Showing papers on "Noise (radio) published in 1982"

Bolometer noise: nonequilibrium theory

Abstract: New theoretical results for noise in cryogenic bolometers are derived. Johnson noise is reduced by as much as 60% by electrothermal feedback from the bias supply. Phonon noise in the thermal link is reduced by as much as 30% relative to the usual equilibrium formula. Photon noise in the Rayleigh-Jeans limit is computed with attention to the attenuation of the photon correlations in the light beam. Basic results on bolometer responsivity, time constant, and thermal properties are presented in a new and convenient form. Excess 1/f and contact shot noise are also discussed.

A systematic study of ULF Waves Above FH+ from GEOS 1 and 2 Measurements and Their Relationships with proton ring distributions

Abstract: A detailed analysis of the ULF noise observed on the GEOS magnetic antennas in the frequency range ∼0.2'12 Hz has revealed the properties of structured emissions occurring just above the proton gyrofrequency whose existence was reported by Russell et al. (1970) and Gurnett (1976). These waves are observed in the vicinity of the geomagnetic equator at all L values between ∼4 and ∼8. They propagate in a direction perpendicular to the dc magnetic field. The waves consist of harmonically related monochromatic emissions. The fundamental frequency is generally of the order of the local proton gyrofrequency. Sometimes the fundamental and first harmonics are missing. If there is more than one fundamental frequency present, nonlinear coupling often occurs between the different emissions. The amplitudes of individual events vary from some tens of milligammas to some hundreds. Their duration ranges from some tens of minutes to some hours. Within the range of sensitivity of the detectors (10−2 γ Hz−1/2 at 1 Hz, 10−3 γ Hz−1/2) at 8 Hz the average probability of emission occurrence during a given hour is 12%, this number increases to ∼30% during the afternoon and in the pre-midnight sectors. Simultaneous observations of proton fluxes, as obtained from the two GEOS particle experiments show that these waves are often associated with distribution functions peaking at some energy (5 ≲ E ≲ 30 keV) for 90° pitch angle particles. This ring-like distribution provides the energy source for the excitation of non-resonant (k∥ = 0) instabilities near nFH+ (n = running number). A theoretical model is presented that qualitatively explains the main characteristics of the observed waves.

The active space of red-winged blackbird song

Abstract: Active space, that distance from the source over which signal amplitude remains above the detection threshold of potential receivers, was determined for Red-winged Blackbird (Agelaius phoeniceus) song in an upland pasture near Ithaca, New York. Song amplitude, the rate of signal attenuation, the amplitude of ambient noise, and the sensitivity of redwings to song masked by noise determine active space and were measured in the field: 1. Maximum root-mean-square song amplitude at 1 m from a singing bird ranged from 88.5–93.5 dB SPL (\(\bar X\)± SE = 90.8±0.21 dB SPL). 2. Close to the source, song attenuates at a rate which closely matches that predicted by spherical spreading (6 dB/doubling of distance) alone. At distances beyond about 30 m, however, excess attenuation becomes important (Fig. 4). The rate of attenuation varies with relative wind direction (Table 1). 3. Ambient noise level is relatively low in early morning, rises in late morning and afternoon as air turbulence increases, and then drops again in the evening as turbulence decreases (Table 2 and Appendix). Measured on a 4 kHz octave filter scale (center frequency = 4 kHz, octave pass-band = 2.83– 5.66 kHz), noise during one day ranges from 15 dB SPL (at 06∶00 h) to 36 dB SPL (at 11∶00 h), more than a 10-fold variation in ambient sound pressure. 4. A 3 dB signal to noise ratio is sufficient for detection of song masked by noise in the field (Figs. 5 and 6). Also, redwings are able to detect differences in this ratio as small as 3 dB. 5. In the absence of wind, the maximum active space of redwing song is about 189 m for a signal SPL of 93.5 dB at 1 m. This distance varies with changes in ambient noise level during the day, and with relative wind direction.

Broadband Shock Noise from Supersonic Jets

Abstract: Broadband shock noise from supersonic jets is investigated through acoustic measurements in both the near and far fields. The peak Helmholtz number of broadband shock noise from unheated convergent nozzles is found to be independent of nozzle pressure ratio when based on the length of the shock cells and the ambient speed of sound. Excellent agreement between power spectral densities measured at various far-field angles is obtained at and above the peak shock noise frequency when source convection and directivity effects are included. The directivity of broadband shock noise is found to be pointed in the upstream direction, with omnidirectionality being approached only at high pressure ratios. For both convergent and convergent-divergent nozzles, the relative importance of shock noise with respect to jet-mixing noise is found to be maximum near the pressure ratio at which a Mach disk begins to form in the jet. Near-field measurements point to a limited portion of the shock cell system as the region of dominant broadband noise emission from a highly underexpanded convergent nozzle.

Voyager 2 plasma wave observations at saturn.

Abstract: The first inbound Voyager 2 crossing of Saturn's bow shock (at 31.7 Saturn radii near local noon) and the last outbound crossing (at 87.4 Saturn radii near local dawn) had similar plasma wave signatures. However, many other aspects of the plasma wave measurements differed considerably during the inbound and outbound passes, suggesting the presence of effects associated with significant north-south or noon-dawn asymmetries, or temporal variations. Within Saturn's magnetosphere, the plasma wave instrument detected electron plasma oscillations, upper hybrid resonance emissions, half-gyrofrequency harmonics, hiss and chorus, narrowband electromagnetic emissions and broadband Saturn radio noise, and noise bursts with characteristics of static. At the ring plane crossing, the plasma wave instrument also detected a large number of intense impulses that were interpreted in terms of ring particle impacts on Voyager 2.

Dynamics of an impinging jet. Part 2. The noise generation

Abstract: The aerodynamic noise generated by a subsonic jet impinging on a flat plate is studied from measurements of near-field and surface-pressure fluctuations. The far-field noise measured at 90° to the jet axis is found to be generated by two different physical mechanisms. One mechanism is the impinging of the large coherent structures on the plate, and the other is associated with the initial instability of the shear layer. These two sources of noise radiate to the far field via different acoustical paths.

Observations of a post-flare radio burst in X-rays

Abstract: More than six hours after the two-ribbon flare of May 21, 1980, the hard X-ray spectrometer aboard the SMM imaged an extensive arch above the flare region which was found to be the lowest part of a stationary post-flare noise storm recorded at the same time at Culgoora. The bent crystal spectrometer aboard the SMM confirms that the arch emission was basically thermal. Variations in brightness and energy spectrum at one of the supposed footpoints of the arch are seen as correlation in time with radio brightness, suggesting that suprathermal particles from the radio noise regions dumped in variable quantities onto the low corona and transition layer.

Centimeter and millimeter wave attenuation and brightness temperature due to atmospheric oxygen and water vapor

Abstract: Calculations are presented for atmospheric absorption and radiation emission for several atmospheric conditions and elevation angles. The calculations are for frequencies in the 1 to 340 GHz frequency range. The calculations are compared to those from other models. Agreement is found to within 15% for absorption coefficient (7.5 g/m/cubed water vapor at 290 K) and approximately the same for total zenithal attenuation. The attenuation and gaseous emission noise curves defined by the International Radio Consultative Committee are found to have minor inconsistencies.

Theory and computer simulations of magnetospheric very low frequency emissions

Abstract: A theory of magnetospheric VLF emission must account for the following features: (a) the triggering of monochromatic emissions by signals of sufficient strength and duration, while the background noise and weak short signals are not amplified, and (b) the occurrence of frequency changes after the emissions have reached a sufficiently large amplitude. A nonlinear mechanism exhibiting these features, with fixed and varying frequencies, is examined analytically and by computer simulation techniques. This mechanism depends on a simultaneous propagation and amplification of wave packets along geomagnetic lines to maintain the nonuniformity ratio RproportionaldelB/sub 0//B/sub w/ in the regime Vertical BarRVertical Barroughly-equal0.5, corresponding to maximum amplitication. (B/sub 0/ is the geomagnetic field and B/sub w/ is the wave magnetic field.) For a constant frequency, this condition yields triggering thresholds which are related to the properties of the magnetosphere. For a varying frequency ..omega..(t), it yields the condition partial..omega../partialtproportional..omega../sup 2//sub t/ for the large-amplitude portion of the risers, where ..omega../sub t/proportionalB/sup 1/2//sub w/ denotes the trapping frequency of the wave.

Optical phase modulation in an injection locked AlGaAs semiconductor laser

Abstract: Optical phase modulation obtained by injecting coherent CW light into a directly frequency-modulated semiconductor laser is reported. Phase modulation at up to a 1 GHz modulation frequency has been obtained without compression for a 1.4 GHz half locking bandwidth. Phase deviation can be represented by the ratio of the original FM deviation to the locking half bandwidth. The phase deviation normalized by the frequency deviation is inversely proportional to the cutoff modulation frequency. A static phase shift of π took place with a 0.48 mA drive current change in the injection locked laser. Reduction in FM noise by means of CW light injection and FM noise accumulation in cascaded injection locked laser amplifiers are discussed, together with the optimum design for an injection locked repeater system.

Superconducting quantum interference device with very low magnetic flux noise energy

Abstract: We have fabricated dc superconducting quantum interference devices incorporating Josephson tunnel junctions with an inductance of about 2 pH and a capacitance per junction of about 0.3 pF. The lowest measured flux noise energy was 3.2 h/ at 1.4 K at a frequency of 202 kHz. When the 1/f noise was subtracted, the white noise energy decreased from around 3h/ at 4.2 K to below 2h/ at 1.4 K.

Optimal Detection of Digitally Modulated Signals

Abstract: In this paper optimal detectors are derived and analyzed for the general class of digitally modulated signals in which the sequence of symbols is unknown a priori and information data are not of interest. The detectors test the signal present condition in background white Gaussian noise versus the null condition of noise alone. Particular attention is focused upon cases in which the SNR per symbol is low compared to unity. The models employed herein are sufficiently general to include most forms of spread-spectrum signals as well as other digital type communication signals.

Low‐level VLF and LF radio emissions observed at Earth and Jupiter

Abstract: Major observational features of magnetospheric VLF-LF radio noise that have been uncovered by the IMP, Hawkeye and ISEE satellites, are examined with emphasis on the capabilities of the noise to diagnose local and remote plasma parameters. The relationship of the radiation to not less than 1-mV/m electrostatic upper hybrid emissions is assessed, and indirect observational evidence suggests that upper noise is associated with the generation of the VLF-LF radiation. Theoretical luminosities of mechanisms including synchrotron radiation, linear mode conversion of upper hybrid waves and nonlinear scattering of upper hybrid waves off plasma density irregularities are estimated, and the wave-wave scattering hypothesis is considered to be a viable mechanism if the presence of low-frequency waves as a scattering agent can be established.

Seismic event discrimination

Abstract: Discrimination of small events begins with detection, association, and location. Recent advances in detection include use of updated measures of the noise variance to control the false alarm rate; and updating the noise amplitude spectrum, N , for application of the optimum detection filter S / N ** 2 where S is the signal spectrum. Postdetection processes of interest include determination of signal azimuth and emergence angle from linear three-component processing. Association programs have recently been improved by implementation of techniques taking advantage of information provided by arrays. Location advances include modification of the a priori travel-time residual covariance matrix to account for the fact that travel-time residuals are correlated at stations near to each other, simultaneous determination of location and phase velocity when using regional phases, and direct use of phases other than P , e.g., Pg , Lg , S , pP , and PKPBC . According to most source theories, small earthquakes and explosions have the same, flat, displacement spectrum for P waves at teleseismic frequencies. Aside from location, discrimination therefore rests on focal mechanism differences and generation of shear waves by earthquakes. The M s : m b discriminant rests on the shear wave generation; but there is overlap of explosions and earthquakes at low magnitudes for dip-slip earthquakes. Low frequency (0.3 Hz) P waves are weak from explosions due to the surface pP reflections; P / pP amplitude ratios for earthquakes vary over the focal sphere in a way impossible for a pair of explosions. Regional discrimination shows promise in the ratio of maximum amplitudes before and after Sn , and in the observed fact that shear phases from explosions detonated near high impedance contrasts are of lower frequency than those from earthquakes in the same region, possibly due to the generation of the explosion shear phases by P to S scattering. Recent experimental analyses show that there is almost no decoupling at high frequencies, in agreement with earlier theories. These frequencies may be observable at regional distances.

Remote sensing of sea state using laser altimeters

Abstract: The reflection of short laser pulses from the ocean surface is analyzed based on the specular point theory of scattering. The expressions for the averaged received signal, shot noise, and speckle-induced noise are derived for a direct detection receiver system. It is found that the reflected laser pulses have an average shape which is closely related to the probability density function associated with the ocean surface profile. The result is used to estimate the mean sea level and significant wave height from temporal moments of the reflected laser pulse.

Intrinsic noise in locust photoreceptors.

Abstract: 1. In locust photoreceptors, the amplitude of the response to light pulses lasting less than 20 ms depends solely upon the number of absorbed photons, which can be estimated at low intensities by counting quantum bumps. Consequently, each receptor can be operated as a calibrated photon counter. 2. Three types of noise in receptor responses have been identified--extrinsic or photon noise and two types of intrinsic noise, dark noise (spontaneous activity) and transducer noise (noise in the transduction mechanism). The methods by which the noise sources are measured and identified involves measuring the responses to a train of flashes of constant intensity and converting these voltage values into a series of equivalent quantum catches. Because photon absorptions follow the Poisson distribution, the variance among equivalent catches due to photon noise equals the mean catch, and any excess variance represents intrinsic noise. 3. Dark noise is negligible: spontaneous signals (quantum bumps produced in darkness) occur less than ten times per hour at 25 degrees C, and the combined effects of membrane and electrode noise are unimportant at all but the highest intensities. 4. At low intensities transducer noise is responsible for more than 50% of all receptor noise (variance), and this rises to 90% when bright stimuli are presented to the dark-adapted eye. 5. Two simple models of transduction indicate that variations in the amplitudes and latencies of responses to single photons are a major source of transducer noise. 6. Transducer noise would be difficult to detect from an analysis of response noise alone, without knowledge of absolute photon catch, because in some important respects it mimics photon noise, e.g. it lowers the quantum efficiency without violating the square root relationship relating increment thresholds to mean intensity.

Multiple modulation for optical pump-probe spectroscopy

Abstract: A system built largely from readily available amateur radio gear which uses radio frequency and audio modulation of pump and probe light beams allows the detection of very weak signals (rms noise ∼3×10−11W for a 10 s time constant) even in the presence of a much larger (3.6×10−2 W) background of pump and probe light.

The self-excited axisymmetric jet

Abstract: The results of a number of investigations suggest that large-scale coherent structures and their interactions play key roles in the transport of heat, mass, and momentum, and in the generation of aerodynamic noise. Investigations related to the study of a large-scale coherent structure in the circular jet are considered, taking into account the advantages of inducing controlled perturbation through self-sustained excitation with the whistler nozzle. The primary objective of the present study was to document the jet response to self-excitation and the sensitivity of this response to the initial condition and the Reynolds number R(D). It was also felt that the results would find use in the control of mixing and aerodynamic noise in a jet.

Correlation of low-frequency intensity and frequency fluctuations in GaAlAs lasers

Abstract: The intensity and frequency noise in single-mode GaAlAs lasers have been measured and the correlation between these instabilities investigated. The measurements were made over the range from 10 Hz to 25 kHz in CSP, TJS, and BH laser structures. It is found that the coherence function for frequency noise and intensity noise from one facet is near unity at the lasing threshold, decreases rapidly with decreasing current below threshold, and also decreases, but more slowly, as the current is increased above threshold. Qualitatively similar behavior is found for the correlation between intensity noise from the two laser facets. Junction voltage fluctuations are not correlated with the other types of noise, except when longitudinal mode hopping is occurring. A model in which both intensity and frequency noise are related to local current variations and optical backscattering in the diode active region is developed to explain the results.

Generation-recombination noise in p-type silicon

Abstract: The generation-recombination ( g - r ) noise spectra of p -type planar silicon devices were measured for frequencies between 40 and 5 × 10 6 Hz in the temperature range 78–300 K. The resistance of these devices was measured in the same temperature range. Computer calculations involving the use of seven energy levels made it possible to explain the resistance, and to interpret the magnitude and the characteristic time of the various g - r noise components in the spectra as a function of temperature. From these calculations the energy position in the forbidden band, the hole capture coefficient and the concentration of the different impurities were determined. Four different impurity levels with energies at 0.135, 0.165, 0.325 and 0.400 eV above the top of the valence band, were found from g - r noise data.

Effect of thickness on airfoil surface noise

Abstract: Noise emission from very small chord and very large chord airfoils was measured with eleven 0.63 cm microphones placed along a horizontal semicircle (4.57 m radius) that was centered at the leading edge of the test airfoil. The noise signals were analyzed by an automated spectrum analyzer which yielded 1/3-octave band sound pressure level spectra for each microphone, and the data were corrected to remove the effects of atmospheric attenuation and jet noise. It is found that the effect of thickness is large and must be accounted for in any fundamental airfoil noise theory that attempts to describe the noise emitted from real airfoils. Incident mean velocity gradients and compressibility must also be taken into account. The effect of thickness increases with frequency, with thick airfoils being quieter than thin ones.

Characterization of phase noise in semiconductor lasers

Abstract: Phase noise in semiconductor lasers has been investigated by many authors in the range of low frequencies (<1 MHz). In this letter we present for the first time phase noise measurements extended up to frequencies greater than 1 GHz. Experimental results showing the power spectral density Sφ(ω) of the instantaneous frequency φ(t) and the variance σ2[Δτφ(t)] of the phase shift Δτφ(t) are presented. The peculiar behavior of Sφ(ω), which presents a sharp peak at the same frequency of the amplitude noise peak, can account for the excessive broadening of the linewidth of single‐mode injection lasers.

Some Noise Comparisons of Data-Collection Arrays for Emission Tomography-Systems Having Time-of-Flight Measurements

Abstract: The noise performance of an emission tomography system having time-of-flight measurements is shown in several examples to be superior for a confidence-weighted data array compared to a most likely position data array. The examples range from a point to a planar distribution of radioactivity, and include a crude model of the left ventricle of a heart containing radioactive palmitate.

Noise and flow structure of a tone-excited jet

Abstract: Detailed and well-controlled acoustic as well as turbulence measurements are made of a tone-excited jet to obtain an understanding of the mechanism of broadband noise augmentation. Results are presented for a range of excitation frequencies and levels both with and without flight simulation. Results of measurements of the large-scale instability waves within the excited jet are also described. Changes in the radiated noise and the flow structure and their interrelationship are then discussed.

The possible production of lower hybrid parametric instabilities by VLF ground transmitters and by natural emissions

Abstract: A parametric instability at the lower hybrid frequency which is known as the ion quasi-mode instability may be excited in the ionosphere. The instability considered is a three-wave interaction in which an incident whistler mode wave near the lower hybrid frequency decays into a lower hybrid wave and an ion acoustic type of oscillation. Threshold calculations are made at intervals along the L = 4 magnetic field line starting at the earth's surface (the geomagnetic position of Siple Station, Antarctica) and proceeding to the equatorial plane. A model is developed for the ionospheric plasma parameters needed to evaluate the expression for threshold field strength. The fields directly radiated by the Siple transmitter are not strong enough to meet the requirement for threshold. However, it is possible that the threshold can be met for Siple transmissions which are amplified by natural processes and also for natural emissions.

Low frequency muffler

Abstract: A muffler comprises two Helmholtz resonators, one tuned for high frequency and the other for low frequency. There is a resonance of the high frequency resonator volume's compliance and the effective tail pipe gas inertance which produces an undesirable low frequency noise. This is matched by the resonant frequency of the low frequency resonator, and suppressed thereby. A secondary effect to suppress noise at frequencies above said low frequency resonance due to the tailpipe gas inertance increasing linearly with frequency.

A real-time fading simulator for mobile radio

Abstract: A test equipment for simulating the Rayleigh-distributed fading encountered in mobile radio is described The fading is achieved by in-phase and quadrature modulation of the radio signal by two independent Gaussian noise signals The noise formation and filtering are performed using a microprocessor for which the clock frequency can be varied to control the fade rate The simulator has been designed to handle amplitude modulated signals and is being used in the development of ssb mobile-radio receivers

Two-second variability in AM Herculis binaries

Abstract: AN UMa and the newly identified AM Her star E1405--451 both show excesses of optical variability in the broad frequency range from 0.4 to 0.8 Hz (2.5--1.25 s). The optical fluxes from AN UMa and E1405--451 must vary by 2.4% and 1.2% rms, respectively, in order to produce the level of noise observed in the power spectra. The noise is not produced during any one section of time or orbital phase but is approximately a constant proportion of the net optical light from E1405--451. The feature is possibly bimodal in AN UMa with a minimum of power at 0.65 Hz. Time series observations of AM Her made during both its ''bright'' and ''faint'' phases show that no such noise feature is present to less than 0.25% and 1.3% rms of the stellar light. The predictions of a model for this phenomenon, recently advanced by Langer, Chanmugam, and Shaviv and based on a thermal instability in the height of a standoff accretion shock, are tested with inconclusive results.

Quarter micron low noise GaAs FET's

Abstract: New quarter-micron gate GaAs MESFET's fabricated with optical lithography have yielded the best noise figures ever reported for FET's at frequencies between 12 and 32 GHz.