Showing papers on "Noise (radio) published in 1976"

Plasma waves in the distant magnetotail

Abstract: The results of an extensive study of plasma waves in the distant magnetotail on the basis of measurements from the Imp 8 spacecraft are discussed. The plasma measurements are compared with plasma and magnetic field measurements described by Frank et al. (1976) to study the relationship of the plasma waves to the various plasma regimes found in the distant magnetotail. Three distinctly different types of plasma wave turbulence in the distant magnetotail are detected. The first, most frequently occurring type of turbulence, consists of broadband electrostatic noise at frequencies between 10 Hz and a few kHz. The second, less frequent type of plasma wave turbulence consists of intense (100 milligamma) bursts of low frequency (10 to 300 Hz) magnetic noise. The third, least frequent type of turbulence consists of electrostatic waves near harmonics of the electron gyrofrequency.

Plasma wave interactions with energetic ions near the magnetic equator

Abstract: An intense band of electromagnetic noise is frequently observed near the magnetic equatorial plane at radial distance from about 2 to 5 Re. Recent wideband wave-form measurements with the IMP-6 and Hawkeye-1 satellites have shown that the equatorial noise consists of a complex superposition of many harmonically spaced lines. Several distinctly different frequency spacings are often evident in the same spectrum. The frequency spacing typically ranges from a few Hz to a few tens of Hz. It is suggested that these waves are interacting with energetic protons, alpha particles, and other heavy ions trapped near the magnetic equator. The possible role these waves play in controlling the distribution of the energetic ions is considered.

Experiments on Supersonic Jet Noise

Abstract: The noise generated by a 1-in. supersonic jet was investigated at nominal jet Mach numbers of 1.5, 2.0, and 2.5. In particular, a quantity W, referred to as the apparent source strength per unit length, was determined along the jet axis using a directional microphone system. The integrated value of W along the jet axis was found to agree with the sound intensity obtained by a conventional microphone. This result is consistent with the a priori assumption that the jet may be described in terms of independent, spatially compact acoustic sources. The main finding of the investigation is the discovery of two distinct intense noise-producing regions in a jet having supersonic source velocities: the upstream region associated with Mach wave radiation, and a zone, located downstream of the potential cone, exhibiting radiation similar in character to that of a subsonic jet. An estimate of the radiated intensity associated with the Mach waves also is made.

Noise response of cavities of varying dimensions at subsonic speeds

Abstract: An expression for the Strouhal number of lengthwise cavity oscillations is obtained which includes the effect of length-to-depth ratio. This expression, which agrees well with the experimental data, is also used to predict the Mach number at which cavity acoustic response is maximum. Interaction between lengthwise and depthwise modes is seen to occur at Mach numbers from 0.1 to 0.5. Cavity shape is shown to affect the noise spectra in generating either a broadband or narrowband signal.

Ambient sea noise in waters near Australia

Abstract: Ambient sea noise has been measured at 40 positions in mainly tropical waters near Australia: in the Indian Ocean; the Arafura and Timor Seas; and in the Coral and Tasman Seas (Pacific Ocean). Wind‐speed dependence was observed at all frequencies of measurement from 22 to 5000 Hz. At hydrophones suspended at depths between 8 and 25 m, the wind‐ dependent noise appeared to be independent of bottom depth which varied from 26 to 6700 m. The rate at which wind‐dependent noise increased with increasing wind speed was found to be significantly less than has been observed in the North Atlantic Ocean. Non‐wind‐dependent noise, observed at frequencies below 250 Hz, varied by about 10 dB within a particular sea and by up to 20 dB between seas, and could be broadly related to the relative shipping densities and propagation conditions in the individual seas. In the shallow Arafura and Timor Seas north of Australia, there is evidence that noise of distant shipping is not a significant component of the ambient noise. A...

Fan with noise reduction

Abstract: A fan for moving a gaseous fluid, e.g., air, is described in which the high audible frequency noise resulting from the phenomenon occurring at the trailing edges of the blades, known as vortex shedding, is reduced. This is accomplished by notching an edge of each of the blades so that the pattern of vortices leaving the blade, which causes the noise, is disturbed and a turbulence condition engendered. The turbulence distributes the pressure fluctuations resulting from movement of the blades through the fluid over a relatively broad band of frequencies and reduces the annoying noise frequencies. Various notch configurations are disclosed.

The Generation and Radiation of Supersonic Jet Noise. Volume 3. Turbulent Mixing Noise Data

Abstract: : The characteristics, both spectral and directivity, of the sound field of supersonic shock-free jets are studied by measuring the turbulent mixing noise in the far field from four two-inch diameter nozzles, namely a convergent nozzle for pressure ratios up to critical and three convergent- divergent nozzles having nominal design Mach numbers of 1.4, 1.7, and 2.0, respectively. The measurements were conducted in a free-field environment. The results from 75 jet exit conditions are presented in a systematic manner in the form of tabulated one-third octave spectra. The test program was carefully planned in order to obtain results (i) at constant jet efflux temperature with varying jet velocity, and conversely (ii) at constant exit velocity while varying jet efflux temperature. The effects of jet velocity and exhaust temperature on turbulent mixing noise can therefore be assessed independently.

On the elementary conductance event produced by L-glutamate and quanta of the natural transmitter at the neuromuscular junctions of Maia squinado.

Abstract: 1. The membrane potential of giant muscle fibres of Maia squinado was measured with an intracellular wire electrode. On applying L-glutamate to the fibre the cell deplorized and fluctuations of the membrane potential around its mean level--glutamate noise--were seen. 2. The variance of the glutamate voltage noise is proportional to the mean level of depolarization. The noise can be regarded as being caused by numerous exponentially decaying elementary voltage events about 5 X 10(-10) V in amplitude. The miniature excitatory junctional potential (min.e.j.p.) is approximately 6000 times the amplitude of the elementary voltage event produced by L-glutamate. 3. The power spectrum of glutamate voltage noise is a Lorentzian with a half-power frequency of approximately 20 Hz. 4. Min. e.j.p.s. decay exponentially with a time constant that coincides with the average lifetime of the elementary glutamate voltage event. 5. When glutamate is applied locally to a spot where extracellular min. e.j.p.s. can be recorded with a focal glass pipette, extracellular glutamate noise is seen. Glutamate noise could not be detected from elsewhere on the fibre. 6. The variance of the extracellular noise is proportional to the mean extracellular potential, and its power spectrum is a Lorentzian with a half-power frequency of about 110 Hz. 7. The extracellular min. e.j.p.s decay exponentially with a time constant that coincides with average lifetime of the elementary glutamate current event. 8. It is suggested that the decay of the quantal currents flowing at the excitatory junction is limited by the closure of the conductance channels in the post-synaptic membrane and not by the relaxation of the transmitter concentration in the synaptic cleft.

A very-long-baseline interferometer system for geodetic applications

Abstract: A very-long-baseline interferometer system was designed and built for geodetic applications. Each interferometer terminal records a 360-kHz spectral band of noise from a compact extragalactic radio source. The center frequency of the spectral band can be selected to sample sequentially bands covering a much wider frequency range to obtain subnanosecond accuracy in group-delay measurements. A tunnel-diode pulse generator is used to calibrate the delays in the receiver. The necessary sets of algorithms and computer programs have been developed to analyze the data and have allowed the system to be employed to make accurate determinations of vector baselines, radio-source positions, polar motion, and universal time.

Vortex noise from nonrotating cylinders and airfoils

Abstract: An experimental study of vortex-shedding noise was conducted in an acoustic research tunnel over a Reynolds-number range applicable to full-scale helicopter tail-rotor blades. Two-dimensional tapered-chord nonrotating models were tested to simulate the effect of spanwise frequency variation on the vortex-shedding mechanism. Both a tapered circular cylinder and tapered airfoils were investigated. The results were compared with data for constant-diameter cylinder and constant-chord airfoil models also tested during this study. Far-field noise, surface pressure fluctuations, and spanwise correlation lengths were measured for each configuration. Vortex-shedding noise for tapered cylinders and airfoils was found to contain many narrowband-random peaks which occurred within a range of frequencies corresponding to a predictable Strouhal number referenced to the maximum and minimum chord. The noise was observed to depend on surface roughness and Reynolds number.

Investigation of the transition from tunneling to impact ionization multiplication in silicon p-n junctions☆

Abstract: White noise spectra of diodes breaking down between 1·5 and 5 V have been used to investigate the details of the transition from tunneling to avalanche breakdown in silicon p-n junctions. It is found that the transition and carrier multiplication in these junctions is dominated by the influence of the threshold energies for ionization. Because this influence is not explicitly taken into account in the existing theories of carrier multiplication and noise, they are not applicable to low breakdown voltage diodes. Consequently, a multiplication onset model and alternate schemes for calculating the DC multiplication and noise in low breakdown voltage diodes are developed. Analysis of the noise data indicates that the threshold energies for ionization depend slightly on junction widths and, for the diodes employed in this study, range between 1·66–1·9 eV for electrons and 1·79–2·04 eV for holes. The minimum distance between ionizing collisions is found to range from 190 to 240 A for electrons and 200 to 250 A for holes. Application of the threshold energies for ionization to the multiplication onset model permits evaluation of the doping densities on both sides of the step junctions. From it, it is determined that the solubility of aluminum in silicon is NA = 9·5 ± 0·5 × 1018 cm−3.

Subharmonic pitches of a pure tone at low S/N ratio

Abstract: 2AFC scores were obtained on 50 untrained subjects who had to respond to a 3% increment or decrement of the fundamental frequency of two successive signals. The first signal in each trial was a reference, containing six harmonics of 200 Hz. Main variables were the number of harmonics of the second signal in a trial (either 3, 2, or 1) and the presence or absence of continuous background noise. Low scores were observed in the absence of the noise (typically 65% for the 3‐ and 2‐component signals, and virtually 50% for the 1‐component signals), whereas in the presence of the noise much higher scores were observed (typically 90% and 80%, respectively). The results indicate that background noise has great impact on the perceptibility of a ’’low pitch’’ related to the fundamental frequency, even more so than the number of components presented (within the range of three, two, or one components): at a low S/N ratio, even a single harmonic may give rise to the perception of a subharmonic low pitch. Subsequent exp...

Noise in buried channel charge-coupled devices

Abstract: In this paper we discuss the noise measured at the output of a buried channel charge-coupled device (BCCD) linear shift register. The measured noise arises from four sources; the electrical insertion of signal charge, the output amplifier, dark current, and bulk state trapping. In making these measurements the concept of correlated double sampling was used in an output circuit which had a noise level which was equivalent to less than 3O noise electrons. A critical component in this output was a low noise MOSFET which was achieved by use of the buried channel technology. A low noise input structure for electrical insertion of signal charge was used which introduced a signal which had a noise level which ranged from less than 10 e-to as high as 60 e-depending on the size of the signal charge. The dark current noise was found to be well characterized as a shot noise and levels on the order of 20 e-were measured. The above low noise levels made possible direct measurement of the noise due to bulk state trapping, and depending on the signal size and clock rate noise levels were measured which ranged from less than 10 to over 100 noise electrons. One of the most important bulk traps was found to be due to gold impurities which had a density of ∼ 2 × 1011cm-3.

Systems and methods for determining radio frequency interference

Abstract: The presence, frequency and amplitude of radio frequency interference superimposed on communication links originating from a terrestrial region and including a relay in a geostationary spacecraft are determined by pointing a narrow beam antenna on the satellite at the terrestrial region. The level of noise radiated from the region, in a predetermined frequency band of interest, to the antenna is measured at a terrestrial station that is usually remote from the region. Many of the measurements are performed in each of a plurality of frequency bands across the spectrum. Calibrating radio signals having a plurality of predetermined EIRP's (Effective Isotropic Radiated Power) and frequencies in the spectrum are transmitted from the region through the spacecraft narrow beam antenna back to the station. At the station, the levels of the received calibrating signals are separately measured for each of the frequency bands and EIRP's. In response to comparisons of the measured amplitudes of like frequencies of the calibrating signals and noise, the presence, frequency and amplitude of sources of radio frequency interference originating from the region are determined.

Statistical-Physical Models of Man-made and Natural Radio Noise Part II: First Order Probability Models of the Envelope and Phase

Abstract: Most man-made and natural electromagnetic interferences are highly non-gaussian random processes, whose degrading effects on system performance can be severe, particularly on most conventional systems, which are designed for optimal or near optimal performance against normal noise. In addition, the nature, origins, measurement and prediction of the general EM interference environment are a major concern of an adequate spectral management program. Accordingly, this second study in a continuing series [cf. Middleton, 1974] is devoted to the development of analytically tractable, experimentally verifiable, statistical-physical models of such electromagnetic interference Here, classification into three major types of noise is made: Class A (narrowband vis-a-vis the receiver), Class B (broadband vis-a-vis the receiver), and Class C (=Class A+Class B). First-order statistical models are constructed for the Class A and Class B cases. In particular, the APD (a posteriori probability distribution) or exceedance probability, PD, viz. P1 (e>eo)A,B, and the associated probability densities, pdf’s, w1(e)A,B, of the envelope are obtained; [the phase is shown to be uniformly distributed in (0,2p)]. These results are canonical, i.e., their analytic forms are invariant of the particular noise source and its quantifying parameter values, levels, etc. Class A interference is described by a 3-parameter model, Class B noise by a 6-parameter model. All parameters are deducible from measurement, and like the APD’s and pdf’s, are also canonical in form: their structure is based on the general physics underlying the propagation and reception processes involved, and they, too, are invariant with respect to form and occurrence of particular interference sources. Excellent agreement between theory and experiment is demonstrated, for many types of EM noise, man-made and natural, as shown by a broad spectrum of examples. Results for the moments of these distributions are included, and more precise analytical conditions for distinguishing between Class A, B, and C interference are also given. Methods for estimating the canonical model parameters from experimental data (essentially embodied in the APD) are outlined in some detail, and a program of possible next steps in developing the theory of these highly nongaussian random processes for application to general problems of spectrum management is presented.

Generation-recombination noise in the channel of GaAs Schottky-gate field-effect transistors

Abstract: The low-frequency noise in GaAs m.e.s. f.e.t.s was measured continuously for temperatures varying from 80 to 310K. It presented distinct maxima, suggesting the presence of several g.r. processes. The shift of these peaks at different frequencies (500 Hz to 1 MHz) allowed us to obtain the time constants and the activation energies of four trapping levels. The noise reported here could stem from multilevel trapping in the channel.

Power line carrier noise elimination

Abstract: A method and apparatus are provided for eliminating noise signals from power transmission lines utilized for transmitting information signals in power line carrier systems Use is made of the principle that signals transmitted line-to-ground are more severely attenuated than signals transmitted line-to-line Capacitive impedance is selectively added to the system to effect coupling of the noise signals to the transmission lines in a line-to-ground mode

Effects of forward velocity on turbulent jet mixing noise

Abstract: Flight simulation experiments were conducted in an anechoic free jet facility over a broad range of model and free jet velocities. The resulting scaling laws were in close agreement with scaling laws derived from theoretical and semiempirical considerations. Additionally, measurements of the flow structure of jets were made in a wind tunnel by using a laser velocimeter. These tests were conducted to describe the effects of velocity ratio and jet exit Mach number on the development of a jet in a coflowing stream. These turbulence measurements and a simplified Lighthill radiation model were used in predicting the variation in radiated noise at 90 deg to the jet axis with velocity ratio. Finally, the influence of forward motion on flow-acoustic interactions was examined through a reinterpretation of the 'static' numerical solutions to the Lilley equation.

1/f noise measurements for characterizing multispot low-Ohmic contacts

Abstract: The patchiness of contacts can be determined from measurements of 1/f noise and contact resistance on a number of contacts of different diameters made by the same method. If a contact consists of k conducting spots with average radius a, k and a can be determined if the distance between the spot centers is at least 2.5 times the spot diameter. Using the developed multispot contact model, the results of the 1/f noise analysis are in good agreement with multispot parameters obtained from microscopic investigations.

High-Voltage Damage and Low-Frequency Noise in Thick-Film Resistors

Abstract: The results from a unified study of high-voltage damage and low-frequency noise in thick-film resistors are presented. Material microstructure and the mechanisms for charge transport and I/f noise are discussed to provide a basis for the interpretation of the experimental results. Experimental results showing a definite relationship between high-voltage damage and low-frequency noise are presented and interpreted. It is shown that noise parameters are extremely sensitive to the condition of interfacial regions within the conductive phase of the thick-film material. Because of this sensitivity to conductive-phase microstructure, noise measurements are recommended as a diagnostic tool for research on thick-film materials and as a diagnostic tool for quality-assurance efforts in the production environment.

An improved low‐pressure dc discharge

Abstract: We describe a scheme in which the primary beam electrons are electrostatically confined in a low‐pressure dc discharge plasma which can substantially improve the density (300 or 400% in our case). The electrons are heated to 2 eV with a fraction thermalized to ∼10 eV by the extended presence of the primary beam electrons. Ion noise observed was not changed substantially from that without confinement. (It was slightly better than unconfined.) The plasma noise is strongly dependent on filament geometry. We also describe a second electrostatic confinement scheme in which both the primary electrons and the plasma are confined, resulting in a further density increase.

Low-frequency intensity fluctuations of c.w. d.h. GaAlAs-diode lasers

Abstract: The optical-emission fluctuations and the current noise of cw dh GaAlAs junction lasers have been investigated for the first time in the low-frequency range In both the optical noise spectra and the current noise spectra, a l/f behaviour has been observed It was found that a strong crosscorrelation exists between optical-emission noise and forward-current noise Finally, an optoelectronic feedback method that practically eliminated the relative excess light noise has been demonstrated