Showing papers on "Noise (radio) published in 1973"

Electromagnetic radiation trapped in the magnetosphere above the plasma frequency

Abstract: An electromagnetic noise band is frequently observed in the outer magnetosphere by the Imp 6 spacecraft at frequencies from about 5 to 20 kHz. This noise band generally extends throughout the region from near the plasmapause boundary to near the magnetopause boundary. The noise typically has a broadband field strength of about 5 microvolts/meter. The noise band often has a sharp lower cutoff frequency at about 5 to 10 kHz, and this cutoff has been identified as the local electron plasma frequency. Since the plasma frequency in the plasmasphere and solar wind is usually above 20 kHz, it is concluded that this noise must be trapped in the low-density region between the plasmapause and magnetopause boundaries. The noise bands often contain a harmonic frequency structure which suggests that the radiation is associated with harmonics of the electron cyclotron frequency.

Maximum-Likelihood Estimation of the Number-Flux Distribution of Radio Sources in the Presence of Noise and Confusion

Abstract: The application of the method of maximum likelinood to the determination of the slope of the number- flux-density relationship is extended to include the presence of experimental errors in the flux-density measurements. It is shown that these experimental errors have a significant effect on the number counts at higher natios of flux density to error than is often recognized. The case of noise-limited fiux-density measurements is treated in some detail, and it is found that, provided the lower limit of a survey is chosen to be at least five times the rms noise, the enhancement in the source density as a function of fiux density can be readily calculated. For the case of significart confusion errors in the flux-density measurements the importance of a Monte Carlo approach is emphasized. Several methods that have been used previously are discussed and a number of shortcomings noted. (auth)

Observations of noise bands associated with the upper hybrid resonance by the Imp 6 Radio Astronomy Experiment

Abstract: The intense noise bands occurring near the upper hybrid resonance frequency have been observed with the Imp 6 GSFC radio astronomy experiment in the plasmasphere. The identification of the upper hybrid resonance provides an accurate measure of the local electron density and allows the observed noise data to be fit to the scale of characteristic frequencies in the plasma. The data are consistent with earlier theoretical interpretations in which noise is generated between the upper hybrid and plasma frequencies and propagates to region 4 of the CMA diagram, where it is reflected at the L = 0 plasma cutoff.

Cryogenic cooling of mixers for millimeter and centimeter wavelengths

Abstract: It is shown theoretically that cryogenically cooling a Schottky-barrier mixer only slightly increases the conversion loss while giving a considerable reduction in mixer noise. The d.c. bias and local oscillator drive must be appropriately scaled. Experimental results indicate that in conjunction with a cooled paramp IF amplifier, single-sideband (SSB) receiver noise temperatures of ~350 K at 85 GHz, and ~260 K at 33 GHz are presently obtainable-an improvement by a factor of 6 at 85 GHz and 4 at 33 GHz over current room-temperature mixer receivers. An unexplained source of noise within the diodes has been observed and if this can be eliminated a further factor of 2 improvement in noise temperature will be obtained.

Noise in the superconducting quantum flux detector

Abstract: An expression is derived for noise in the tank circuit of a superconducting quantum flux detector run at a radiofrequency. This noise is limited by the sensing element of the device, the superconducting ring with a weak link, and is therefore intimately related to the thermal fluctuations in the ring. Not considering external noise, the superconducting ring and the tank circuit together with the preamplifier should exhaust the important sources of noise in the flux detector.

Unified theory of high‐frequency noise in Schottky barriers

Abstract: A unified noise‐temperature equation is presented which describes the high‐frequency noise (>1/f noise) characteristic of Schottky barrier diodes from above room temperature to cryogenic temperatures. The unified noise equation takes into consideration the dominant transport mechanism of the barrier and is applicable to any metal‐semiconductor system. Noise‐temperature measurements on fabricated Schottky barrier diodes at 18, 77, and 300 °K confirm the validity of the noise theory presented.

Plasma wave observations near the plasmapause with the S³‐A satellite

Abstract: Description of the electric field noise phenomena observed by the S3-A spacecraft near the plasmapause during the magnetic storm of Dec. 16, 17, 1971. The most striking and unusual feature of these storm time electric field observations is the occurrence of a region of intense low-frequency (20 to 500 Hz) electrostatic noise bursts just outside the plasmapause boundary. These noise bursts occurred concurrently with the rapid decrease in low-energy (E ranging from 20 to 50 keV) ring current protons mirroring near the equator during this storm and may be responsible for the loss of these particles. The characteristics of other phenomena, such as whistlers, ELF hiss, and banded chorus, observed near the plasmapause during this period are also discussed.

Self synchronous noise rejection circuit for fluid velocity meter

Abstract: A noise rejection circuit for use with electromagnetic fluid flow meters. An electromagnetic fluid flow meter with an electromagnet to induce a magnetic flux of periodically changing polarity. Sensors are positioned so as to receive the voltages induced therein by reason of flow of fluid cutting the flux lines. The low level signal of the sensors requires extremely efficient noise elimination circuits.

Si and GaAs 0.5 μm-gate Schottky-barrier field-effect transistors

Abstract: Si and GaAs Schottky-barrier field-effect transistors with gate lengths of 0.5 μm have been experimentally realised. Noise and gain properties were measured in the microwave range up to 20 GHz. When compared with 1 μm-gate f.e.t.s, the devices show considerable improvements in gain and in noise figure. At 10 GHz, the following values were measured: Si m.e.s.f.e.t.: maximum available gain = 5.9 dB, noise figure = 5.8 dB; GaAs m.e.s.f.e.t.: maximum available gain = 12.8 dB, noise figure = 3.7 dB.

Leading edge serrations which reduce the noise of low-speed rotors

Abstract: Acoustic effects of serrated brass strips attached near the leading edges of two different size rotors were investigated. The two bladed rotors were tested in hover. Rotor rotational speed, blade angle, serration shape, and serration position were varied. The serrations were more effective as noise suppressors at rotor tip speeds less than 135 m/sec (444 ft/sec) than at higher speeds. high frequency noise was reduced but the low frequency rotational noise was little affected. Noise reductions from 4 to 8 db overall sound pressure level and 3 to 17 db in the upper octave bands were achieved on the 1.52 m (5.0 ft) diameter rotor. Noise reductions up to 4 db overall sound pressure level were measured for the 2.59 m (8.5 ft) diameter rotor at some conditions.

Effects of time dependence of multiplication process on avalanche noise

Abstract: Theoretical and experimental studies of noise generated due to the randomness of the multiplication process in the avalanche region of a uniform diode are presented. The theory extends the results of McIntyre to include the time dependence of the multiplication process. It also shows the correspondence between the results of McIntyre, Gummel and Blue, Hines and Tager. The space-charge feedback and transit-time effects have been neglected in this analysis. The theoretical and the experimental results described have shown that even at frequencies well below transit-time frequency, the importance of the factor resulting from consideration of the time dependence of the multiplication process cannot be ignored. The measurements of the avalanche noise on uniform p+-n-n+ silicon diodes are found to be in good agreement with the theory presented here.

Noise quality detector for electric motors or other machines

Abstract: Such detector is adapted to detect a transducer signal indicative of the level of noise, or the actual measured noise, produced by an electric motor or the like, particularly for the purpose of quality control in connection with a production line, automated or manual, so that motors which produce excessive noise or are defective may be identified and rejected. The detector employs an accelerometer or some other transducer, i.e. a microphone, for receiving the vibration or noise from each motor and converting the noise into electrical signals. After amplification, the signals are sent through a bandpass filter or a plurality of filters which selectively transmit those signals which are within a particular band of frequencies, while attenuating signals which are outside such band. The filter preferably has controls for adjusting both the upper and lower cutoff frequencies of the filter. The output signals from the filter are compared with a predetermined level. One or more indicating devices are operated by the comparison device to indicate whether the detected noise level is above or below such level. The indicating devices may be in the form of lamps or a meter. It is preferred to detect both the peak signal level and the average signal level so that the motor will be rejected if either level is above the desired level. The disclosed device is adapted to detect and indicate the noise quality of a plurality of motors in sequence on an automatic basis.

Millimeter and submillimeter wave detection and mixing with superconducting weak links

Abstract: A weak link between two bulk superconductors can exhibit Josephson effects and can be used as a fast sensitive detector or mixer for electromagnetic radiation. Emphasis is placed on the millimeter and submillimeter range where such applications are potentially most important, though such devices should also work well at lower frequencies. The peculiar properties of the weak link make possible a variety of modes of operation including one in which it is both local oscillator and mixer. These modes of operation are analyzed and the pivotal experiments are reviewed. Noise and impedance matching are considered and comparison is made with other types of detectors of millimeter and submillimeter waves.

LF radio noise from the earth's magnetosphere

Abstract: Gyro-synchrotron radio noise emitted by electrons trapped in the earth's magnetosphere has been a subject of extensive research. Previous efforts, which considered frequencies greater than 1 MHz, have shown that this noise should not be detectable in the MF to HF range because its intensity is below the cosmic background noise level. The author has investigated the LF range and has found that appreciable noise is generated at these frequencies. In fact, the theoretical results for this LF noise agree very well with experimental data obtained by a radio astronomy experiment aboard the IMP 6 spacecraft. A comparison showed that the model predicted both variation in the observed noise intensity with Kp and the noise spectral characteristics. Consequently, it is concluded that detectable LF radio noise is emitted, by means of the cyclotron-synchrotron mechanism, by electrons trapped in the earth's magnetosphere, and that this noise is observable only for frequencies below about 300 kHz. For higher frequencies, the theoretical model and the experimental data reconfirm that this noise is below that of cosmic origin.

Interaction between high-frequency turbulence and magnetospheric micropulsations

Abstract: The quasi-linear interaction between a high-frequency whistler noise and a drift wave is considered as a possible explanation of the miropulsations observed during the magnetospheric substorms. The drift wave is a so-called ion trapped mode that propagates in a direction almost perpendicular to the magnetic field and has a frequency lower than the ion bounce frequency. Plasma pressure and field line curvature are taken into account in the general frame of the Vlasov equation. The drift wave interacts with the high-frequency turbulence by inducing anisotropy in the electron distribution function. As an example, growth rates and polarizations are computed for frequencies in the range of the Pc 5 micropulsations and compared to the observations. (auth)

Wave mechanics of boundary layer turbulence and noise

Abstract: Recent advances in the understanding of the structure of boundary‐layer turbulence form the basis of a new conceptual model tot the fluctuating velocity field. In this, an “active” mode of small‐scale motion is coupled to a large‐scale “passive” mode by aid of wave‐mechanical considerations. The small‐scale motion originates during the intermittent “bursting” in the wall layer leading to excitation of large‐scale, lightly damped, traveling shear waves. It is argued that the passive wave‐like mode gives the major contribution to the pseudosound, whereas the active bursting mode serves as the predominant source for the radiative noise. It is shown that the bursting mode gives rise to compact noise sources of both dipole and quadrupole type whose strengths are related to the turbulent shear stress production.

The effect of nozzle inlet shape, lip thickness, and exit shape and size on subsonic jet noise

Abstract: Far field noise data were taken for convergent nozzles of various shapes and sizes at subsonic velocities exceeding 400 feet per second. For a circular nozzle, the nozzle inlet shape and lip thickness had no effect on the noise level, directivity, or spectra when compared at the same nozzle exit diameter and peak exhaust velocity. A sharp edged orifice was one exception to this statement. Coannular nozzles can produce additional high frequency noise. Blunt ended centerbodies, where there is significant base drag, also generate significant additional noise. The total noise power generation was essentially the same for circular, slot, and plug nozzles of good aerodynamic shape. The noise radiation patterns were essentially the same for these nozzle shapes except near the nozzle exhaust axis.

Flow velocity measurement

Abstract: A method of detecting the velocity of a flow which comprises deriving from each of a first pair of positions spaced apart in the direction of flow a signal corresponding to radiative noise in the flowing system, multiplying said signals together and either auto-correlating the product or cross-correlating the product with the product signal obtained by multiplying together further signals corresponding to radiative noise in the flowing system and derived from each of a second pair of positions spaced apart in the direction of flow.

Photon-noise-limited Fourier spectroscopy

Abstract: A simple, approximate calculation is made of the effect of photon noise on Fourier spectroscopy when detector noise is small. The usual comment that there is no advantage due to multiplexing in these circumstances is true only for dense spectra, such as line-absorption spectra, whereas for line-emission spectra, especially with narrow lines and high resolution, there is a gain of signal-to-noise ratio over an equivalent scanning spectrometer. For weak spectra there may also be a gain, instead of the usual loss, in the range of observable line intensities.

Small-signal analysis of punch-through injection microwave devices

Abstract: A theoretical small-signal analysis of punch-through injection microwave devices is given. A numerical study of a silicon p + − n − p + structure is performed, which shows good agreement with experimental measurements by Snapp and Weissglas for a diode with a doping density of 1·2 × 10 15 /cm 3 . Negative resistance is also calculated for diodes with doping densities of 0·6 × 10 15 /cm 3 and 5 × 10 15 /cm 3 . A partially analytical mode, including the lowfield region, is developed and compared with the numerical calculation. Ohmic losses for devices with low impurity concentrations and diffusion for devices with high impurity concentrations are shown to be significant factors. The noise spectrum is calculated numerically from the assumption of two noise sources, injection noise and diffusion noise. The noise measure is determined and shown to be in good agreement with experiments by Bjorkman and Snapp.

Prediction and suppression of long-period nonpropagating seismic noise

Abstract: Approximately half the noise observed by long-period seismometers at LASA is nonpropagating; that is, it is incoherent over distances greater than a few kilometers. However, because it is often strongly coherent with microbarograph data recorded at the same site, a large proportion of it can be predicted by convolving the microbarogram with some transfer function. The reduction in noise level using this technique can be as high as 5 db on the vertical seismometer and higher still on the horizontals. If the source of this noise on the vertical seismogram were predominantly buoyancy, the transfer function would be time-invariant. It is not. Buoyancy on the LASA long-period instruments is quite negligible. The noise is caused by atmospheric deformation of the ground and, since so much of it can be predicted from the output of a single nearby microbarograph, it must be of very local origin. The loading process may be adequately described by the static deformation of a flat-earth model; however, for the expectation of the noise to be finite, it is shown that the wave number spectrum of the pressure distribution must be band-limited. An expression for the expected noise power is derived which agrees very well with observations and predicts the correct attenuation with depth. It is apparent from the form of this expression why it is impossible to obtain a stable transfer function to predict the noise without an array of microbarographs and excessive data processing. The most effective way to suppress this kind of noise is to bury the seismometer: at 150 m the reduction in noise level would be about 10 db.