Showing papers on "Noise (radio) published in 1992"

Scattering theory of current and intensity noise correlations in conductors and wave guides.

Abstract: The low-frequency current-fluctuation spectra of phase-coherent electron conductors are related to the scattering matrix of the conductor. Each contact of the conductor is connected to a thermal equilibrium electron reservoir. The current-current correlations of the conductor are compared with the intensity-intensity correlations of a photon wave guide with all its ports connected to blackbody radiation sources. Only two sources of noise are considered: (a) Fluctuations in the occupation numbers of the incident channels that reflect the thermal equilibrium fluctuations of the reservoir states and (b) a shot noise (or partition noise) that originates if a carrier can be scattered into more than one final state and is present even at zero temperature. The theory uses single-particle scattering states to build up multiparticle states with the proper symmetry. Second quantization provides an elegant treatment of this problem if annihilation and creation operators for both the input and output channels of the wave guide are introduced. At equilibrium, in the absence of transport, the correlations of flux fluctuations measured at two different contacts are negative for both fermions and bosons. Away from equilibrium, in the presence of a net flux, the fluctuations are related to transport coefficients which invoke products of four scattering matrices.The transport portion of the correlation of the flux fluctuations at two different contacts is negative for fermions but is positive for bosons. The transport portion (shot noise) is very sensitive to the transmission behavior of the wave guide. Both for fermions and for bosons completely open transmission channels give no contribution. In addition to two-terminal conductors, we consider four-probe conductors in high magnetic fields which have the property that carriers transmitted and reflected at a barrier reach separate contacts. We discuss a four-terminal experiment which explicitly shows that the correlation function in the presence of two particle sources is not an incoherent sum of correlations generated by particles originating in one of the sources but contains exchange terms due to the indistinguishability of identical particles. We discuss the conditions for such exchange terms to be sensitive to a quantum-mechanical phase and the possibility to tune this phase with the help of an Aharonov-Bohm flux.

High-sensitivity magnetometer based on index-enhanced media

Abstract: The large dispersion of a phase-coherent medium, at a point of vanishing absorption, is applied to interferometric measurements of detuning between atomic and radiation frequencies. It is shown that, under certain conditions, the interferometer quantum-limited operation is determined by vacuum-fluctuation shot noise while the noise introduced by the interaction of the probe field with the phase-coherent atoms can be made negligible. As a possible application, an optical magnetometer is analyzed whose sensitivity is shown to be potentially superior to the present state-of-the-art devices.

Performance of a spread spectrum packet radio network link in a Poisson field of interferers

Abstract: Results on the modeling of interference in a radio communication network and performance measures for the link as a function of distance are presented. It is assumed that a transmitter-receiver pair in a radio network is affected by a set of interferers, using the same modulation and power, whose positions are modeled as a Poisson field in the plane. Assuming a 1/r/sup gamma / propagation power loss law, the probability distributions for the noise at the receiver are found to be the stable distributions. Results are given for the probability of symbol error and link capacity as a function of the distance between the transmitter and receiver for direct sequence and frequency hopping spread spectrum schemes. It is found that the frequency hopping schemes are inherently superior and their performance is not dependent on the synchronization of the hopping times for the different users. >

A calculation of confusion noise due to infrared cirrus

Abstract: General expressions are developed for the statistical errors to be expected in photometric measurements due to confusion in a background of fluctuating surface brightness. Backgrounds actually observed in the far IR by the IRAS satellite are used to calculate tables of these error expressions for two simple measurement techniques. The confusion-noise-limited sensitivities for NASA's planned Space IR Telescope Facility and the ESA's IR Space Observatory are estimated at a wavelength of 100 microns from these tables.

The effects of temperature on supersonic jet noise emission

Abstract: This paper examines the generation of sound produced by high temperature supersonic jets. In particular, the question of the importance of supersonic instability waves to noise emission is considered relative to the role of Kelvin-Helmholtz (K-H) instability waves. Here, these waves are taken to be synonymous with the Mach emission process. Jet total temperatures from 313 to 1534 K are investigated using an axisymmetric water cooled supersonic nozzle designed for Mach 2. The aerodynamic and acoustic results of this study indicate that the dominant noise contributors are the K-H waves over the entire temperature range. Good agreement between measured and numerically predicted plume properties are obtained and an elliptic nozzle is used to demonstrate reduction of the K-H waves.

Generalization in a linear perceptron in the presence of noise

Abstract: The authors study the evolution of the generalization ability of a simple linear perceptron with N inputs which learns to imitate a 'teacher perceptron'. The system is trained on p= alpha N example inputs drawn from some distribution and the generalization ability is measured by the average agreement with the teacher on test examples drawn from the same distribution. The dynamics may be solved analytically and exhibits a phase transition from imperfect to perfect generalization at alpha =1, when there are no errors (static noise) in the training examples. If the examples are produced by an erroneous teacher, overfitting is observed, i.e. the generalization error starts to increase after a finite time of training. It is shown that a weight decay of the same size as the variance of the noise (errors) on the teacher improves on the generalization and suppresses the overfitting. The generalization error as a function of time is calculated numerically for various values of the parameters. Finally dynamic noise in the training is considered. White noise on the input corresponds on average to a weight decay, and can thus improve generalization, whereas white noise on the weights or the output degrades generalization. Generalization is particularly sensitive to noise on the weights (for alpha (1) where it makes the error constantly increase with time, but this effect is also shown to be damped by a weight decay. Weight noise and output noise acts similarly above the transition at alpha =1.

Low-frequency noise properties of N-p-n AlGaAs/GaAs heterojunction bipolar transistors

Abstract: The low-frequency noise characteristics of N-p-n Al/sub x/Ga/sub 1-x/As/GaAs heterojunction bipolar transistors (HBTs) have been investigated as a function of bias current, device geometry, extrinsic-base-surface condition, Al mole fraction in the emitter, and temperature in order to identify the dominant noise mechanisms. These measurements show the existence of three distinct regions in the noise spectra: a 1/f noise line shape, a Lorentzian spectrum (noise 'bump'), and a white-noise region. The 1/f noise is attributed to fluctuations in the extrinsic-base surface recombination current. The noise bump is generated by an AlGaAs trap in the emitter-base junction. The DX center was identified as a possible candidate for this trap. It is shown that for 4- mu m*10- mu m emitter AlGaAs/GaAs HBTs, the use of a depleted, AlGaAs passivation ledge over the extrinsic-base surface typically reduced the 1/f base noise current by a factor of 10, and the reduction of the Al mole fraction from 0.3 to 0.2 decreased the magnitude of the noise bump by a factor of 3. >

Interference suppressor for a radio receiver

Abstract: The RF input of a radio receiver is converted to IF including received signal, noise and interference and fed to an interference detector and to an interference canceler of an interference suppressor. A fast Fourier transform (FFT) at the front end of the interference detector detects samples of all the interferences in the frequency domain. The frequency domain samples are scaled for gain control and detection of interferences above a predetermined threshold. The interference detector suppresses interferences to the predetermined threshold level. The scaled frequency domain interference samples are phase rotated and fed to an inverse fast Fourier transform to obtain a time domain replica of the interferences which are phase shifted replicas of the detected interferences. Such digital time domain replicas are converted to a continuous analog replica of the interferences, converted back to IF and summed with the original IF which included received signals, noise and interferences in order to cancel the interferences at the output of the interference suppressor.

Noise characterization of a self-mode-locked Ti:sapphire laser.

Abstract: The phase and amplitude noise spectra of a self-mode-locked Ti:sapphire laser are measured and compared with the noise characteristics of a colliding-pulse mode-locked (CPM) dye laser. The timing jitter of the Ti:sapphire laser is found to be approximately half that of the CPM laser for jitter rates above 50 Hz. The amplitude noise of both lasers is comparable for frequencies of as much as several hundred kilohertz. However, between 0.5 and 5 MHz the noise spectrum of the Ti:sapphire laser is 126–134 dB below the carrier, while the CPM laser noise is limited by the pump laser at 115–131 dB below the carrier.

Earthquake site response estimation: A weak-motion case study

Abstract: Using weak-motion recordings of aftershocks of the 1989 Loma Prieta earthquake recorded in Oakland, California, near the failed Nimitz Freeway, two methods have been applied to estimate the site response of an alluvium site and three mud-over-alluvium sites. The first estimate is the traditional spectral ratio, and the second utilizes the cross spectrum. Recordings obtained at a nearby bedrock site are used as estimates of the sediment site input motions. While the two site response estimates produce similar peaks and troughs, there is an approximate factor of 2 difference in amplitudes. This discrepancy is evidence that there is a much greater level of noise than would be expected from the pre-event ambient noise. We interpret this as signal-generated noise produced by scattering from heterogeneities, which causes the true sediment site input to differ significantly from the bedrock site recording. Given this level of noise, the cross-spectrum estimate suffers a severe downward bias (by a factor of 2 in this study) and should probably not be used when the input motion is estimated from a bedrock site recording. The spectral-ratio estimates are relatively unbiased, but the level of noise introduces a large degree of uncertainty. Therefore, inferences about site response from individual spectral ratios should probably be avoided. On the other hand, ensemble averages of the estimates significantly reduce the scatter to reveal resonances that agree quite well in frequency and overall shape with those of one-dimensional models whose parameters were determined independently. A discrepancy of higher observed amplitudes than predicted by theory remains unexplained but most likely results from the effects of boundary layer topography, which are not accounted for by the simple one-dimensional models.

Investigation of the mechanisms of low‐frequency wind noise generation outdoors

Abstract: Simultaneous measurement of wind noise and the instantaneous wind speed were performed for bare and screened microphones outdoors. Analysis of these measurements demonstrates that the dominant source of pressure fluctuations at the microphone outdoors is the intrinsic turbulence in the flow. This is in contrast to the results of measurements performed in low‐turbulence environments by Hosier and Donavan [Natl. Bur. Stand. Rpt. NBSIR79‐1599 (Jan. 1979)] and by Strasberg [J. Acoust. Soc. Am. 83, 544–548 (1988)]. For low‐turbulence conditions the fluctuating wake of the screen is the dominant noise source. This finding has important implications for windscreen design for outdoor measurements since the principles described by Hosier and Donavan apply only to low‐turbulence conditions.

Understanding noise in SIS receivers

Abstract: A simple laboratory test receiver has been built and used to examine some interesting aspects of heterodyne mixing with superconductor-insulator-superconductor tunnel junctions Experiments made over the frequency range 220 GHz–490 GHz using junctions of both the Pb/Bi/In-type and the Pb/In/Au-type indicate that, in the majority of cases, a receiver may be characterized through a few simple measurements made external to it's dewar We also show that, at high local oscillator drive levels, excess mixer noise may be generated which may be removed by the application of a magnetic field This is of particular relevance to high frequency mixing where Josephson interference may be strong Finally, it is observed that even in the presence of severe interference a stable bias point, free from excess mixer noise, is often possible

Location of low-frequency noise sources in submicrometer bipolar transistors

Abstract: Expressions are derived for the low-frequency noise (1/f noise, shot noise, Nyquist noise) in bipolar transistors. Particular attention has been paid to the influence of the internal base and emitter series resistance. The expressions have been compared with experimental results from submicrometer silicon bipolar transistors. It is found at low forward currents that the 1/f noise is dominated by 1/f noise in the base current. At high-forward currents the 1/f noise probably stems from the 1/f noise in the internal base series resistance. How to locate the low-frequency noise sources is demonstrated theoretically by analyzing the results for the common-emitter and the common-collector configurations both at low and high currents and at different values of the external resistances. >

Infinite-distance soliton transmission with soliton controls in time and frequency domains

Abstract: It is shown by computer runs and simple analysis that one hundred million km soliton transmission is possible by means of soliton transmission controls in the time and frequency domains. This means that limitless transmission is possible. The key to success is to reduce noise by the synchronous modulation technique which can also reduce timing jitter and nonlinear interaction forces. The accumulated noise converges to a fixed low level even after limitless transmission.

Spatial noise phenomena of longitudinal magnetic recording media

Abstract: The authors have studied the spatial noise characteristics of uniformly magnetized media and implemented a synchronous magnetic pulse detection technique to measure waveforms from precise radial and azimuthal locations on a magnetically recorded disk. They introduce a correlation analysis which includes measuring noise waveforms from the same position on the disk under different remanent magnetic states. Results from this analysis demonstrate that at remanence only minor changes occur in the output waveform and its (presumed) corresponding magnetic structure from write to write, while at DC demagnetization large variations are seen for successive writes. This sensitive correlation technique is used to analyze the random and deterministic components of media and head noise. >

A fixed tuned broadband matching structure for submillimeter SIS receivers

Abstract: We have designed, fabricated, and tested a quasi-optical spiral antenna mixer with a Nb/AlO(x)/Nb tunnel junction. This design incorporates a hybrid antenna fed by a planar logarithmic spiral antenna to couple to the radiation field, as previously done with Pb based devices, as well as a newly designed matching circuit. This matching circuit is a relatively complex structure requiring several layers of photolithographic processing on top of the actual tunneling device. Computer modeling of the device predicted the measured bandwidth to within 8 percent, making scale model measurements unnecessary. We have obtained a good match from 210 GHz to 460 GHz between the antenna and a relatively large area (1.25 by 1.25 sq microns) tunnel junction with omega R(sub N) C approx. = 2 - 4.4. This compares to simple inductive stubs that attain only a few percent of total bandwidth in the submillimeter band or inductively tuned SIS arrays with an upper limit of operating frequencies well below the submillimeter band. Noise temperatures were measured at 345 GHz, 426 GHz, and 492 GHz yielding double sideband noise temperatures at 200 K, 220 K, and 500 K, respectively.

High density recording on CoSm/Cr thin film media

Abstract: SmCo/Cr thin films with coercivity up to 3000 Oe were prepared by RF-diode sputtering. Hard disks were fabricated using glass substrates and the recording properties, carrier-to-integrated-noise ratio, media noise, and overwritability were evaluated using conventional thin-film inductive heads. The influence of the thickness of the Cr underlayer, SmCo magnetic layer, and Cr overlayer on media noise was studied. The intergranular exchange and magnetostatic interaction effects present in SmCo/Cr media were measured from their remanence magnetization curves and correlated with media noise values obtained from recording measurements. The media noise in SmCo/Cr disks varied little with increasing linear density, and no supralinear increase in noise as explained by a transition noise model was observed. A thin Cr underlayer ( >

Low noise and less vibration vacuum cleaner

Abstract: A vacuum cleaner with noise and vibration greatly reduced is disclosed The vacuum cleaner includes a blower assembly which comprises a vibration absorbing assembly for absorbing vibrations occurring due to high speed revolutions of electric blower, a noise shielding assembly for shielding the noise so as to prevent the noise from being propagated from the electric blower to outside of the vacuum cleaner, a flow path changing assembly for curving and extending a flow path by bending the flow path of air after passing through the electric blower, and a noise absorbing assembly suppressing the noise by absorbing the noise propagated through the flow path The vacuum cleaner further includes a blower assembly receiving section, which shields and absorbs the noise Further, a vibration absorbing assembly is disposed on a contact portion between a vacuum cleaner main body and the blower assembly, and an air suction hole is formed on a partition wall which separates the dust collecting room and the blower receiving room from each other, so that the noise generated by the electric blower should be shielded without giving any increased resistance to the flow path of air

Low-noise operation of a diode-pumped intracavity-doubled Nd:YAG laser using a Brewster plate

Abstract: The authors demonstrate noise reduction which has been attained in a diode-pumped intracavity-doubled Nd:YAG laser by using a novel birefringent filter formed by a Brewster plate and a birefringent KTP crystal. It is confirmed that single longitudinal mode operation is obtained by the birefringent filter. As a result, the relative intensity noise obtained is less than -135 dB Hz. This noise reduction technique should play an important role for application of laser-diode pumped intracavity-doubled solid-state lasers. >

Excess phase noise in self-heterodyne detection

Abstract: The interpretation of self-heterodyne spectra is difficult if, apart from spontaneous emission, additional noise sources are presented. Measurements on an external-cavity semiconductor laser show how, for a relatively long delay, the high-frequency (Lorentzian) wings of the self-heterodyne spectrum are a sensitive measure for the quantum-limited (Schawlow-Townes) laser linewidth. The quantum-limited laser linewidth is shown to be inversely proportional to the output power. Values below 5 kHz are routinely measured. The full width at half maximum (FWHM) laser linewidth is larger than this due to excess low-frequency fluctuations, which are shown to result from the presence of side modes. >

Simulations to demonstrate reduction of the Gordon–Haus effect

Abstract: The superposition of spontaneous emission noise on a train of soliton pulses produces a random change of the center frequency of the soliton spectrum that causes a change of the group velocity of individual solitons, which in long-light-wave systems translates into a random jitter of the position of the pulses at the receiver. This phenomenon is known as the Gordon-Haus effect. If uncontrolled, the Gordon-Haus effect sets a definite limit on the permissible data rate or on the length of soliton-based light-wave systems. Recently Kodama and Hasegawa [Opt. Lett. 17, 31 (1992)] have shown that the Gordon-Haus effect can be suppressed by placing filters along the fiber that reduce the frequency jitter and the concomitant group-velocity changes. We demonstrate the reduction of the Gordon-Haus effect by computer simulations.

A low noise 230 GHz heterodyne receiver employing 0.25- mu m/sup 2/ area Nb-AlO/sub x/-Nb tunnel junctions

Abstract: The authors report recent results for a full-height rectangular waveguide mixer with an integrated IF matching network. Two 0.25 mu m/sup 2/ Nb-AlO/sub x/-Nb superconducting-insulating-superconducting (SIS) tunnel junctions with a current density of approximately=8500 A/cm/sup 2/ and omega RC of approximately=2.5 at 230 GHz have been tested. One of these quasiparticle tunnel junctions is currently being used at the Caltech Submillimeter Observatory in Hawaii. Detailed measurement of the receiver noise have been made from 200-290 GHz for both junctions at 4.2 K. The lowest receiver noise temperatures were recorded at 239 GHz, measuring 48 K DSB at 4.2 K and 40 K DSB at 2.1 K. The 230-GHz receiver incorporates a one-octave-wide integrated low-pass filter and matching network which transforms the pumped IF junction impedance to 50 Omega over a wide range of impedances. >

1/f noise and incipient localization.

Abstract: The magnitude of the 1/f noise in indium oxide films and ZnO accumulation layers is studied as a function of static disorder. The noise level, of both systems, is much higher than in typical metals even for the diffusive regime. The correlation between noise and static disorder leads us to suggest that incipient localization may play an important role in determining the noise characteristics of diffusive systems near an Anderson transition

Detection of a submerged object insonified by surface noise in an ocean waveguide

Abstract: A theory is developed and applied to determine whether an object submerged in an ocean waveguide and insonified only by surface‐generated noise can be detected with conventional sensing arrays. An expression for the total noise‐field covariance of a stratified waveguide with a submerged object present is derived using full‐field wave theory. This is evaluated by numerical wave‐number integration for a spherical object in a shallow water waveguide. The Cramer–Rao lower bound on detection error is computed for several realistic shallow water scenarios at both low and high frequency. The results indicate that cross‐range localization is possible when the array aperture is sufficient to resolve the object scale. This conclusion is supported by beamforming simulations. Range localization is possible at greater distances. However, this requires high correlation between direct and scattered noise fields at the sensor, which is difficult to replicate via matched field processing. In addition, wave theory indicates that high resolution imaging of reflected ambient noise is generally most effective within the deep shadow range of the object. Beyond the deep shadow range, diffractive interference from the total forward field may overwhelm reflections, depending upon the incident noise directional spectrum and measurement range. Overall, present analysis indicates that the proposed detection scheme presses the limits of current technology.

Electrostatic waves in the Earth's magnetotail and in comets, and electromagnetic instabilities in the magnetosphere and the solar wind

Abstract: The role of electrostatic ion-beam instabilities in the generation of the broadband electrostatic noise in the Earth's magnetotail and in electrostatic bursts in comets is discussed. The theory of electromagnetic ion-cyclotron instabilities in multicomponent plasmas is reviewed by using the semicold approximation. The method is applied to ring current proton precipitation and to the generation of electromagnetic ion-cyclotron waves at the geostationary altitude. The semicold approximation is also applied to the propagation of electromagnetic ion-cyclotron waves in the solar wind. Due to the large proton thermal anisotropy in the core of the proton distribution function in the fast solar wind, strong electromagnetic ion-cyclotron waves can be generated. The waves can resonate with the alpha particles which, because of the resonant interaction, can be accelerated to velocities well in excess of the proton bulk velocity, provided that initially the alpha-proton drift velocity is larger than zero. >

Manifestations of intense noise stimulation on spontaneous otoacoustic emission and threshold microstructure: experiment and model.

Abstract: Comparison between changes that occur simultaneously on spontaneous otoacoustic emissions (SOAEs) and on other cochlear origin phenomena can contribute to the understanding of cochlear micromechanical activity The temporary changes that arise after short noise exposure are investigated in the following paper The effects of noise exposure on the threshold microstructure near an SOAE and on the amplitude and frequency of the SOAE were measured These experimental results indicate the following: (1) exposure to wideband noise for a short time causes a temporary reduction in the SOAE frequency and amplitude, and alters reversibly the threshold microstructure in the vicinity of the SOAE The difference between the minimum and maximum in the threshold microstructure is reduced, and the frequency that yields the minimum threshold decreases; (2) the threshold at the SOAE frequency is most sensitive to noise exposure; (3) intense stimulation causes a relatively small increase, or even a decrease, in threshold at frequencies near the SOAE The experimental results are interpreted in terms of a nonlinear transmission line model which includes nonlinear amplifiers The effect of the noise exposure is modeled by reduction in the cochlear partition amplification term Most of the experimental results are predicted by this model