Showing papers on "Noise (radio) published in 1981"

VHF radio pictures of cloud flashes

Abstract: Five ground-based VHF receivers were used to trace paths followed by flashes of lightning. Every lightning flash radiates a succession of radio noise pulses, and three Cartesian coordinates of the positions of pulse sources were determined by measuring the times at which the noise pulses arrived at each of the spaced receivers. Lightning channels were mapped in space and time by locating a large number of radio noise sources for each flash, whose shape and position then became apparent despite the presence of intervening hydrometeors. A center frequency of 253 MHz was chosen for the receivers and their bandwidths were wide enough for time differences to be measured with rms errors of 140 ns. Hence two of the three spatial coordinates of a particular source could be determined to an accuracy of 25 m rms and the vertical coordinate could be found to an accuracy which was typically 140 m rms, but the actual height accuracy depended on source position. Some factors that affect the design of the receivers are discussed and case studies of five cloud flashes are presented. Cloud flashes could be classified into two types according to the rates at which they emitted pulses in the VHF part of the radio spectrum. One class radiated pulses at rates that approximated 103 pulses per second when received in bandwidths of 10 MHz. These pulses were often nearly rectangular in form, lasted approximately 1 ms on average, and occurred in synchronism with pulses received at HF and at UHF. The second class emitted much shorter pulses (median durations of 0.2 to 0.4 ms were measured) at higher rates typically 105 pulses per second, and pulses were generally not in synchronism with those received at other radio frequencies. Diameters of the channels occupied by radio sources varied from 100 m to several hundred meters, and were enlarged by the extents of the sources themselves. It was possible to measure the principal extents of many individual sources active during low-pulse-frequency cloud flashes. Average sizes near 300 m were measured for low-pulse-frequency flashes, and sizes near 60 m were estimated for sources active during high-pulse-frequency flashes. It was found that pulses originated in regions near streamer tips, and that pulses were associated with initial ionization. First streamers progressed at speeds that ranged from 0.9 × 105 m/s to 2.1 × 105 m/s except for one extensive, positive flash that moved at 5 × 105 m/s for the first few kilometers. We define a positive flash as being one which conducts excess positive charge in the same direction as that in which the streamer progresses. Subsequent discharges along paths that had been ionized previously seldom radiated much noise but noise was radiated by channels that were several tens of milliseconds old, and then we measured streamer speeds that were at least an order of magnitude higher than first streamer speeds. Relatively few cloud flashes were found to have been oriented vertically. Most were horizontal and often consisted of several streamers that extended from a common origin. Measurements of electric field change were used to estimate quantities of charge involved. These estimates were made by adopting various models for the distribution of charge along the known paths, and the quantities were not greatly dependent on the form of the distribution. Most estimates yielded line densities near 10−3 C m−1. Only one of the cloud discharges had been a positive flash. Trains of band-limited noise which lasted for times that ranged approximately from 10 μs to about 1.5 ms were also emitted by lightning flashes of all kinds, but in cloud flashes this type of noise, which was distinctly different from the pulsed emission, occurred more frequently and with longer durations during the J-type portions or final stages of cloud flashes. It accompanied streamers which progressed at speeds near 107 m/s, often accompanied K changes, which were usually delayed after the start of this noise by tens of microseconds, and the streamers that caused the noise were positive streamers, with the single exception of one train which was found to have been due to a negative streamer that formed near the origin of an extensive, positive cloud flash. There was evidence that events that produced this noise sometimes triggered dart leaders and recoil streamers and most sources of this band-limited noise were located near the origins of the flashes, and streamers which caused the noise served also to discharge the origins but not usually by way of the paths formed by first streamers. The principal results of this work are listed at the end of the paper.

Semiconductor laser noise in an interferometer system

Abstract: The noise of semiconductor laser light after passing a Michelson interferometer has been measured for gain guided as well as index guided double-heterostructure injection lasers. This noise is mainly due to the partition noise and the frequency noise of the laser emission. Unless the interferometer is perfectly balanced, the observed noise is several orders of magnitude larger than the usual intensity noise of semiconductor lasers.

Phase noise of single-mode diode lasers in interferometer systems

Abstract: Measurements have been made of the phase noise of six different types of single‐mode diode laser in an unbalanced Michelson interferometer, as a function of optical path difference. The frequency dependence of the phase noise has also been determined. Possible origins of the frequency fluctuations which result in the phase noise are discussed. The detrimental effect of the phase noise on the sensitivity and dynamic range of optical fiber interferometer sensors is briefly discussed.

Low-noise fiber-optic rotation sensing

Abstract: A major source of noise in fiber-optic Sagnac interferometers was identified to be the fluctuation of the phase of Rayleigh backscattering resulting from minute temperature variations and vibration. This noise can be reduced either by introducing a phase modulation into the fiber ring or by the use of a low-coherence source. A noise-equivalent rotation rate of 8 x 10(-4) deg/sec was achieved.

Altitude dependent model of the auroral beam and beam-generated electrostatic noise

Abstract: A height dependent model of the auroral beam and ionosphere is used to investigate the electrostatic noise generated by the auroral beam. The beam pitch angle is assumed to evolve with the first adiabatic invariant conserved. The beam generates whistler noise over most of the field line and upper hybrid noise only at low altitudes. Only a very narrow range of beam source densities generates noise without producing such large power fluxes that nonlinear effects are important. The strongest power fluxes in the auroral arc are likely to occur in the altitude range of a few thousand kilometers.

The signature of auroral kilometric radiation on Isis 1 ionograms

Abstract: Auroral kilometric radiation (AKR) appears on the Isis 1 topside sounder ionograms as intense noise bands between the electron cyclotron frequency and 700 kHz A variable gap occurs between the cyclotron frequency and the lowest AKR frequency As Isis 1 traverses the source region, the gap narrows, and the AKR signals at higher frequencies weaken This signature suggests that the AKR waves are generated directly in the extraordinary mode at frequencies just above the local cutoff frequency and that the radiation is initially perpendicular to the magnetic field

Single-mode diode laser phase noise

Abstract: Measurements have been made of the phase noise of a single‐mode diode laser in an unbalanced Michelson interferometer, as a function of optical path difference. The noise increased linearly with increasing optical path difference. The origin of the phase noise is discussed.

On the computational noise of finite‐difference schemes used in ocean models

Abstract: As a guide for the choice of finite-difference schemes for use in ocean modeling, different distributions of variables over the horizontal array of grid points in an ocean circulation model are investigated using the shallow water equations. Numerical and analytical techniques are used to study the types of computational noise present in each grid system. It is shown that the B-scheme (in which the horizontal velocity is carried at the center and the height field is carried at each corner of a rectangular grid) with diffusive dissipation successively suppresses numerical noise in a coarse grid (> 100 km) ocean model. For fine-scale resolution (<50 km), as in mesoscale ocean eddy models, it is shown that the C-scheme (in which the zonal velocity is carried at points to the east and west of the point of a rectangular grid where the height is carried, with the meridional velocity carried to the north and south of the height point) can be free of noise for the gravest mode (as in a two-layer model). DOI: 10.1111/j.2153-3490.1981.tb01761.x

External Noise Effect on the Onset of Williams Domain in Nematic Liquid Crystals

Abstract: In order to examine the effect of external noise on the electrohydrodynamic instability, we have measured the wave number spectrum of the pattern of Williams domain in nematic liquid crystals MBBA in the presence of external noise superposed to the a c voltage. The threshold voltage for the onset of Williams domain is found to shift to higher values with increasing noise power. Its noise dependence is interpreted in terms of a theory presented by Dubois-Violette et al.

Dispersion patterns of the ground roll in eastern Saudi Arabia

Abstract: Seismic surveys on land must be designed so that the source‐generated noise, such as ground roll, is preferentially attenuated before P‐wave signal amplification and recording. The correct specification of spatial and frequency filters requires prior knowledge of the noise properties in the area. We show that the strong Rayleigh wave component of source‐generated noise has a wavelength range which is predictable on a regional scale, using widespread P‐wave velocity measurements in shallow upholes. This predictive capability decreases the number of noise analyses required to map the boundaries between areas with different Rayleigh wave properties. The case history presented is for northeastern Saudi Arabia, an area of roughly 150,000km2. The data comprise 80 noise analyses and a data base of over 10,000 up‐hole measurements of P‐wave velocities, supplemented by maps of topography and geologic outcrops. Examples show that the frequency‐wavenumber transforms of time‐offset records can be interpreted in detai...

Current-generated noise recorded on ocean bottom seismometers

Abstract: High-amplitude, anrrow band noise that correlates with periods of high ocean bottom currents and the tidal cycle is occasionally observed on ocean bottom seismometers (OBS). The geophones on OBSs of different configurations are not equally sensitive to this noise and hydrophones are almost unaffected. With a suitable design, it should be possible to eliminate this noise problem.

Modal noise in single-mode fibres operated slightly above cutoff

Abstract: If a single-mode fibre is operated slightly above the theoretical cutoff frequency, at every discontinuity like a misaligned joint, a part of the LP01-mode will be converted to the LP11-mode. At a following discontinuity, the LP11-mode will be partly reconverted to the LP01-mode, e.g. a source wavelength shift will then cause modal noise.

Electron precipitation induced by VLF noise bursts at the plasmapause and detected at conjugate ground stations

Abstract: A new type of wave-induced electron precipitation event has been identified. During observations at conjugate stations Siple, Antarctica, and Roberval, Canada (L ∼4.2), VLF noise bursts were found to be associated on a one-to-one basis with amplitude perturbations of subionospheric radio propagation. The amplitude perturbations are attributed to patches of enhanced ionization that extended below ∼80 km in the nighttime ionosphere and that were produced by precipitating electron bursts. Similar amplitude perturbations seen previously were correlated with whistlers that propagated within the plasmasphere. For the new events the driving waves were structured collections of rising elements that propagated just beyond the plasmapause at roughly 5-min intervals over a several-hour period. These noise bursts were of relatively long duration (∼10 s) and strong intensity (inferred to be >30 pT at the equator). Triggering of the noise bursts appears to have been mostly by whistlers but changed in character with time. Some later bursts had narrowband precursors at constant frequencies possibly locked to power line harmonic radiation. The burst initiation characteristics suggest the existence of a variable threshold for rapid temporal growth in the magnetosphere controlled by the trapped electron dynamics. The temporal signatures of the amplitude perturbations show that precipitation was maintained over multiple bounces of the trapped magnetospheric electrons. In some cases these signatures included a new undershoot effect during the recovery phase lasting 2–5 min. This effect may have been related to cutoff of background drizzle precipitation. Precipitation effects were observed on both long (∼10 Mm) and short (∼½ Mm) subionospheric paths and were monitored simultaneously at the conjugate stations. Similarities in the perturbation signatures on long and short paths suggest that the form of the signatures was governed by ionospheric changes and was not distorted by the subionospheric propagation mechanism. Some differences in the conjugate perturbation signatures are believed to be caused by the difference in loss cone widths for precipitation in the northern and southern hemispheres. Existence of this loss cone gap produced an estimated ∼30-pT noise amplitude threshold for significant northern precipitation and possibly caused saturation of southern precipitation.

Ultra low noise Nb DC SQUIDs

Abstract: Electron-beam lithography has been used to produce ultra low noise Nb dc SQUIDs (Superconducting QUantum Interference Devices) with Josephson elements consisting of either small area tunnel junctions or very narrow variable thickness bridges (nanobridges). Detailed voltage noise and transfer function measurements have been made as a function of temperature and flux and current bias. These measurements allow a computation of the intrinsic energy resolution e vs bias parameters as well as comparison with models of intrinsic SQUID noise. The best e obtained was of order Planck's constant h. Moreover, the detailed characteristics of the SQUID voltage noise are in agreement with a small signal analysis that depends only on the current noise in each of the Josephson elements and measurable parameters.

Amplification of electrostatic noise in cyclotron resonance with an adiabatic auroral beam

Abstract: The adiabatic evolution of the auroral beam in the convergent geomagnetic field gives rise to a region of ∂F/∂ν⊥ >0 in the electron distribution function. The altitude dependence of this source of free energy is examined, and its ability to drive electrostatic waves toward instability is considered. General linear properties of the possible instabilities are discussed, and a detailed numerical calculation based on a collisionless height dependent model of an auroral arc (Maggs and Lotko, this issue) is described. Of the three wave modes considered, the upper hybrid mode is optimally amplified at altitudes below about 1000 km. Oblique electron Bernstein modes may be weakly unstable at the lowest altitudes, and the whistler mode appears to be stable at all altitudes. Wave amplification is limited by a finite resonance time which is determined by refraction of parallel wavenumbers by the topside ionospheric density gradient. Slow beams originating at relatively high altitudes and nearly perpendicularly propagating waves with frequencies away from cyclotron harmonics are favored by this instability mechanism.

Low-frequency noise characteristics of channel substrate planar GaAlAs laser diodes

Abstract: The influence of external feedback on low‐frequency noise generation (100 Hz–10 kHz) in single longitudinal and transverse mode channel substrate planar laser diodes is reported. An increase in noise by as much as 60 dB was induced by external feedback as small as 0.04%. The induced intensity variations were observed to be sensitive to both phase and amplitude of the externally reflected light.

External noise effects on the fluctuation line width

Abstract: We calculate stationary state correlation functions of the anharmonic overdamped oscillator driven by multiplicative (white Gaussian) noise of strengthQ together with additive noise of relative strengthq. (i) We donot observe a particular slowing down at the so-called noise induced transition. But there is a region of the oscillator's stiffness parametera neara≈0 in which the decay time is typically enhanced. (ii) If the phase transition point is defined by the minimum of the decay rate, it lies within the ordered phasea>0 forq⋟1 and shifts down toa=0 forq→0. In our approximation it is even in the disordered regiona<0 for very smallq. (iii) As a function of the multiplicative noiseQ the decay ratedecreases with increasingQ if the system is well above or well below threshold. There seem to be experimental indications of this behavior. But within the proper threshold regime of smalla increasing noiseQ increases the decay rate. The valuesa c where the cross-over occurs depend on the fluctuating variable and onq.

Observation of narrow-band charge-density-wave noise in Ta S 3

Abstract: We report the observation of narrow-band noise in the linear-chain compound Ta${\mathrm{S}}_{3}$. The noise frequency is proportional to the charge-density-wave (CDW) current, providing direct evidence for CDW motion in Ta${\mathrm{S}}_{3}$.

Conduction mechanisms and 1/f noise in thick‐film resistors with Pb3Rh7O15and Pb 2Ru2O7

Abstract: Thick‐film resistors with Pb3Rh7015 and Pb2Ru20 7 as conductors are considered. These compounds are chemically stable during the firing process of the resistors. The variation of sheet resistivity R ⧠ with volume fraction conductor follows a simple power law. This result cannot be explained by percolation theory in its simple form. The conductivity is found to be independent of frequency up to 10 GHz. In both cases a small positive Hall coefficient is observed which is interpreted as being due to mixed electron and hole conduction. Finally the 1/ f noise is considered, and is found to vary approximately linearly with R⧠. A theoretical model for the 1/f noise in thick‐film resistors is presented that accounts for this linear relationship. The observed deviations from linearity indicate that probably tunneling through barriers at the interfaces between the conducting grains is an important factor in determining the conduction.

An Improved Prediction Method for Noise Generated by Conventional Profile Coaxial Jets

Abstract: A semiempirical model for predicting the noise generated by conventional velocity profile jets exhausting from coaxial nozzles is presented and compared with small scale static and simulated flight data. Improvements to the basic circular jet noise prediction are developed which improve the accuracy, especially at high jet velocity and near the jet axis.

Device for the activation of an apparatus for measuring acoustic emission by detection of background noise

Abstract: A DEVICE FOR THE ACTIVATION OF ANAPPARATUS FOR MEASURING ACOUSTIC EMISSION BY DETECTION OF BACKGROUND NOISE Abstract of the Disclosure An apparatus for measuring acoustic emission is activated only at critical periods which are determined by detection of the background noise from a reference level. An acoustic emission transducer continuously transmits a signal to a detection circuit which delivers the top modulation envelope of the background noise to a comparator at a reference level delivered by a control unit. When the mean level of the background noise exceeds the reference level, the comparator delivers a signal which activates the input/output assembly of the measuring apparatus.

Experiments on the nonlinear characteristics of noise propagation from low and moderate Reynolds number supersonic jets

Abstract: A series of experiments were conducted on high-speed model jets to identify and quantify the distortion of radiated noise by nonlinear propagation effects. The jets were operated in a low to moderate Reynolds number range, and the radiated noise was either broadband or discrete in frequency. Three condenser microphones were used to measure the waveforms propagated by the axisymmetric, cold model jets of Mach numbers 2.1 and 2.5. Relatively low Reynolds numbers were obtained by exhausting the jets into a low pressure anechoic test chamber. Nonlinear propagation distortion effects, such as wave steepening, harmonic generation and wave merging, were easily quantified. At a moderate Reynolds number, low frequency production and wave amalgamation, measured by a decrease in the zero crossings per unit time, were quantified in the propagation of sound away from the jet.

Wing Effect on Jet Noise Propagation

Abstract: Jet aircraft with engines under the wings may produce higher flyover noise levels than similar aircraft with other engine mounting arrangements. To determine the cause of higher flyover noise of such aircraft, an experimental investigation was performed in an anechoic chamber. Basic experimental apparatus consisted of an ASME 15.24 cm (6 in.) diameter converging nozzle and a wing section which corresponded to the horizontal projection area of a portion of a wing of a typical jetliner. Results of this experiment indicate that the presence of the wing in the vicinity of the jet enhances the noise produced by the jet alone. This noise enhancement may be attributed to two sources. The boundary layer generated on the surface of the wing as the result of entrainment of the air into the region between the wing and the jet is believed to be responsible for the low-frequency noise enhancement. Reflection of jet noise incident on the wing surface contributes to enhancement of noise primarily at high frequency. The jet is found to have considerable effect on noise enhancement at high frequency where strong refraction effects on sound waves occur. The substantial enhancement of high-frequenc y noise measured in planes at oblique angles to the wing surface may require consideration in aircraft noise prediction and design. Based on static test results, it appears that the wing effect may increase the sideline noise levels of aircraft during takeoff.

Noise analysis of the K+ current through the apical membrane of Necturus gallbladder.

Abstract: Current noise power spectra of the voltage-clamped(V=0) Necturus gallbladder, exposed to NaCl-Ringer's on both sides contained a relaxation noise component, which overlapped with a 1/f α noise component, with α being about 2. Substitution of all Na+ by K+ on either the serosal or mucosal side increased the relaxation as well as the 1/f α noise component considerably. InNecturus gallbladder both noise components are reduced by addition of 10mm, 2,4,6-triaminopyrimidine (TAP) or 5mm Ba2+ to the mucosal side, as well as by acidification of the mucosal solution to pH5 and lower. Fivemm of tetraethylammonium (TEA+) added to the mucosal solution, abolished K+ relaxation noise and decreased the 1/f α noise component. Applying a Cs+ concentration gradient across the epithelium did not yield relaxation noise. However, if Rb+ was substituted for all Na+ on one side, a Lorentzian noise component appeared in the spectrum. Its plateau was smaller than with KCl-Ringer's on the respective side. These data confirm the existence of fluctuating K+ channels in the apical membrane of theNecturus gallbladder. Furthermore it can be concluded that these channels have the permeability sequence K+>Rb+>Cs+. The inhibition of the fluctuation by mucosal acidification indicates the existence of acidic sites in the channel. The singlechannel conductance was estimated to be between 6.5 and 40 pS.