Showing papers on "Noise (radio) published in 1988"

A microphone array with adaptive post-filtering for noise reduction in reverberant rooms

Abstract: The author presents a self-adapting noise reduction system which is based on a four-microphone array combined with an adaptive postfiltering scheme. Noise reduction is achieved by utilizing the directivity gain of the array and by reducing the residual noise through postfiltering of the received microphone signals. The postfiltering scheme depends on a Wiener filter estimating the desired speech signal and is computed from short-term measurements of the autocorrelation and cross-correlation functions of the microphone signals. The noise reduction system has been tested experimentally in a typical office room. The system produces an enhanced speech signal with barely noticeable residual noise if the input SNR is greater than 0 dB. The received noise power-measured in the absence of the speech signal-can be reduced by 28 dB. >

VHF radio pictures of lightning flashes to ground

Abstract: We present information obtained from VHF radio pictures of 47 lightning flashes that struck ground 1430 m above mean sea level (msl) to the north of Johannesburg, over a period of 16 years. Radio pictures were obtained using a hyperbolic method, i.e., by taking differences between times at which radio noise from lightning arrived at five widely spaced stations. These data were supplemented by recordings of electric field change. We found that stepped leaders and intracloud streamers emitted pulses and progressed at speeds that averaged 1.6×105 ms−1. Stepped leaders ranged in length from 3 to 13 km. BIL (breakdown-intermediate-leader) waveforms of electric field change were caused by stepped leaders whose branched channels followed complicated paths. The electric field changes they recorded in an effective bandwidth of 3.5 kHz could be reproduced faithfully by calculations based on an assumption that the radio sources became charged at the expense of a region near the origin of each leader. Some return strokes radiated trains of noise, called Q noise, whose sources extended at speeds near 108 ms−1. Sources of this Q noise retraced leader channels before extending them at these high speeds. Interstroke processes radiated Q noise whose sources extended at speeds that averaged 8.7×107 ms−1. Most Q sources (98%) were directed vertically, but they pervaded regions that retrogressed at “speeds” that averaged 2.2×104 ms−1 in directions away from the starting points of the flashes. This retrogression was directed horizontally, but the vertical Q streamers caused J changes to have positive or negative slopes according to the relative positions of Q sources and the reversal cone. Sixty percent of our sample of ground flashes were endowed with portions that were cloud flashes or were themselves minor components of cloud flashes. A more detailed summary appears at the end of the paper.

Geophysical aspects of man-made electromagnetic noise in the earth—A review

Abstract: The paper gives a summary of geophysical aspects of man-made electromagnetic noise in the Earth as follows: - EM distortion effects of man-made constructions below and over the Earth's surface defined as ‘passive-noise’, - field observation of EM disturbances due to ‘active’ man-made sources, - EM source mechanism of some important active sources from a geophysical point of view, - efforts in order to improve the signal-to noise ratio by instrumental, methodological and dataprocessing ways, - application of man-made EM noise for geophysical prospecting.

Observation of coherent Rayleigh noise in single-source bidirectional optical fiber systems

Abstract: The authors report noise in a directional optical fiber system that they believe arises from the interference of the signal light with the Rayleigh backscattered light. This noise, called coherent Rayleigh noise (CRN), is the dominant noise source in this system. The interference provides a mechanism for translation of laser phase fluctuations into receiver photocurrent fluctuations. A number of noise reduction schemes are proposed. The authors demonstrate that CRN can be reduced, but not eliminated, by providing a small drive current modulation signal to the source laser to broaden its linewidth. Systems using this single-source-bidirectional architecture must take this noise source into account. >

Noise spectroscopy of deep level (DX) centers in GaAs‐AlxGa1−xAs heterostructures

Abstract: We have measured the generation‐recombination noise from the donor‐related DX centers in current biased GaAs/AlxGa1−xAs heterostructures from 1 Hz to 25 kHz and from 77 to 330 K. A significant noise contribution from these traps is observed even at Al mole fractions below 0.2, where the trap level is resonant with the conduction band. The activated behavior of the noise spectrum from this resonant level is very similar to that observed at higher Al mole fractions, when the level lies deep in the fundamental gap. This result can be predicted, based on the recently elucidated relationship of the trap level to the band structure of AlxGa1−xAs. In accordance with other experimental results, the noise spectra demonstrate that the emission and capture kinetics of the level are unperturbed by its resonance with the conduction band. We briefly discuss some implications of these results for heterostructure transistor design.

VLF wave stimulation experiments in the magnetosphere from Siple Station, Antarctica

Abstract: The Earth's magnetosphere is host to remarkable very low frequency (VLF) electromagnetic signals of natural origin. One of these, called a whistler, originates in lightning. Others, such as hiss and chorus, originate within the plasma itself. They are important for at least three reasons. First, they reveal the properties of the plasma through which they travel and thus can be used as remote sensing tools. Second, their high intensity and narrow bandwidths indicate the presence of a previously unknown kind of wave particle interaction that converts the kinetic energy of charged particles to coherent electromagnetic radiation. This process is called the coherent wave instability (CWI). Third, energetic charged particles are precipitated into the ionosphere through resonant scattering by these same waves, causing enhanced thermal ionization, X rays, light, and heat. To better understand and use the CWI, controlled VLF signals have been injected into the magnetosphere from Siple Station, Antarctica and received on satellites and near the conjugate point in Quebec, Canada. In addition to reproducing many puzzling natural phenomena, these experiments have provided critical new data on the CWI, laying a foundation for various theories and computer simulations. Key findings are as follows: 1) Coherent VLF signals often exhibit exponential temporal growth (∼30 dB) and saturation at levels estimated to be of order 5 pT. 2) Temporal growth requires that the input signal exceed a threshold that varies widely with time. The probable cause of the growth threshold is in situ background noise that reduces the efficiency of phase bunching by a coherent input signal whose intensity is comparable to the noise level within the frequency band of the interaction (∼100 hz). 3) Narrowband triggered emissions can be entrained by Siple frequency ramps of different slope but of much lower (−20 dB) amplitude. The mechanism of entrainment is not yet understood. 4) For two equal amplitude input waves spaced 20 Hz apart, the temporal growth of each component is almost totally suppressed. For larger spacings, 40–100 Hz, the lower frequency is more suppressed than the upper. For 10 < Δƒ < 100 Hz, unsymmetrical sidebands at integer multiples (up to seventh order) of Δƒ are created, along with subharmonics. The integer sidebands are attributed to emission growth triggered by one beat and suppressed by the next. Taken together, the spectrum of the stimulated sidebands and sub-harmonics is thus more noise-like than the transmitted spectrum. 5) Simulated hiss shows coalescence of selected noise wavelets into longer and stronger chorus-like emissions, suggesting that chorus and hiss originate in the same mechanism. Future objectives of a VLF wave injection facility include (1) new experiments on the physics of wave growth and wave-induced particle scattering and precipitation, (2) testing of the predictions of theories of VLF wave-particle interaction, (3) development of new techniques for remote sensing and control of space plasmas using VLF techniques, and (4) improvements in the design and operation of VLF communication and navigation systems.

A broad-band low-noise SIS receiver for submillimeter astronomy

Abstract: A quasi-optical heterodyne receiver using a Pb-alloy superconductor-insulator-superconductor (SIS) tunnel junction as the detector and a planar logarithmic spiral antenna for the RF coupling is described, and its performance is compared with the predicted performance of a theoretical model. Noise measurements were made in the laboratory at frequencies between 115 GHz and 761 GHz, yielding double-sideband noise temperatures ranging from 33 K to 1100 K. The receiver has also been used for astronomical spectroscopy on the Caltech Submillimeter Observatory (Mauna Kea, Hawaii) at 115, 230, 345, and 492 GHz. >

Ultra-low-noise cryogenic high-electron-mobility transistors

Abstract: Quarter-micrometer gate-length high-electron-mobility transistors (HEMTs) for cryogenic low-noise application with very low light sensitivity have been developed. At room temperature, these exhibit a noise figure of 0.4 dB with associated gain of 15 dB at 8 GHz. At a temperature of 12.5 K the minimum noise temperature of 5.3+or-1.5 K has been measured at 8.5 GHz, which is the best noise performance observed to date for any microwave transistors. The results clearly demonstrate the potential for low-temperature low-noise applications. >

Intermediate period response of water levels in wells to crustal strain: sensitivity and noise level

Abstract: The response of water levels in wells to earth tides indicates that wells can be used to detect small crustal strain. Vertical groundwater flow between the well intake and the water table can significantly attenuate this sensitivity. The attenuation of strain sensitivity as a function of frequency can be inferred from the response of water wells to atmospheric loading. For the wells examined in this study, significant attenuation due to water table drainage can occur when strain accumulates gradually over periods of days to weeks. Despite the presence of attenuation the wells are still sensitive strain meters over this range in period. At a frequency of 2.5 cycles/day, the noise level of the water level records examined is −130 dB relative to 1 strain2/Hz. This noise level is about the same as that reported for dilatometers and wire strain meters, but is at least 10 dB higher than that reported for laser strain meters. In the frequency band of 0.025 to 2.5 cycles/day, the noise level of the water level records examined increases roughly 25 dB-per-decade decrease in frequency. Some of this noise is due to the influence of atmospheric loading. When the effects of atmospheric loading are removed from the record, the noise level is reduced to roughly 20 dB-per-decade for frequencies above 0.08 cycles/day, a rate typical of high-quality strain meters. For periods slightly less than a month, the wells have a lower noise level in terms of areal strain than that of the best geodetic distance measurements.

Low frequency noise and DLTS as semiconductor device characterization tools

Abstract: The technique of low frequency noise vs temperature measurements is shown to be a powerful diagnostic technique for determining Generation Recombination (G.R.) trapping parameters in MOSFETS. From computer controlled measurements of low frequency noise vs temperature, the trapping parameters are extracted in a manner similar to that of Deep Level Transient Spectroscopy (DLTS). The trapping parameters are also extracted by curve fitting of the low frequency noise vs temperature curves. These noise-determined trapping parameters are compared with those measured by DLTS. The agreement between parameters determined by the spot frequency noise Arrhenius plot with those determined by DLTS is close, while the agreement between those determined by noise curve fitting is reasonable for noise peaks near room temperature, but becomes poor for low temperature noise peaks. We have found that the low frequency noise vs temperature technique appears to show higher sensitivity than our DLTS measurements. By the use of PECVD silicon nitride as a passivation material, the sensitivity to measure G.R. traps by the low frequency noise vs temperature technique can be increased. This improved sensitivity is due to the reduction of interface state noise. The PECVD silicon nitride passivation had no effect on reducing the G.R. noise peaks thus indicating that these traps are probably located in the bulk space-charge region.

An analysis of whistler mode radiation from the Spacelab 2 electron beam

Abstract: During the Spacelab 2 mission the plasma diagnostics package (PDP) was released from the Shuttle to free fly. At times during this free flight, when the PDP was magnetically connected to the Shuttle, Stanford's fast-pulsed electron generator, located in the Shuttle cargo bay, ejected a 1-keV 50-mA electron beam. The PDP plasma-wave instrument 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 traversed the 200 m 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 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.

1/f-noise-power measurements of copper oxide superconductors in the normal and superconducting states

Abstract: The first measurements of the 1/f-noise spectrum in copper oxide superconductors are presented. The key features of our results are (1) no noise is found in the superconducting state, (2) in the normal state the noise is large, comparable in magnitude to that in metal-insulator composites, and (3) the temperature dependencies of the noise and the resistivity are opposite, unlike either metals or semiconductors. Implications for possible conduction models are considered.

Low-noise metal medium for high-density longitudinal recording

Abstract: A low‐noise medium that shows potential for future high‐density longitudinal magnetic recording applications has been prepared and studied. The disk consists of a CoCr‐based ternary alloy and a few percent of a third transition metal sputter deposited on a Cr underlayer. The media noise power is low and nearly frequency independent up to 80 kfci, in sharp contrast to the noise power spectra typical of metal films. Because of the exceptionally low media noise, we have obtained signal‐to‐noise ratios (SNR) greater than 35 dB up to 30 kfci on media having a 70% amplitude density (D70) of 20 kfci. Other characteristics of the media include overwrite better than −35 dB, saturation magnetization of 500 emu/cm3, and coercivity of 1300 Oe. The grain diameter, coercivity, and D70 depend strongly upon the Cr underlayer thickness. The data suggest that the low‐noise results from reduced intergranular exchange coupling.

Real-time in situ measurements of atmospheric optical absorption in the visible via photoacoustic spectroscopy. 1: Evaluation of photoacoustic cells.

Abstract: Four configurations of resonant photoacoustic cells were evaluated for maximum signal sensitivity to light absorption by the sample, with minimal noise and background. Theoretical and experimental data are discussed. Azimuthal and radial resonant modes were compared for one cell. Of the four, the best cell was a brass cylinder, 2.5-cm radius and 9.5-cm length, which was operated in the azimuthal mode. An argon-ion laser (λ = 514.5 nm) was the light source. A continuous sample flow through the cell, required for real-time in situ atmospheric measurements, gave an acceptable noise level and time for signal response at ~500 cc/min. Linearity of the photoacoustic signal was checked in the range applicable to atmospheric absorption. At a signal-to-noise ratio (SNR) equal to 1, a light absorption detection limit of 4.7 × 10−6 m−1 could be achieved.

A solar‐wind “trigger” for the outer heliosphere radio emissions and the distance to the terminal shock

Abstract: The solar wind data from the plasma science experiment on the Voyager spacecraft are examined to search for the source of the 2 to 3 kHz radio noise detected in the outer heliosphere. It is found that two anomalous high speed streams passed Voyager 2 before the noise was initially observed. It is suggested that the interaction of these streams with the terminal shock is responsible for the more intense emission. Using a time-of-flight argument, the distance to the shock is estimated at about 70 AU-140 AU. The larger value is consistent with an estimate using Voyager data and a standard pressure-balance argument.

Distributed noise cancellation system

Abstract: A method and system for cancelling noise from sources that are distributed over a region, whereby two sensors are located so that a first sensor will detect both voice signals and noise signals, and a second sensor will detect only the noise signals. The voice signals picked up at the second sensor are negligible, and the noise signals picked up at both sensors are correlated. The signals output from each sensor are connected to a predetermined number of narrowband filters in order to divide each respective signal into a predetermined number of frequencies, such as 15 for example. Thereafter, both signals are combined to cancel effectively the noise component from the signal output having both voice and noise to leave a voice signal that is substantially noise free.

Spatiotemporal characteristics of the inductively coupled plasma

Abstract: A previously unrecognised source of noise in inductively coupled plasma (ICP) atomic emission spectrometry was identified with a combination of high-speed motion picture photography and noise spectrum analysis. The noise mechanism is a fluid mechanics phenomenon and involves axisymmetric oscillations of the plasma as the plasma gases flow from the torch into the surrounding static atmosphere. The oscillations develop into vortex rings with increasing height above the torch. As these plasma oscillations pass through the optical axis of the measurement system they produce periodic variations in the analytical signal, typically in the 100–600 Hz range. The frequencies of the oscillations observed in the films agreed with the frequencies of the major noise peaks in the noise power spectra. Knowledge of this noise phenomenon is relevant to studies of the fundamental properties of the ICP and its applications.

Effect of noise on spinodal decomposition

Abstract: The long-time behaviour of two-dimensional systems undergoing spinodal decomposition is studied numerically with the aid of a cell-dynamical approach both without and with noise. In both cases, the representative length scale of the pattern behaves as l(t) approximately tphi , where the exponent phi crosses over from approximately 0.28 to approximately 0.33. The crossover time increases with an increase in amplitude of the noise.

Noise suppression apparatus

Abstract: A noise suppression apparatus has a main microphone for mainly picking up a voice and for outputting an input signal including an audio signal and a first noise component generated from a noise source, a reference microphone for picking up a second noise component generated from the noise source, a filter bank for band-dividing the input signal from the main microphone and the second noise component from the reference microphone, and a noise cancel circuit for obtaining a phase difference between the input signal and the second noise component with respect to each divided band of the filter bank so as to correct the input signal based on the phase difference and for cancelling the first noise component in the input signal by use of the corrected input signal.

Quantum-noise quenching in the correlated spontaneous-emission laser as a multiplicative noise process. I. A geometrical argument.

Abstract: We show, via simple geometrical arguments, the quantum-noise quenching in a correlated (spontaneous) emission laser (CEL). This noise quenching is a consequence of the correlation between noise sources which results in a multiplicative noise process. The steady-state distribution for the phase difference between the two electric fields in a CEL is compared and contrasted to that of a standard phase-locked laser. Noise quenching is shown to occur in the case of the CEL via an explicit solution of the Fokker-Planck equation.

Signals and Noise in Measurements of Low‐Frequency Geomagnetic Fields

Abstract: The apparent magnetic noise, obtained from the coherency function for two parallel magnetic sensors, generally overestimates sensor noise because the sensors do not measure the same signal. The different signals result from the nonparallel alignment of the sensors and from the additional magnetic signal induced in each sensor by its motion in the Earth's magnetic field. A magnetometer array experiment was completed in Grass Valley, Nevada, to determine the minimum magnetic signal that could be detected in the presence of background natural field variations and motion of the sensor. Superconducting quantum interference device (SQUID) magnetometers with internal biaxial tiltmeters were used to record the magnetic fields and the motion of the sensors. A least squares fitting program enabled the field at one site in the array to be predicted from a remote site and to produce a residual field with a standard deviation of 8 pT over a 1-hour period with a low-pass filter at 0.3 Hz. Consistent field-to-residual ratios of 40–60 dB were achieved, with some ratios exceeding 70 dB. The least squares fit uses only a linear combination of the magnetic and tilt fields at a remote site to predict the observed magnetic field. This procedure allows for correction of calibration and orientation errors as well as the removal of the apparent fields originating from sensor movement. Misalignment and motion of the sensor are shown to be the major sources of magnetic field noise. The orientation error is typically of the same magnitude as the noise induced by sensor motion. In order to achieve ratios better than 20–40 dB one must include both the orthogonal fields and the tiltmeter outputs. Inclusion of a frequency-dependent transfer function should increase the prediction ability of the least squares model, as evidenced by the improvement to 70 dB obtained with simple band limiting of the original data. These techniques should be applicable to any type of artificial source survey where natural field fluctuations are the noise-limiting factor. The ability to describe the observed signals should allow a dramatic increase in one's ability to detect an artifically generated signal, allowing signal-to-noise improvements of 40–60 dB without increasing transmitter power or the averaging time.

A low noise SIS receiver covering the frequency range 215-250 GHz

Abstract: We have developed a heterodyne receiver incorporating an SIS mixer for use on a radiotelescope operating at 1.3 mm wavelength. The mixer has a minimum conversion loss of <2 dB and contributes less than 60 K to a total double side band receiver noise temperature of about 80 K at 220 GHz and 230 GHz. To our knowledge this represents the lowest receiver noise ever reported in this frequency range.

Method and apparatus for improving voice intelligibility in high noise environments

Abstract: The output of a noise canceling first or second order electret gradient microphone in a very high noise environment has background noise attenuated and certain phonemes shaped to improve intelligibility by means of a slow action automatic gain control circuit (22) which matches dynamic range of the microphone (10) with dynamic range of the following communication channel, and a fast action automatic gain control circuit (24) in parallel with the first circuit, that reshapes puff noise into plosive, fricative and affricative speech component signals that produced the puff noise.

Plasma wave turbulence around the shuttle: Results from the Spacelab‐2 flight

Abstract: During the Spacelab-2 flight, which occurred from July 29, to August 6, 1985, a spacecraft called the Plasma Diagnostics Package (PDP) was released from the shuttle to explore the plasma environment around the shuttle. The plasma wave instrument on the PDP detected a region of intense broadband turbulence around the shuttle at frequencies extending from a few Hz to about 10 kHz. The noise has broadband intensities ranging from 1 to 5 mV/m and was observed at distances of up to 400 m from the shuttle. The highest intensities occurred in the region downstream of the shuttle and along magnetic field lines passing near the shuttle. The intensities also tended to increase during periods of high thruster activity, which provides strong evidence that the noise is caused by an interaction of the ionosphere with gaseous emissions from the shuttle, similar in many respects to the interaction of a comet with the solar wind. Antenna interference patterns observed in the wideband data show that the wavelength of the turbulence is very short, a few meters or less.

Fundamental 1/ƒ noise in silicon bipolar transistors

Abstract: For the first time flicker noise in the collector of p + - n-p Ge82-185 silicon bipolar transistors is observed unobstructed by the amplified base noise sources. The 1/ f spectrum from the collector is attributed to the scattering of carriers during diffusion through the forward biased base rather than their recombination. Both agreement and disagreement is found between α Hp and α Hn as given by the theory of acoustic phonon scattering and our experimental Hooge's constants. No evidence is found to indicate either Umklapp or intervalley scattering as possible sources for the 1/ƒ noise.

Ground‐level detection of low‐ and medium‐frequency auroral radio emissions

Abstract: Natural radio emissions in the frequency range of several hundred kHz were detected at ground level in the auroral region during a ground-level observing program conducted near Fairbanks, Alaska in the spring of 1986 in an attempt to detect auroral cyclotron-maser-generated whistler modes. Most of the strongest emissions were observed in a small time interval around midnight magnetic local time, suggesting a close tie-in with auroral processes. The results support earlier reports of radio noise from the aurora at frequencies of about 100 kHz and indicate the importance of extending the upper frequency of ground-based high-latitude VLF stations to the vicinity of 1 MHz.

Noise in a thin metallic medium: the connection with nonlinear behaviour

Abstract: At high transition densities the noise in thin metallic films with longitudinal orientation shows an anomalous increase. At the same time, the medium becomes nonlinear, that is superposition of isolated pulses no longer applies. Measurements of both effects suggest that they are related. The anomalous increase in the noise is seen to be the consequence of the increase in nonlinearity as transition spacing is reduced. Nonlinear behavior is also responsible for the significant dependence of the noise on the recorded data pattern. >