Showing papers on "Noise (radio) published in 1984"

The influence of feedback intensity on longitudinal mode properties and optical noise in index-guided semiconductor lasers

Abstract: Theory and experiments on optical feedback effects in index-guided single-mode semiconductor lasers are presented. Evidence is found for the existence of a characteristic parameter C which indicates the relative strength of the optical feedback. Near the transition ( C \approx 1.0 ) from low to high feedback, the feedback-induced low-frequency intensity noise shows a maximum. At higher feedback hysteresis and instabilities are dominant, whereas the feedback-induced noise is low again.

Correlated low‐frequency electric and magnetic noise along the auroral field lines

Abstract: Dynamics Explorer 1 measurements of intense low-frequency electric and magnetic noise observed at low altitudes over the auroral zone are described. The intensity of both the electric and magnetic fields decreases rapidly with increasing frequency. Most of the energy is at frequencies below the O(+) cyclotron frequency, and some evidence is found for a cutoff or change in spectral slope near that frequency. The magnetic to electric field ratio decreases rapidly with increasing radial distance and also decreases with increasing frequency. The polarization of the electric field in a plane perpendicular to the earth's magnetic field is essentially random. The transverse electric and magnetic fields are closely correlated, with the average Poynting flux directed toward the earth. The total electromagnetic power flow associated with the noise is substantial. Two general models are discussed to interpret these observations, one based on static electric and magnetic fields imbedded in the ionosphere and the other based on Alfven waves propagating along the auroral field lines.

Receiver design for high-speed optical-fiber systems

Abstract: The technology of optical-fiber systems is advancing rapidly. Parallel to the development of long-haul telecommunication systems in the gigabits per second data rates operating in the long-wavelength region is the wide penetration of optical-fiber systems in local area networks, video trunking and distribution, sensors, etc. These diversified applications impose different and often conflicting constraints on the optical receiver. This paper re-examines the optical receiver design in view of these different requirements, namely, high receiver sensitivity, wide dynamic range, transparent to the operating bit rate, unrestricted data format, and fast acquisition time. Design tradeoffs between conflicting receiver requirements are considered in detail. In particular, the sensitivity of high-capacity long-wavelength transmission systems is emphasized. The state-of-the-art performance of photodetectors and low-noise amplifiers is discussed. We show that dark current of avalanche photodiodes (APD's) is the main factor limiting the sensitivity of long-wavelength optical receivers. In addition, as an example, we report on the design and experimental performance of a hybridized low-noise optical receiver amplifier capable of more than 2-Gbits/s operation. The input noise spectral density achieved is 9 pA/ \sqrt{Hz} with a noise corner frequency of 920 MHz, corresponding to an equivalent noise resistance of 120 Ω.

Generation of broadband electrostatic noise by ion beam instabilities in the magnetotail

Abstract: Particle data from ISEE 1 sampled in the earth's magnetotail show the presence of energetic ion beams in the boundary layer of the plasma sheet. A theory of instabilities driven by the beams is developed and compared with wave data sampled simultaneously to the particle data. It is concluded that the ion beams generate broadband electrostatic bursts of noise. The electrostatic noise correlates well with the occurrence of the beams, and the spectrum is consistent with that predicted from a negative energy beam instability under magnetotail conditions. The theory predicts that a spectrum of growing waves can be driven for frequencies from 0.001 omega(pe) up to omega(pe), the electron plasma frequency, with a spectral peak typically near 0.01 omega(pe) or lower, in agreement with the wave data. Furthermore, as one moves away from the source region perpendicular to the magnetic field, the high frequency components of the observed wave spectra are predicted to disappear gradually, leaving the low frequency part of the spectrum, also as is observed. Evidence is given for significant pitch angle scattering of the beams by the broadband electrostatic noise, leading to more isotropic ion distributions.

Noise on Residential Power Distribution Circuits

Abstract: This paper presents electromagnetic noise measurements on power distribution circuits in the frequency range of 5-100 kHz. Since this noise is due primarily to appliances such as universal motors, light dimmers, television receivers, and certain switching power supplies located on the secondary side of the distribution transformer, the noise varies dramatically with time as these appliances are turned on and off. Measurements of the noise inserted into the distribution circuit by several representative appliances are presented. The noise measurement technique itself, as well as the resulting measurements, are described. Background noise which is present in the absence of such appliances was also measured. These noise measurements are important for the design of communications systems, such as distribution-line carrier, which operate in the 5-20 kHz region and use the distribution line as a communications medium.

Magnetic field fluctuations arising from thermal motion of electric charge in conductors

Abstract: A model is developed for the source of magnetic field fluctuations emanating from thermal agitation of electric charge in conductors. The calculation of the thermal magnetic noise with the model involves the solution of a general volume conductor forward problem. Frequency‐dependent equations for this problem are derived from Maxwell’s equations. The model is applied to calculate the magnetic noise generated by infinite conducting slabs. A good agreement between theoretical predictions and experimental results were found. Thermal magnetic noise fields inside magnetically shielded rooms are estimated and noise in biomagnetic measurements is discussed.

Intensity discrimination: A severe departure from Weber’s law

Abstract: These experiments were designed to assess the importance of different types of information which might be used in detecting intensity changes for pure tones. Thresholds for detecting an intensity change, expressed as 10 log (ΔI/I), were measured over a wide range of frequencies and levels under conditions where one or more sources of information was either present or was removed. Spread of excitation was restricted by using bandstop noise centered at the signal frequency. Information conveyed by dynamic responses to signal onsets and offsets was eliminated by masking onsets and offsets with bursts of bandpass noise. Phase‐locking information was eliminated by using high‐frequency signals (above 5 kHz). Dynamic responses to signal onsets and offsets appear to play little role in intensity discrimination. Phase locking does appear to be important since Weber’s law or a near‐miss to it was observed at low frequencies, whereas at high frequencies performance deteriorated at moderate sound levels, and improved again at high levels. A preliminary experiment, using 225‐ms stimuli revealed only a small midlevel deterioration at high frequencies. However, when 30‐ms stimuli were used a large deterioration was observed, performance being worse when bandstop noise was presented with the tone. Hence at short durations and high frequencies spread of excitation seems to be important: When it is restricted by bandstop noise values of 10 log (ΔI/I) observed at moderate levels it can be as large as 14 dB. The results of the experiments are consistent with a bimodal distribution of thresholds in primary auditory neurons; at intermediate levels neither population will operate effectively. The absence of a level effect at low frequencies can be explained by phase‐locking cues extending the range over which VIIIth‐nerve fibers can signal changes in intensity.

Theory of noise in semiconductor lasers in the presence of optical feedback

Abstract: We derive analytical expressions for the power spectral densities of intensity and frequency noise of single mode semiconductor lasers in the presence of an optical feedback. By explicitly taking into account the spontaneous emission processes into the laser mode, we obtain a behavior which, at high frequencies significantly differs from the usual one, and is in good agreement with recent experimental results. In particular, we are able to show in which way the intensity and frequency noise, besides being influenced by the external cavity length, are affected by the presence of the well known resonant peaks in the noise spectra of the solitary laser and how a substantial lowering and flattening in the noise spectra can be obtained with external cavity round trip times shorter than the inverse of the peak resonant frequency. We also present experimental results in good agreement with the theoretical ones.

The Influence of Bubbles on Ambient Noise in the Ocean at High Wind Speeds

Abstract: Observations of ambient noise in the ocean at high wind speeds reveal significant departures in spectral shape from previously reported values at lower wind speeds. The observations were made in open ocean conditions in Queen Charlotte Sound, British Columbia, in bands centered at frequencies of 4.3, 8.0, 14.5 and 25.0 kHz; surface wind speeds up to 25 m s−1 were recorded. Ambient noise at 4.3 kHz displayed a logarithmic relationship with wind speed throughout the observed speed range, similar to that reported previously. However, at the two higher frequencies (14.5 and 25.0 kHz), the noise spectrum levels did not increase with increasing wind speed at the same rate and for winds above about 15 m s−1 the noise levels actually decreased with increasing wind speed. Similar though less extreme behavior was observed at 8.0 kHz. We attribute this effect to the presence of bubbles which are known to be entrained at the surface of the ocean. The hypothesis is explored with a model in which the noise sou...

A Deep-Sea Differential Pressure Gauge

Abstract: A pressure gauge configured to respond to the difference between the ocean pressure and the pressure within a confined volume of compressible oil is found to be especially useful for detecting pressure fluctuations in the frequency range from a few millihertz to a few hertz. In the middle of the range its noise level is lower than any other known gauge. The limitation of the gauge at the lower frequency limit is caused by unpredictable thermal expansion in the confined oil and at the upper limit by thermal agitation noise in the resistance of the strain gauge transducer. The gauge is insensitive to acceleration and tilting. Measurements with this gauge on the deep seafloor show two principal features in the spectrum of pressure fluctuations. At frequencies below 0.03 Hz there is evidence of pressures generated directly by long surface gravity waves. Above 0.11 Hz the pressures associated with microseisms are predominant. Between 0.03 and 0.11 Hz there is a spectral gap where the pressure level dr...

Sound generated aerodynamically revisited: large-scale structures in a turbulent jet as a source of sound

Abstract: The theoretical capability of identifying the source of turbulent jet noise is assessed in comparison with experimental data. Account is taken of axisymmetric and spiral turbulence modes in Lighthill's (1952) formulation of turbulent noise. Coherent structures interacting with the mean flow and the fine-grained turbulence are the primary noise sources, modeled as an oscillating streamwise distribution. Low-frequencies arise farther downstream while high-frequencies congregate close to the nozzle lip. Previous measurements at various exit velocities, angles with respect to the nozzle axis, the Strouhal number and downstream distance are discussed. The model successfully predicted the angular distribution of noise frequency due to coherent structures. Further work is indicated on compressibility effects.

Acoustic near-field properties associated with broadband shock noise

Abstract: Shock noise associated with unheated supersonic jets was investigated using a near-field microphone array and a single-sensor wedge-shaped hot-film probe. Both over- and underexpanded cases were investigated using Mach 1.45 and 1.99 convergent-divergent nozzles. Correlation measurements through each shock cell of a single underexpanded case with the Mach 1.45 nozzle were obtained between the hot-film probe and the microphone array. The results of the hot-film/near-field microphone correlations show general agreement with certain theoretical models as to the location for shock noise production, and provide evidence for the existence of some large-scale flow structure that collectively interacts and phases the motion of the downstream shocks. The nearfield microphone correlations demonstrate that downstream shocks dominate shock noise production and suggest the existence of a Doppler effect in the near field of the sources. In addition, broadband shock noise is found to propagate at small angles to the jet axis.

Deep-level analysis in (AlGa)As—GaAs 2-D electron gas devices by means of low-frequency noise measurements

Abstract: Low-frequency noise of (AlGa)As-GaAs heterostructures grown by molecular-beam epitaxy was investigated. The temperature of the samples was varied between 100 and 400 K. In the frequency range from 1 Hz to 25 kHz noise spectra can be described as superposition of several generation-recombination (GR) noise components. Four deep levels (E = 0.40, 0.42, 0.54, 0.60 eV) were detected, three of which are in agreement with those measured independently by deep-level transient spectroscopy (DLTS).

Observations of amplitude scintillations on a low-elevation Earth-space path

Abstract: Results of amplitude-scintillation measurements at 11 GHz on an 8.9° elevation path from an INTELSAT V satellite are presented and briefly compared with those obtained from OTS at 30° of elevation. The variability of the standard deviation of the process is illustrated and discussed in the context of the long-term distribution. Spectral density analysis reveals the familiar bandlimited noise type of spectrum with a 3 dB corner frequency of about 60 mHz.

Measurements on the origin of the wind-dependent ambient noise variability in shallow water

Abstract: Contributions of environmental quantities related to the noise source field and the propagation path are derived from the comparison of measurements of the wind‐dependent noise by omnidirectional receivers at a fixed North Sea station with shipborne measurements in the Baltic Sea. The influence of propagation loss on the wind‐dependent shallow water noise appears to be only marginal, even at extremely different sea areas. The quantity governing the noise spectrum level under uncontaminated conditions is the wind speed at the sea surface for which the second power law relation has been verified between 50 Hz and 20 kHz and above a ‘‘threshold’’ wind speed of ≊5 kts. Neither the characteristic height of the sea waves nor the wind turbulence at the reference height are relevant to the noise production, but both may indicate wind profile changes which originate an essential portion of the noise variability for a given wind speed. Further deviations from the second power law are attributed to a bubble layer effect under storm conditions reducing or enhancing the high frequency noise level thus yielding a spread of the average spectrum level of more than 20 dB.

Direct Phase Noise Measurements of SAW Resonators

Abstract: The most significant contribution of SAW resonators is in the frequency control of low phase noise oscillators at UHF frequencies Techniques for measuring the phase noise of oscillators are well known, but the contribution to that phase noise of the frequency determining element is usually estimated or found by indirect methods We present a method for directly measuring the phase noise in a SAW resonator (or any other linear circuit element) at or near its resonant frequency The correlation between the phase noise measured in a SAW resonator by this method and that measured in an oscillator containing this device is excellent The single-sideband noise spectrum contributed by SAW resonators is found to follow a l/fy trend away from the measurement frequency, where Y is close to 1

Interaction between VLF waves and the turbulent ionosphere

Abstract: The effects of broadening of frequency spectra of quasi-monochromatic very low frequency (VLF) signals were observed in the polar ionosphere when VLF transmitter signals were registered on the satellite "Intercosmos-19". Broadening of the spectrum was mainly registered on the electric antenna to the extent of Δf ≤ ± 1 kHz. Broadening of the VLF signal and intensive extra low frequency (ELF) noise with f < 1 kHz were observed simultaneously. Such effects can be explained by a transformation of the initial whistler-wave, into whistler-mode waves with wave-normals very near the resonant cone, by small-scale ionospheric irregularities. These irregularities can be caused by ion-acoustic or ion-cyclotron waves. The effect of the broadening of the VLF signal spectrum can be used as a diagnostic of ionospheric turbulence and field-aligned electric currents in the upper ionosphere.

The implementation of digital filters using a modified Widrow-Hoff algorithm for the adaptive cancellation of acoustic noise

Abstract: The active reduction of noise is an application of the principle of superposition in which an unwanted noise signal is detected by a microphone and processed by an electronic controller to produce an equal amplitude, 180° out-of-phase cancellation signal. The signal is then appropriately amplified and injected back into the space by a loudspeaker. This paper describes an electronic controller based on a digital implementation of transversal filters using a modification of the Widrow-Hoff Least-Mean-Squared Algorithm. In the case of active noise reduction there is no signal to enhance and all of the detected input must be cancelled. Further, the time required for the physical acoustic wave must be taken into consideration for the system to work.

Intensity noise suppression and modulation characteristics of a laser diode coupled to an external cavity

Abstract: Static and dynamic properties of a GaAlAs laser diode with an external cavity (1.5-50 cm) have been studied experimentally, it is found that the optical feedback induced intensity noise is strikingly suppressed by 30 dB when a single frequency oscillation is realized with good phase matching. The threshold of modulation depth, over which intensity noise increases abruptly, has been measured as a function of the modulation frequency in a range of 10-400 MHz. It has the maximum at about 40-70 MHz and increases with the decrease of the external cavity length. On the basis of the compound cavity model the phase and gain conditions for the single frequency oscillation are discussed and the threshold modulation depth is calculated, which is shown to be in good agreement with the experimental results.

Bifurcating, chaotic, and intermittent solutions in the rf-biased Josephson junction

Abstract: Extensive numerical simulations of the rf-biased Josephson junction are presented. It is shown that, as the amplitude is increased for fixed damping and frequency, an apparently endless sequence of bifurcating-chaotic trees separated by periodic solutions exists. The state diagram characterized in the amplitude-damping plane at fixed frequency (..cap omega.. = 0.65) shows a complex set of solutions. A detailed study of the transition in the high-damping limit is presented, indicating the bounds for asymmetric, bifurcating, and chaotic solutions for low ..beta../sub c/. It is shown that strange attractors are common to trapped and free-running chaotic solutions for nearby amplitudes due to intermittency among the various basins of attraction. Return maps at high amplitudes are found to be essentially one dimensional. Experimental consequences of our simulations are presented in terms of equivalent noise temperatures at low frequencies, resulting in maximum noise temperatures of the order of 10/sup 6/--10/sup 7/ K.

Diode laser noise at video frequencies in optical videodisc players

Abstract: Video frequency noise in diode laser output and its influence on reproduced picture quality have been studied in an optical videodisc player. It was found that laser noise is related to an instability of oscillating longitudinal modes and has a strong correlation with picture quality. A single-mode laser easily oscillates in two or more modes by temperature change and optical feedback, which causes mode-hopping noise and results in white dots randomly appearing on a picture. A multimode laser has a consistently high noise level, and this results in a rough picture quality. Using high frequency superimposed current to power a single-mode laser enables multimode oscillation to be maintained at a steady level. This produces a relatively low noise level resulting in good picture quality. With regard to the video frequency laser noise specifications, it is determined that relative intensity noise should be <6 × 10−14 (Hz−1) in a practical player and <3 × 10−14 (Hz−1) in a pickup simulating noise measuring system.

High-frequency seismic noise at Lajitas, Texas

Abstract: A preliminary analysis of high-frequency (1 to 20 Hz) ambient seismic noise at a very quiet site in southwest Texas is presented. At frequencies greater than 1 Hz, the displacement noise power decreases at a rate between f −4 and f −5. The ambient seismic noise is also characterized by persistent narrow-band peaks (Δ f < 0.2 Hz) of unknown origin. The noise appears to be propagating at phase velocities 2.5 to 4 km/sec or greater which suggests higher mode surface or body waves. Estimates of coherence and degree of polarization of three-component array data indicate that the high-frequency ambient noise is relatively well organized over distances of at least 600 to 700 m for a single component but is relatively unorganized in a three-component sense. A principal result of this preliminary analysis is that the use of three-component, high-frequency (1 to 20 Hz) arrays at very quiet sites, coupled with three-component processors, may result in substantial improvement of the thresholds for detection and discrimination of weak seismic events.

Energization of ions in the auroral plasma by broadband waves - Generation of ion conics

Abstract: Nonresonant heating of plasma ions flowing along geomagnetic field lines by VLF waves is shown to generate ion conics. Ions with energies of less than 1 eV are proven capable of resonance with ionospheric VLF waves excited by electron beams with energies over 100 eV at lower hybrid frequencies (LHF) of 1-1.01. The variation of resonant ion energy with LHF wave frequency is formulated and an expression is derived for the heating rate. The heating rate is then applied to model ion acceleration along a dipole geomagnetic field in terms of the power spectral density of broadband waves, i.e., the VLF noise. Attention is given to ion energization at lower altitudes than the auroral acceleration region and examples of conic acceleration are cited.

Oceanic winds measured from the seafloor

Abstract: Acoustic ambient noise in the ocean at frequencies of 4.3, 8.0, and 14.5 kHz has been shown to be highly correlated with surface wind speed. Measurement of noise at the sea floor yields estimates of wind speed to within ±1 m/s (r2≥0.845) when compared with coincident surface observations. The frequency dependence of the wind speed-noise relation reported by others (17 dB/decade) has been confirmed, except that separate calibrations are required above and below the speed characterizing whitecap formation. There is also some evidence of local hydrography influencing the absolute calibration.

Ambient light and electromagnetic noise reduction circuit

Abstract: A circuit for reducing noise and increasing the reliability of touch panels involves the use of a supplemental noise or electromagnetic interference pickup lead (63) which extends generally coaxtensively with the parallel connected outputs from a series of successively coupled photodetectors (42) and high pass filters (49, 58 and 60 and 64, 65 and 66) for substantially reducing the noise signals created by variations in the level of ambient light. When the photodetectors output (46) is connected to one of the inputs (-) of a differential operational amplifier (56), and the compensating pickup lead (63) is connected to the opposite polarity other input (+), the noise is substantially cancelled out, and a cleaned-up photodetector pulse is provided. A pulse forming circuit (74, 75 and 77) squares up the photodetector output signal pulse; and a hysteresis type storage or buffer circuit (76 and 78) is employed to indicate the presence or absence of photodetector output signals, from successive photodetectors, thereby indicating whether or not the light beams are interrupted either fully or partially.

Effect of fiber‐far‐end reflections on intensity and phase noise in InGaAsP semiconductor lasers

Abstract: The effect of optical feedback on the intensity‐noise spectrum and the longitudinal‐mode line shape of a 1.55‐μm InGaAsP laser is investigated for reflections arising from the far end of a 7.5‐km‐long fiber. The intensity noise near the relaxation‐oscillation frequency is significantly enhanced by the reflection feedback. Owing to the intercavity coupling, the enhancement is, however, frequency dependent and the noise spectrum exhibits high‐contrast modulations with a period corresponding to the fiber‐cavity longitudinal‐mode separation. A simple rate equation model is used to explain the observed behavior.

Min - max detection of weak signals in phi -mixing noise

Abstract: Detection of weak signals in a special \varphi -mixing noise class is considered. The detector structure is restricted to sums of memoryless nonlinear transformations of the observations, correlated with the data sequence and compared to a fixed threshold. Using the efficacy to measure performance, the nonlinearity that has min-max performance is derived.

Pace pulse identification apparatus

Abstract: A pace pulse identification apparatus in which low frequency signals are rejected by a filter and the pace pulses and high frequency noise are applied to one input of a comparator. A threshold wave having an amplitude that is not significantly affected by pace pulses or spikes of noise is provided that has a minimum value when steady high frequency noise is absent and a value equal to the sum of the minimum value and a value that varies with the amplitude of high frequency noise when high frequency noise is present and which is applied to the other input of the comparator. The state of the comparator is changed when a pace pulse is present at the output of the filter because its amplitude exceeds that of the threshold wave.

Noise transmission characteristics of advanced composite structural materials

Abstract: Theoretical and experimental results from a study of noise transmission properties of large unstiffened panels which simulated aircraft outer skins and interior trim are reported. The investigation was performed to define the effects of composite structures on fuselage noise transmission relative to the transmissivity of aluminum structures. One-third octave band measurements were obtained in a two-room facility for measuring transmission loss. Center frequencies of at least 100 Hz were used, and 14 different composite panels, including samples of Kevlar, fiberglass, and graphite, were examined. Details of the composites fabrication techniques are provided, and an infinite panel theory transmission loss model is defined. The flexural rigidities of tape and fabric panels are calculated, as are the transmission losses, the coincidence frequency, and the critical frequency. The theory was determined to be accurate to within 1 dB of the measured transmission loss for mass-controlled specimens.