stiff equation of state, as suggested by the vortex creep model of the neutron star interior. The timing-noise observations also contribute to the evidence for the existence of superfluid in the core of the neutron star.

Underwater noise due to rain, hail, and snow

Abstract: The spectra of underwater noise generated by rain, hail, and snow have been measured in a lake at a depth of 35 m, for a variety of atmospheric conditions. Rain noise spectra, for light winds (<1.2 m s−1), have a sharp peak at 13.5 kHz with a steep falloff (∼60 dB/oct) on the low‐frequency side and a more gradual falloff (9 dB/oct) on the high‐frequency side. A quasi‐flat spectral regime exists in the frequency interval 2–10 kHz. Wind, for speeds increasing above 1.2 m s−1, progressively rounds the peak. The spectral level at 15 kHz (i.e., near the peak) shows a linear dependence on the log of the rain rate with wind speed as a parameter. Correlation of the rain noise spectra with raindrop‐size distributions suggests that low frequencies are generated by the larger drops, although this aspect of the problem needs further work. Hail noise spectra have rounded maxima appearing between 2 and 5 kHz with an approximately 10‐dB falloff on each side. The spectrum of underwater sound generated by gently falling snow shows a linear increase in level, averaging 5 dB/oct, when plotted against the log of frequency.

GaAs FET's with a flicker-noise corner below 1 MHz

Abstract: GaAs MESFET's with significantly reduced low-frequency noise are demonstrated through application of an understanding that the dominant noise source is generation-recombination (g-r) noise from deep level traps in the gate and backside depletion layers. A 1/f noise spectrum measured from 100 Hz to 10 MHz is modeled as the sum of Lorentzian noise spectra from a few traps subject to the temperature distribution inherent in a GaAs MESFET. The noise associated with a single mid-bandgap trapping level does not appear as an ideal Lorentzian, but rather as 1/f over nearly a decade frequency range by virtue of a time constant that is a strong function of temperature ( exp[E a / kT]) and an estimated temperature distribution of 22°C across the active region. The major g-r trap was characterized as having an activation energy of 0.75 eV. By reducing the g-r noise, flicker noise was decreased by more than 15 dB compared to our conventional GaAs MESFET's and the noise corner was reduced to less than 1 MHz from a typical 40 MHz. This significant reduction was achieved by using MBE layers designed to have lower trap concentrations and high channel doping. These results are within 10 dB of the estimated 1/f noise limit due to the quantum mechanics of carrier scattering [5], [14].

Transition on swept leading edges at Mach 3.5

Abstract: Data and correlations for transition from laminar to turbulent flow on 45and 60-deg swept cylinders are presented. The data were obtained at Mach 3.5 in the Pilot Low-Disturbance Wind Tunnel at NASA Langley. Freestream noise levels were varied during the test program from extremely low values that were essentially in the instrument noise range to much higher values approaching those in conventional wind tunnels. The results show that end plates or large trips near the upstream end of the cylinders cause turbulent flow along the entire attachment line of the models over the freestream test Reynolds number range (based on cylinder diameter) of approximately 1.0 X10

Four-mode squeezing.

Abstract: The four-wave-mixing interactions produced by pumping at two well-separated frequencies can couple four different radiation modes to generate a new kind of squeezed state of light. A dual-frequency homodyne detection scheme is described and is used to observe this type of nonclassical correlation. Initial experimental results in an optical fiber show a noise level 20 percent below the vacuum noise level for the output of the dual-frequency detector, even though the noise at each individual detector remains above the vacuum level.

Interferometric conversion of laser phase noise to intensity noise by single-mode fibre-optic components

Abstract: The intrinsic phase noise of semiconductor lasers is interferometrically converted into high levels of intensity noise by reflections at single-mode connectors and other fibre-optic components. This noise can have significant adverse impact on both direct detection and coherent transmission systems, causing bit-error-rate floors in extreme cases.

Low-altitude ISIS 1 observations of auroral radio emissions and their significance to the cyclotron maser instability

Abstract: Low-altitude ISIS 1 observations are presented which support the cyclotron maser instability as the generation mechanism of most, but not all, auroral radio emissions. The observations confirm the ability of the mechanism to generate different wave modes and harmonics as the ratio of the plasma frequency to gyrofrequency f(N)/f(H) increases depending on the form of the energetic electron distribution function. The present observations correspond to conditions where f(N)/F(H) is always larger than 0.2 and is typically about 1.0. The observations and instability growth rate calculations indicate that second harmonic radiation in the extraordinary and the Z modes is directly generated at low altitudes. By far the most common signals observed near ISIS 1 perigee, however, are in the fundamental Z and whistler modes. The observations and calculations in the case of the Z mode suggest that the pertinent process involves a maser that does not saturate, i.e., that group velocity effects are important. The whistler mode is the most intense signal observed at low altitudes. Since the cyclotron maser generated whistler mode waves have peak growth rates for downward field-aligned propagation, they should be able to penetrate the ionosphere. This process may provide the explanation of occasional reports, dating back to the International Geophysical Year time period, of ground level detection of radio noise associated with the aurora.

Transition noise model for longitudinal thin-film media

Abstract: The integrated noise power in longitudinal thin-film media increases with transition density. This behavior has been attributed to transition noise predominating over the more conventional amplitude modulation noise. At low recording densities the total noise power increases linearly with the transition density. At higher recording densities the noise increases more rapidly. The departure from the linear noise power increase with transition density has been attributed to correlation between noise in adjacent transitions. A model is presented for the noise process in longitudinal thin-film media, assuming that each transition's position fluctuation is correlated to the previous transition's position fluctuation by the demagnetization field. The dominant effect of the demagnetization fields is to increase the inherent noise in each transition and broaden the transitions as the recording density increases. It is the inherent noise increase in each transition, rather than the contribution from the adjacent transition correlation noise power term, that explains the experimentally found nonlinear noise power increase at higher transition densities. The frequency dependence of the integrated noise power is shown to be a strong function of the transition shape.

Dark noise in microchannel plate X-ray detectors

Abstract: We present a detailed study of dark noise sources in microchannel plate (MCP) X-ray detectors. Previously postulated noise mechanisms are critically reviewed. Noise measurements carried out in the light of the review are then reported. The sea-level background count rate in two-stage MCP detectors is shown to have two principal components. The first — variable with position across the MCP and decaying with time under vacuum in a manner dependent on the plate history — is attributed to outgassing of the channel plate structure. The second — isotropic, independent of detector bias voltages, time and temperature — is shown to be consistent, in magnitude and in terms of its output charge spectrum, with the results of beta decay from the radioactive potassium content of the multiplier lead glass. Based on this identification of noise sources, prospects for the production of ultralow-noise MCP detectors for imaging X-ray astronomy are discussed. Measurements of noise reduction by coincidence methods using a partitioned anode are described.

Could glitches inducing magnetospheric fluctuations produce low-frequency pulsar timing noise

Abstract: The mechanism by which the fluctuations in pulsar periods, known as the timing noise, are produced was investigated in the framework of the vortex creep theory. It is argued that the two-component timing-noise power spectra of pulsars with composite spectra (namely, blue noise in high frequencies and red noise in low frequencies) could be caused by two related mechanisms. It is proposed that the low-frequency component could be caused by the sudden change of current braking torque, which is perturbed by the microglitches. The perturbed torque will remain unchanged until the next microglitch; hence, the step-function-like variations in torque will give rise to a random walk in the frequency derivative (or red noise in the noise power spectra of the frequency derivative). 32 references.

Quiet Sun and slowly varying component at meter and decameter wavelengths

Abstract: Comparison of maps of the Sun obtained over the period June 29 to July 8, 1982 at 169 MHz with the Nancay Radioheliograph and at 73.8, 50, and 30.9 MHz with the Clark Lake Radioheliograph shows that the slowly varying component at meter and decameter wavelengths is not always thermal emission. During the period under study weak noise storm continua were the most frequent sources of slowly varying component at 169 and 73.8 MHz. Most filaments show no radio counterpart on the disk. A streamer has been detected on the disk from 169 to 30.9 MHz with an optimum observability at 50 MHz. The brightest source of the slowly varying component from 73.8 to 30.9 MHz for most of the period was located above an extended coronal hole in a region where a depression was observed at 169 MHz. In favorable cases, electron densities can be derived from the positions of noise storms and radio streamers; these are in agreement with previous K-corona eclipse observations.

Electron/ion whistler instabilities and magnetic noise bursts

Abstract: Two whistler instabilities are investigated by means of the linear Vlasov dispersion equation. They are called the electron/ion parallel and oblique whistler instabilities, and are driven by electron/ion relative drifts along the magnetic field. It is demonstrated that the enhanced fluctuations from these instabilities can explain several properties of magnetic noise bursts in and near the plasma sheet in the presence of ion beams and/or field-aligned currents. At sufficiently high plasma beta, these instabilities may affect the current system in the magnetotail.

Electron trap states and low frequency noise in tunnel junctions

Abstract: We present the results of measurements of the low frequency excess, or 1/f, noise in small area Nb/Nb 2 O 5 /PbBi and Nb/Nb 2 O 5 /PbAuIn tunnel junctions. Our study shows that the low frequency noise in all of these devices arises from fluctuations in the tunnel barrier transmission coefficient due to changes in the barrier shape. These time dependent barrier deformations appear to be due to charge capture and emission by electron traps in the tunnel barrier material. We find that Nb 2 O 5 barriers prepared either by Reactive Ion Beam Oxidation (RIBO) or by the Raider-Drake Plasma Oxidation process have one particular type of charge trap which dominates the noise spectrum. The density of these noise sources can be controlled in a variety of ways: Improved processing techniques result in higher quality Current-Voltage characteristics and a simultaneous decrease in the density of noise sources. Further, we find that ion implantation of the barrier with Boron results in a decrease in the density of noise sources. The detailed energy density of states for the noise sources also can be modified by thermal cycling and by application of large bias voltages. We will discuss the implications that our single tunnel junction measurements have for the production of extremely low noise SQUID magnetometers.

The effect of cross-spectrum correlation on the detectability of a noise band.

Abstract: An experiment was performed to study the interaction of two narrow‐band noises having correlated temporal envelopes. The detection threshold of a 100‐Hz‐wide noise‐band signal was measured at different center frequencies in the presence of a continuous 100‐Hz‐wide noise band having a center frequency of 1000 Hz. The two noise bands had either correlated or independent temporal envelopes. Measured signal detection thresholds are lower when the two noise bands are independent, but the magnitude of this difference is not a simple function of the frequency separation between the two noise bands.