Showing papers on "Noise (radio) published in 1974"

Noise characteristics of gallium arsenide field-effect transistors

Abstract: Small signal and noise characteristics for GaAs field-effect transistors are derived with the saturated drift velocity of the carriers underneath the gate taken into account. The noise contributed by the saturated carriers is nonnegligible and in most cases, exceeds the noise generated by the unsaturated region. Parasitic elements contribute importantly by preventing the full cancellation of the correlated noise of the intrinsic transistor and by adding their own Johnson noise. The theory predicts the experimentally observed trend of noise figure dependence on drain current and on source-to-drain voltage. The present theory doesnot take into account the effects of a possible short negative resistance region underneath the gate.

A multi-frequency resonant tag circuit for use with an electronic security system having improved noise discrimination

Abstract: An electronic security system adapted for use in a controlled area such as a retail store and employing a multi-frequency resonant tag circuit having distinct frequencies for detection and discrimination. An electromagnetic field is provided within the controlled area at a frequency which is swept through a range including the detection frequency of the resonant tag circuit. In the presence of a tag circuit within the controlled area, pulses are detected by a receiver which includes noise rejection circuitry for discriminating true signals from noise. The noise rejection circuitry is sensitive to expected signals and operative to discriminate true signals from noise on the basis of several different criteria, resulting in substantially reduced false alarm actuation.

Plasma wave turbulence at distant crossings of the plasma sheet boundaries and the neutral sheet

Abstract: The orbit of IMP 7 frequently takes the spacecraft across the neutral sheet in the geomagnetic tail at a downstream distance of about 35 earth radii. This preliminary report discusses the detection of strong low-frequency electromagnetic signals and moderate electrostatic noise enhancements near local midnight. Data used were obtained during a substorm period on Oct. 28, 1972, when there were multiple crossings of the neutral sheet and the boundary of the plasma sheet.

Spectrum of rotor noise caused by atmospheric turbulence

Abstract: The spectrum of blade leading and the associated noise caused by interference of a propeller, helicopter rotor, or fan rotor with inlet turbulence are studied experimentally and theoretically. One test with hot‐wire anemometers in a static inlet and another test with pressure transducers on the blades of a fan rotor reveal inlet turbulence to be highly anisotropic. The intensity of the transverse velocity component was found to be 2.5% of the mean flow and the streamwise component was 0.9%, with ambient winds of about 2 mph. The transverse integral scale of the turbulence is a fraction of an inlet diameter, while the streamwise integral scale is over 100 diameters. Evidence indicates the source of these disturbances is atmospheric turbulence. The associated noise is partially coherent with spectrum peaks which are so narrow as to be difficult to distinguish from true harmonics. A novel blade loading model is developed using concepts from random pulse modulation theory. Theoretical spectrum predictions ind...

Threshold excitations, relaxation oscillations, and effect of noise in an enzyme reaction

Abstract: We study a deprotonation reaction by an enzyme with activity dependent on pH. The rate and transport equations are simplified with a number of assumptions, are analyzed according to the presence of different time scales, and are solved numerically to show relaxation oscillation and threshold excitation, for different choices of parameters. The imposition of fluctuations (noise) on the deterministic equations for threshold excitation conditions leads to random occurrence of an excitation and return to steady state at low noise level and to large, random variations in concentrations at high noise level. At intermediate noise levels (of the order of the threshold excitation), however, we find quasi-periodic concentration oscillations. Thus, critical values of external constraints necessary for oscillations are altered by the presence of noise.

Spatial variability of underwater ambient noise at the Arctic ice‐water boundary

Abstract: Quasisynoptic measurements were made to determine the spatial variability of ambient noise at the Arctic ice‐water boundary over a frequency range from 100 to 1000 Hz. The results of this investigation show that the ice‐water boundary acts as a spatially well‐defined source of ambient noise with measured noise levels near a compact edge about 12 dB higher than open water levels and about 20 dB higher than levels far in the ice field. Measured noise levels near a diffuse ice edge were about 4 dB higher than open water levels, and about 10 dB higher than levels far in the ice field. The relatively high noise levels at the ice edge are probably generated by wave and swell interactions with individual ice floes.

On the distribution of source coherency in subsonic jets

Abstract: Recent experimental and theoretical studies on the sources of far field noise generated by subsonic circular jets have given convincing indications that a coherent wave motion does exist in the potential core of subsonic jets. Cross-covariance measurements using single and cross hot-wire probes in the potential core as well as the turbulent mixing region and microphone measurements in the far field have resulted in cross-correlation coefficients of sufficiently high magnitudes to indicate that significant, if not dominant, noise sources may be located in the potential core. The paper presents the analytical and experimental results of this research.

9. The Instrumentation and Techniques of Infrared Photometry

Abstract: Publisher Summary This chapter examines the instrumentation and techniques of infrared photometry. It presents the design of telescopes and modulators used in infrared. Incoherent detectors, such as bolometers and photoconductors, respond only to the incident power level or number of incident quanta, but they provide the highest possible signal/noise when observing continuous sources. Excess noise across the junction can be eliminated by reducing the photovoltaic potential to nearly zero by drawing off the charge through a low-input impedance current-measuring amplifier. One of the great advantages of the interference filters is the fact that they can usually be repeatedly cooled to low temperatures without degradation. Interference filters have been constructed in the far infrared using metal mesh grids in place of semitransparent metal films. For conventional telescopes used in the infrared, the field stops usually also serves as the defining stop in order to minimize the background from the oversized secondary and sky baffle. It is observed that when the telescope is in good collimation, extremely small stable offsets can be achieved and modulation noise remains below true sky noise for beam separations less than 30 arcs.

Importance of Amplitude Scintillations in Millimetric Radio Links

Abstract: Amplitude scintillations in millimetric radio propagation are not usually considered by systems engineers In this paper, the experimental measurements of these scintillations in a 36 GHz radio link have been fitted into a model suitable for the millimetric range. From it, the amplitude scintillations can be regarded as a noise component superimposed on the mean value, with some expected mean rate fading fo. The r.m.s. value of this noise is then used to determine the outages for the scintillation-type fading, as a function of the frequency and link distance. The implications of the results in the power budget are discussed and reference is made to the limiting performance of digital transmission.

1/f noise of point contacts affected by uniform films

Abstract: An experimental investigation has been carried out to confirm that contact noise of two crossed semiconductor bars with native oxide films in between can be understood as a volume 1/f noise. Noise and contact resistance have been measured as functions of the force F on the crossed bars. F varies between 6 N and 6×10−5 N. From a simple model, an equation is derived which relates the 1/f noise intensity C to the contact resistance R. The calculations are in agreement with the experimental C‐R plots. Two extreme situations are possible: constriction dominated and film dominated. Which situation actually occurs can be seen in the C‐R plot as well as in the R‐F plot.

Signal Detection in the Presence of Atmospheric Noise in Tropics

Abstract: A new model of HF atmospheric radio noise (ARN) that leads to observed probability distributions of noise is briefly described. The new ARN model is then applied to the detection of known signals in the presence of noise to determine optimal receiver structure for ARN. Probabilities of errors for a linear matched filter and nonlinear (-1/log) and inverted (log)^{2} correlator receivers and a phase coherent binary detector are determined and compared. It is shown that the inverted (log)^{2} correlator combined with the linear matched filter is an optimal receiver in the presence of impulsive noise in tropics. The time statistics of burst noise is applied to design of signals for improved reliability.

Intensity discrimination of noise in the presence of band‐reject noise

Abstract: Weber's law holds over at least an 80‐dB range for intensity discrimination of 200‐msec bursts of noise. Weber's law holds over a comparable range when information regarding intensive differences is effectively restricted to a limited frequency region by the addition of a relatively intense band‐reject noise. In particular, no failure of Weber's law is observed at intensities for which the discharge rates of fibers innervating the frequency region of the stopband are presumably saturated. These results do not support the generally accepted notion that a spread of excitation along the cochlear partition with increasing intensity is necessary for the auditory system to maintain its large dynamic range.

Electric field recording on the sea floor with short span instruments.

Abstract: Oceanic electric fields originate from (1) induction by magnetic pulsations in the ionosphere, (2) water motions across the earth's field and (3) electrochemical processes associated with sea floor materials and with tissues of marine organisms. Precise recording of these signals is difficult because of (1) their small size, and (2) a large and unavoidable noise occurs at the contact points between sea water and measuring devices.The spectrum P of average natural electric activity at the sea floor in the band 10-3 to 10+1 cph roughly follows the trend P=kf-1, f=frequency, k centered at 10-2 μV2 m-2. The noise spectrum of typical Ag-AgCl electrode pairs P′ on the sea floor environment approximates k′= 10-1 μV2 with tenfold variations in both directions. Thus, achieving a 101 signal to noise ratio requires an electrode separation of 10 meters. Even so, the unavoidable electrode voltage mismatch, typically 102 to 103 μV, fatally eradicates the signal datum unless considerably longer lines area used.Rejection of electrode bias and noise to allow short electrode separations can be done by physically inverting the electrodes positions, which is most practically achieved with rotating instruments, or (2) by switching back and forth the sea water connections between electrodes and the salt bridge pipes which perform electrical contact with the ocean. A method to achieve this “water chopping” is described.Examples of ionospheric and barotropic velocity signals recorded on the sea floor with 5m span instruments and presented.

Man-Made Radio Noise – Part 2: Estimates for Business, Residential, and Rural Areas

Abstract: The Office of Telecommunications, Institute for Telecommunication Sciences (OT/ITS), over the past several years, has accumulated a data base of man-made radio noise measurements in the frequency range from 250 kHz through 250 MHz taken in a number of geographical areas. This data base has been analyzed to provide estimates of the expected characteristics of man-made radio noise in business, residential, and rural areas. The parameters used are the average available power spectral density, the ratio of the rms to the average voltage of the noise envelope, and the ratio of the rms to the average logarithm of the envelope voltage. The variation of these parameters as a function of frequency, location, and time are shown and discussed. Examples of amplitude and time statistics of the received man-mad e radio noise process also are shown and discussed. The use of the estimates is shown (principally by references in Part II, Bibliography) in the solution of problems encountered in frequency management and telecommunication system design.

The processes of formation of ultrashort laser pulses

Abstract: This paper describes and reviews experimental results on both the processes involved in the establishment of ultrashort light pulses during their build-up from noise and in their subsequent growth and narrowing. Schemes for the improvement of the USP train are suggested.

Time-amplitude correlation effect between large Barkhausen discontinuities (in the magnetization noise)

Abstract: A theory of the power spectrum of the Barkhausen noise is presented, accounting for the frequency behaviour of the experimental spectra in the whole range of analysis frequency between 1 Hz and 20kHz, when the magnetizing frequency is kept relatively low (below 0.1 Hz). The theory assumes the existence of a correlation between the number of clustered pulses in a large Barkhausen discontinuity and the time interval from such a discontinuity to the preceding one: the longer this time interval, the larger the number of clustered pulses. An experimental check of the dependence of the noise power spectrum on the magnetizing frequency in rings of Fe−Si 4% is given. It is shown that the theoretical results fit the experimental data without the introduction of any arbitrary constant.

Investigation of the aerodynamic noise generating region of a jet engine by means of the simple source fluid dilatation model

Abstract: An experiment was conducted on a full‐scale jet engine to investigate the aerodynamic noise generating regions in the free jet. Cross‐correlation measurements were made between the static pressure fluctuations and the farfield radiated sound. These measurements were made for two different static pressure probe positions and a large number of farfield positions (at various angles). In addition, each test geometry was run for four different jet exit velocities. The measured, normalized cross‐correlation functions varied between 0.004 and 0.155. A new Q‐function, based on the above normalized cross correlation is defined and plotted. This function represents the source strength per unit volume within the jet region. This Q‐function shows dependence on the probe position, the angular position of the farfield microphone, and the jet exit Mach number. Third‐octave analyses of both the probe signal and the farfield radiated sound were made. The results show that cross‐correlation techniques are a valuable tool in the investigation of the aerodynamic noise generating regions of an actual jet engine.

Intrinsic fluctuations in the output intensity of double‐heterostructure junction lasers operating continuously at 300°K

Abstract: Measurements of the intrinsic noise fluctuations in the light intensity from a stripe‐geometry double‐heterostructure junction laser operating continuously at room temperature are reported. The frequency spectrum of the fluctuations has been measured from 10 MHz to 4 GHz for currents both above and below threshold. The above‐threshold noise spectra show the resonant peak due to excitation of the spiking resonance by the quantum shot noise intrinsic to the lasing process. Near threshold the frequency of this resonance varies between 200 and 1000 MHz as a function of laser current. In addition, as the pumping current is increased above threshold, the low‐frequency ( 2 GHz) noise continues to increase in apparent contradiction with theoretical predictions.

The Apple Grove 750 kV Project Statistical Analysis of Audible Noise Performance of Conductors at 775 kv

Abstract: This paper presents the results of a statistical anaylsis of conductor corona generated audible hoise as measured at the Apple Grove 750 kV Test Project, a joint field investigation sponsored by the American Electric Power Service Corporation and the Westinghouse Electric Corporation. A comparison of simultaneous audible noise data is made on three sizes of four conductor bundles [diameter of 1.382 in. (35.1 mm), 1.196 in. (30.4 mm), 1.0 in. (25.4 mm)]. Also a correlation is given of simultaneous readings of audible noise, radio noise and corona loss.

Intensity, spectrum, and directivity of turbulent boundary layer noise

Abstract: The problem of noise radiated from a turbulent boundary layer is studied. It is found that the farfield noise intensity, spectrum, and directivity can be determined completely if the nearfield pressure cross‐correlation function is known. The role of supersonic and subsonic pressure components associated with the turbulent boundary layer in relation to noise radiation is examined. By using an empirical model of the wall pressure cross‐correlation function it is found that the directivity pattern of turbulent boundary‐layer noise differs considerably from that of a free dipole. One principal reason for this difference is the fact that the noise sources of a turbulent boundary layer are constantly in motion. The effect of mean flow outside the boundary layer on the radiated noise intensity and spectrum is also investigated. Numerical results indicate that this effect is very important for low‐frequency noise components and for high subsonic flow Mach number.

Low-Noise Diodes and Mixers for the 1--2-mm Wavelength Region

Abstract: Ultralow capacitance low-noise Schottky-barrier diodes have been developed for use in the 1--2-mm-wavelength region. The diodes have been fabricated using both photolithographic and electron-beam lithographic techniques. Use of the latter technique to make diodes shaped as crossed stripes of width 0.25 mu m and 0.4 mu m on epitaxial GaAs resulted in a 30-percent reduction in spreading resistance over that of photolithographically formed circular diodes with approximately the same junction area and capacitance. Because of this reduction, it is suggested that in order to minimize receiver noise figure at frequencies greater than about 200 GHz, it will prove advantageous to use such shaped diodes rather than the conventional circular ones. The diodes were used in mixers operating at 140, 175, and 230 GHz. At 140 and 175 GHz the diodes were mounted in conventional Sharpless wafers. At 230 GHz, in addition to Sharpless wafers, a mixer of unique design was used which incorporated an RF matching element, low-pass filter, and IF output transmission line, all on stripline, whose performance was optimized using low-frequency scaling techniques. Mixer double side-band (DSB) noise figures of 3.8, 8.1, and 12.6 dB were measured at the three frequencies, respectively. At 175 and 230 GHz, however, mixer performance is degraded due to a lack of sufficient local oscillator (LO) power and this is currently the principal limitation to their performance.

A Model of HF Impulsive Atmospheric Noise

Abstract: Determination of optimal receiver or detector and suboptimal estimator in the presence of additive atmospheric noise depends on the application of a mathematically tractable model of noise. In the tropics the atmospheric radio noise occurring mostly in the burst form above a relatively small continuous background does not deliver energy at a constant rate. This type of noise is non-Gaussian and has a very large dynamic range. The noise bursts consist of a number of short impulses. They are modelled here as the product of a narrow-band Gaussian noise and the reciprocal of a non-Gaussian random process. This paper includes the derivation of statistical information for the above noise viz, the probability distribution of amplitudes, and of separation between pulses, required for determining the error probabilities using various digital methods. This information can be used in filter optimization in digital systems, where the error probability is to be minimized.

A preliminary report on infrasonic waves as a source of long-period seismic noise

Abstract: Earth motion in response to atmospheric pressure changes is a major contributor to the ambient seismic background in the 20- to 100-s period range. During windy intervals at the surface the quasi-static deformations caused by wind-generated pressure changes are often quite large in comparison to ambient vibrations from other sources. Recent studies [Sorrells et al., 1971; Ziolkowski, 1973] have demonstrated that the wind-related noise can be eliminated by installing the sensors at depths of the order of several hundred meters. When such action is taken, it is found that the noise levels obtained approximate those observed at the surface during calm intervals. The calm interval noise level thus constitutes the current minimum threshold. The calm interval noise level in the 20- to 100-s period range is largely the result of earth motion from atmospheric pressure sources of undetermined origin [Savino et al., 1972]. If noise levels are to be reduced below the currently realizable minimums, it will be necessary to establish the origin of the atmospheric pressure variations with some precision. In this paper, evidence is presented that supports the hypothesis that much of the earth noise observed during calm intervals in the 20- to 100-s period range is the result of atmospheric infrasonic waves. If this hypothesis is borne out by additional studies, then efforts to reduce further the current long-period noise threshold by increasing installation depths will not yield satisfactory results unless depths of the order of 1–3 km are reached. The relatively high costs of installing systems at such depths would rule out this particular method for obtaining further reductions in the long-period seismic noise threshold. Thus if the current noise threshold is largely the result of infrasonic waves, then the practical limits of using depth of installation for the suppression of seismic noise have already been reached.

Effects of forward velocity and acoustic treatment on inlet fan noise

Abstract: Flyover and static noise data from several engines are presented that show inlet fan noise measured in flight can be lower than that projected from static tests for some engines. The differences between flight and static measurements appear greatest when the fan fundamental tone due to rotor-stator interaction or to the rotor-alone field is below cutoff. Data from engine and fan tests involving inlet treatment on the walls only are presented that show the attenuation from this treatment is substantially larger than expected from previous theories or flow duct experience. Data showing noise shielding effects due to the location of the engine on the airplane are also presented. These observations suggest that multiringed inlets may not be necessary to achieve the desired noise reduction in many applications.