Showing papers on "Very low frequency published in 2001"

Study of the Temporal Behavior of 4U 1728–34 as a Function of Its Position in the Color-Color Diagram

Abstract: We study the timing properties of the bursting atoll source 4U 1728-34 as a function of its position in the X-ray color-color diagram. In the island part of the color-color diagram (corresponding to the hardest energy spectra), the power spectrum of 4U 1728-34 shows several features such as a band-limited noise component present up to a few tens of Hz, a low-frequency quasi-periodic oscillation (LFQPO) at frequencies between 20 and 40 Hz, a peaked noise component around 100 Hz, and one or two QPOs at kHz frequencies. In addition to these, in the lower banana (corresponding to softer energy spectra) we also find a very low frequency noise (VLFN) component below ~1 Hz. In the upper banana (corresponding to the softest energy spectra), the power spectra are dominated by the VLFN, with a peaked noise component around 20 Hz. We find that the frequencies of the kHz QPOs are well correlated with the position in the X-ray color-color diagram. For the frequency of the LFQPO and the break frequency of the broadband noise component, the relation appears more complex. Both of these frequencies increase when the frequency of the upper kHz QPO increases from 400 to 900 Hz, but at this frequency a jump in the values of the parameters occurs. We interpret this jump in terms of the gradual appearance of a QPO at the position of the break at high inferred mass accretion rate, while the previous LFQPO disappears. Simultaneously, another kind of noise appears with a break frequency of ~7 Hz, similar to the NBO of Z sources. The 100 Hz peaked noise does not seem to correlate with the position of the source in the color-color diagram but remains relatively constant in frequency. This component may be similar to several 100 Hz QPOs observed in black hole binaries.

Low-Frequency Modes of Tropical Ocean Dynamics

Abstract: Basinwide very low-frequency (VLF) modes with zonal uniformly deepening and shoaling of the equatorial thermocline are found as solutions of linear shallow-water equations with the meridional basin boundaries under an equatorial β plane. They can be understood as the free heat-content recharge oscillations. The interannual VLF modes had been recognized as the essential part of the coupled recharge oscillator theory for the El Nino–Southern Oscillation. It is suggested that the decadal VLF modes may also be transformed into coupled modes relevant to the Pacific decadal climate variability through the ocean–atmosphere interaction in the Tropics.

Dual-radio communication apparatus, and an operating method thereof

Abstract: A dual-radio communication apparatus has a first radio device, such as a Bluetooth radio, for use in a first frequency band, and a second radio device, such as a Globalstar satellite radio, for use in a second frequency band, which is proximate to the first frequency band. The communication apparatus also has a controller coupled to the first and second radio devices. The first radio device may comprise a frequency-hopping spread-spectrum transmitter. Moreover, the first radio device has a first operating mode employing a first frequency range. The first radio device also has a second operating mode employing a second frequency range, which is smaller than the first frequency range. The controller is adapted to set the first radio device in its second operating mode, when the second radio device is in operation, and otherwise set the first radio device in its first operating mode.

An attempt to delineate very low frequency electromagnetic signals associated with earthquakes

Abstract: We report on our observation of pulse-like electromagnetic signals in the frequency range 1∼10 kHz that we associate with earthquakes. The severest difficulty in separating earthquake-associated VLF signals from those originating in lightning discharges stems from the circumstance that the latter signals are overwhelming in number compared with the former. While claims have often been made of observation of electromagnetic signals in association with earthquakes, most of the claims, excepting a few, heavily rely on temporal correlation. By means of simple instrumentation and data processing software, our method by and large enables us to isolate VLF signals whose direction of arrival is well focussed at the epicenter direction. In this preliminary report we present several examples that positively demonstrate the existence of a class of VLF signals having a peak frequency of occurrence 1∼4 days prior to earthquakes of Magnitude 4∼6. With an accumulation of experience the technique described in this paper would seem to offer a promising approach towards earthquake prediction.

Continuous gradient VLF measurements: a new possibility for high resolution mapping of karst structures

Abstract: The Centre of Hydrogeology Neuch‚tel (CHYN) investigates and monitors karst aquifers, which are highly vulnerable to contamination. One of our particular research tasks within this scope is the characterization and localization of the 'epikarstic zone' (0-20 m depth) (Mangin 1975; Kiraly 1995). Density, size and aperture of fractures that connect the water reservoir with the surface, are features which characterize the vulnerability of karst aquifers. Very local scale mapping of these types of high-permeability fracture zone over extensive areas is therefore of interest for karst hydrogeology. In order to investigate this zone the very low frequency (VLF, 15-30 kHz) method has been adapted at CHYN for investigation at shallow depths with high lateral resolution. Geophysical surveying using VLF radio signals began in the 1960s as a tool for prospecting conductive minerals (McNeill & Labson 1991; Paal 1965). The CHYN equipment uses VLF and low frequency (LF, 30-300 kHz) radio transmitters, which are located at several positions around the world and are used principally for navigation and submarine communication. Consequently the CHYN has developed a radio frequency-resistivity device (RF-R), also called radio magnetotelluric (RMT, 12-240 kHz), and a continuously measuring radio frequency-electromagnetic (RF-EM, 15-300 kHz) device (Turberg et al. 1994; Turberg & Barker 1996; Stiefelhagen 1998). In fissured limestone with a resistivity of several hundreds of Wm, the penetration depth for these frequencies is about 10-100 m. However, when investigating small structures at very shallow depths (0-10 m) in the epikarstic zone, these devices have limited use due to their excessive penetration depths and their poor horizontal resolution. In this paper we introduce a method that is based on measuring the vertical gradient of the horizontal magnetic field component of VLF radio signals (VLF-EM-vGRAD(Hy)). According to our knowledge, such a technique has not been previously published.

Automatic detection of slow-wave-sleep using heart rate variability

Abstract: In this study, we used heart rate variability parameters to first characterize and then automatically detect slow-wave sleep (SWS). First, a wavelet transform was used to decompose equally sampled R-R interval series into their time-dependent spectral components: very low frequency (VLF) 0.005-0-04Hz, low frequency (LF) 0.04-0.15 Hz, and high frequency (HF) 0.15-0.45Hz. Then, the known decrease in LF power during SWS was confirmed and a linear relation between the average LF/HF balance throughout the night and the balance during SWS was found. Also, similar behaviour was found with the VLF power and the VLF/HF ratio. Finally, a decision algorithm with two criteria was defined using a training set of ECG recordings and applied to a test set. The results amounted to an 80% correct identification of SWS. The limitations of the study, as well as inherent differences between SWS definitions based on EEG and ECG, are discussed.

Millimeter wave radio transmitter, millimeter wave radio receiver and millimeter wave band communication system

Abstract: PROBLEM TO BE SOLVED: To provide a millimeter wave radio transmitter, a millimeter wave radio receiver and a millimeter wave band communication system, capable of improving phase noise characteristics of a local oscillator, accurately controlling respective levels of radio signal waves to be transmitted, local oscillation waves and unwanted suppression signal waves with excellent reproducibility and increasing the radio transmission distance. SOLUTION: By using a local oscillation waves 106 outputted from a local oscillator 105, modulated signal waves 108a are frequency up-converted to a millimeter wave band by an image rejection type frequency mixer 101. Unwanted side band signal waves on the lower side of side bands on both sides generated at the time of the frequency up-conversion are suppressed. The radio signal waves 107 which are the side band signal waves on the upper side frequency up-converted by the image rejection type frequency mixer 101 and the local oscillation waves 106 for frequency down-converting the radio signal waves 107 on a reception side are transmitted simultaneously by transmission parts (103 and 104). COPYRIGHT: (C)2003,JPO

Wave power studies of cusp crossings with the Polar satellite

Abstract: Burst data from the electric field instrument (EFI) on the Polar satellite are used to classify cusp wave properties and the association of the observed waves with particle populations. The most intense wave type (most commonly triggering burst mode data collection) are the broadbanded waves which are then studied in detail using the three-dimensional resolution of the EFI. Finite wavelength effects are removed using two techniques: time-shifting of the sphere potentials to a spacecraft body reference time and spin angle selection. The corrected electric field signal exhibits a uniform spectrum in two dimensions. The wave properties are consistent with two-dimensional turbulence, with phase velocities smaller than the spacecraft velocity and frequency as low as (or lower than) the oxygen gyrofrequency. Owing to their low frequency, we conclude that these waves cannot resonate with and thus heat the ions. The broad k-spectrum and very low frequency in the plasma frame points toward drift or shear instabilities for wave excitation.

A comparison of performances of linearized and global nonlinear 2-D inversions of VLF and VLF-R electromagnetic data

Abstract: The performances of linearized (local) and global nonlinear joint 2-D inversions of very low frequency (VLF) and VLF resistivity electromagnetic measurements are analyzed. A stable iterative inversion scheme is used in linearized inversion while the very fast simulated annealing approach is used in global nonlinear inversion. Synthetic noise‐free and noisy data due to three different models in complexity and two field examples are considered. Synthetic examples show that linearized inversion reveals the subsurface structure better than global nonlinear inversion provided the model has only a few parameters under inversion. Both linearized and global nonlinear inversions must be performed combining all available data in order to obtain the most reliable estimates of the subsurface parameters. Complex models with a large number of parameters are better to invert using global nonlinear inversion although the CPU time needed is always much longer than the one used in linearized inversion. Contrary to global n...

Plane wave electromagnetic measurements for imaging fracture zones

Abstract: Electromagnetic fields from distant radio transmitters interact with the resistivity structures in the earth. They can be regarded as plane-wave fields since their penetration depth is much smaller than their wavelength. In this thesis, plane wave methods have been applied in a number of novel ways to map the resistivity of the bedrock and the sedimentary overburden. The main objective of the field measurements carried out was to map and characterise fault and fracture zones in several areas in northern Sweden. Two different techniques have been applied, the radiomagnetotelluric (RMT) method and the radiofrequency electromagnetic (RF-EM) method. These methods utilise frequencies in both the very low frequency band (VLF, 3-30 kHz) and the low frequency band (LF, 30-300 kHz). They are extensions of the traditional VLF method, which has been widely used for detection of fracture zones in the bedrock and for groundwater exploration. The broader frequency range used in the RMT and RF-EM methods makes it possible to resolve variations of resistivity with depth in addition to lateral variations. A recently developed 2D-inversion scheme for plane-wave excitations of the ground was applied for the interpretation of the data. The computer program was originally developed for the interpretation of low frequency magnetotelluric data. It was found that the 2D-code is a fast and convenient way for quantitative interpretations of both the RMT and the RF-EM data. The results show that detailed information concerning resistivity distribution in the ground down to several hundred meters in the bedrock can be obtained. Fracture zones in the bedrock can be identified and their resistivity, width and dip can be determined.

Interhemispheric propagation of VLF transmissions in the presence of ionospheric HF heating

Abstract: Recent experiments carried out at the Arecibo Observatory in Puerto Rico and the magnetically conjugate location in Argentina suggest a connection between the prevalence and intensity of the interhemispheric propagation of VLF transmissions and HF heating of the ionosphere. During several nights of the study the O-mode CW heater appeared to enhance the coupling of VLF transmissions into the affected region of plasma. These signals presumably then entered plasmaspheric ducts more efficiently and led to conjugate measurements of relatively high amplitude. No such effects were seen with pulsed or X-mode heating. Short time resolution measurements of fading transmissions were marked by transit delays and amplitudes that remained nearly constant as the interhemispheric VLF signals diminished, consistent with the model of a plasmaspheric duct complemented by favorable plasma structures extending well into the F region but producing the majority of the total transit delay at plasmaspheric altitudes. It is also suggested that weak long-delay receptions often observed in these data are most probably prolongitudinal whistlers that are not ducted but which represent a potential contaminant in ducted VLF experiments of all types.

Propagation of Z-mode and whistler-mode emissions observed by Interball 2 in the nightside auroral region

Abstract: A case study of VLF (very low frequency) natural emissions propagating at high altitude above the nightside auroral zone is presented. The analysis is based on data of the MEMO experiment on board the Interball 2 spacecraft. In its VLF band the device records waveforms of three magnetic and two electric field components covering a frequency range 1-20 kHz. Several analysis methods are applied in order to obtain the details on the wave propagation and mode structure and to identify possible source regions. We demonstrate that observed band-limited emission consists of Z-mode and whistler-mode waves. Downgoing Z-mode waves are found just above the lower cutoff at the L=0 frequency. A possible source may be connected with the electron cyclotron resonance mechanism taking place in the nightside sector at altitudes above 21,000 km. Upgoing whistler-mode waves with upper cutoff at the local plasma frequency are probably generated by upgoing electrons in the auroral region at altitudes lower than the actual satellite position, i.e., below 18,000 km.

Assessment of noise with low frequency line spectra – practical cases

Abstract: Many small power plants have been built in Denmark in recent years. These plants are often situated near dwellings giving rise to complaints about annoyance of low frequency noise and vibration indoors. Normally the annoying low frequency noise is in the frequency range 10–200 Hz. Since it is created by combustion engines the noise has a line spectrum. In Denmark the A-weighted total level in the frequency range 10–160 Hz LpA,LF is used for assessment and legislation about low frequency noise exposure in dwellings. From our experience solving practical low frequency noise and vibration problems LpA,LF seems to overestimate the annoyance of very low frequency noise whereas noise with the major spectral content near 160 Hz seems to be underestimated. This problem is discussed based on our experience.

Measurement of radiated electromagnetic field levels before and after a changeover to energy-efficient lighting.

Abstract: An energy-efficient lighting retrofit at the Food and Drug Administration (FDA) Winchester Engineering and Analytical Center (WEAC) presented the opportunity to measure the electromagnetic (EM) environments in several rooms before and after changing the fluorescent lighting systems and to compare the changes in EM fields with the proposed standard EM immunity levels. Three rooms, representing the types of work areas in the laboratory, were selected and measured before and after the lighting changeover. Electric and magnetic field measurements were taken in the extremely low frequency (ELF), very low frequency (VLF), and radio frequency (RF) ranges of the EM spectrum. In 2 rooms, ELF electric fields were reduced and VLF and RF electric fields were increased as a result of the changeover to high-frequency fixtures. A third room received low-frequency, energy-efficient fixtures during this changeover, and this change resulted in only a slight increase of the ELF electric fields. The ELF magnetic fields were greatly reduced in 2 but only slightly reduced in the third room. No significant change was seen in VLF or RF magnetic fields for any of these rooms. Some field-strength measurements exceeded the proposed immunity levels recommended in the draft International Electrotechnical Commission standard IEC 60601-1-2 (rev. 2). The data show that increasing the separation distance from the fluorescent light fixtures greatly reduces the field-strength levels, limiting the potential for EM interference.

Low-Frequency Electromagnetic Exploration for Groundwater on Mars

Abstract: Water with even a small amount of dissolved solids has an electrical conductivity orders of magnitude higher than dry rock and is therefore a near-ideal exploration target on Mars for low frequency, diffusive electromagnetic methods. Models of the temperature- and frequency-dependent electrical properties of rock-ice-water mixtures are used to predict the electromagnetic response of the Martian subsurface. Detection of ice is difficult unless it is massively segregated. In contrast, liquid water profoundly affects soundings, and even a small amount of adsorbed water in the cryosphere can be detected. Subcryospheric water is readily distinguishable at frequencies as low as 100 Hz for fresh water to 10 mHz for brines. These responses can be measured using either natural or artificial sources. Ultra low frequency signals from solar wind and diurnal-heating perturbations of the ionosphere are likely, and disturbances of regional crustal magnetic fields may also be observable. Spherics, or extremely to very low frequency signals from lightning discharge, would provide optimal soundings; however, lightning may be the least likely of the possible natural sources. Among the active techniques, only the time-domain electromagnetic (TDEM) method can accommodate a closely spaced transmitter and receiver and sound to depths of hundreds of meters or more. A ground- or aircraft-based TDEM system of several kilograms can detect water to a depth of several hundred meters, and a system of tens of kilograms featuring a large, fixed, rover- or ballistically deployed loop can detect water to several kilometers depth.

Dc offset correction circuit and agg in zero-if-wireless receivers

Abstract: In Direct Conversion receivers the baseband analog modulated signal is produced directly from the high frequency signal received at the antenna As the baseband signal is extended to zero intermediate frequency, an error signal (at DC or of very low frequency) is added The spectrum of the modulated signal overlaps with the one of the error signal Consequently saturation at the following stages or determination at the performance of the demodulator could occurThe level of the error signal depends upon the frequency and the power of the received signal at the antenna, the frequency and the level of the LO signal, the power supply level, the temperature, as well as the environment that surrounds the receiver (high frequency signal leaks to the antenna, radiated and reflected from moving objects back to the receiver)The basemant signal is sampled and converted to digital by ADC converters The method proposed includes a dynamic calculation of the error signal value, using as input each digital sample, through an algorithm that is implemented in digital circuity The value of the calculated error function is updated in every sample and through DAC converters is subtracted at the output of the mixers The correction of the error signal is combined with the AGC control signal which is calculated also through the digital algorithm and can also be updated in every sampleIn this way any low frequency component at the error signal could be tracked and corrected This is very important when the low level signal is present as the amplification gain through the AGC is high In that case any error signal added (usually its level is much higher than the one of the received signal) could overflow the following stagesAnother point is that in the case of a significant change at both the values of the error signal and the power of the received signal the algorithm converges to the valuesThe algorithm includes also the correction of the static DC offset (at the case that no signal is present at the antenna) through a