Showing papers on "Radio frequency published in 1992"

IEEE standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3kHz to 300 GHz

Abstract: IEEE C95.1-1991 gives recommendations to prevent harmful effects in human beings exposed to electromagnetic fields in the frequency range from 3 kHz to 300 GHz. The recommendations are intended to apply to exposures in controlled, as well as uncontrolled, environments. They are not intended to apply to the purposeful exposure of patients under the direction of practitioners of the healing arts. The induced and contact current limits of IEEE C95.1-1991 are modified in this edition. In addition, field strengths below which induced and contact currents do not have to be measured are specified, spatial averaging and measurement distance requirements are clarified, and more precise definitions for averaging volume and radiated power are provided.

Evaluation of microwave and radio frequency catheter ablation in a myocardium-equivalent phantom model

Abstract: The use of 2450-MHz microwave energy applied via a miniature coaxial cable-mounted helical coil antenna was investigated as a means to increase the treated volume of cardiac tissue in a saline-perfused, tissue-equivalent manner during ablation. Using an array of fiber-optic temperature probes implanted in a saline-perfused, tissue-equivalent gel phantom model designed to simulate the myocardium during ablation, the heating pattern from the microwave antenna was characterized and compared to that induced by a commercial RF electrode catheter at 550 kHz. Effects of variable contact angle between the heat source and heart wall were assessed. Heating patterns from the RF electrodes dropped off much more abruptly both radially and axially than the microwave antenna. The volume of effectively heated tissue was more than ten times larger for the microwave antenna when the heat sources were well-coupled to the tissue, and more than four times larger for the microwave antenna when the sources were angled 30 degrees away from the tissue surface. >

Closed-loop class E transcutaneous power and data link for MicroImplants

Abstract: The use of a multifrequency transmitter coil driver based on the class E topology is described. The development of a high-Q approximation, which simplifies the design procedure is presented. A closed-loop controller to compensate for transmitter and receiver variations, and a method of data modulation using synchronous frequency shifting are described. The closed-loop class E circuit shows great promise, especially for circuits with unusually low coefficients of coupling. Currents of several amperes, at radio frequencies, can easily and efficiently be obtained. >

Optical and radio variability in blazars

Abstract: An analysis is presented of the optical (0.44 micron) and radio flux density variations (4.8, 8.0, and 14.5 GHz) of the five best observed variable AGN, of which three are BL Lac objects and two are optically violent variable (OVV) quasars. For each object the nature of the variability at individual wavelengths is determined by using a structure function analysis and the autocorrelation function as calculated from the discrete correlation technique. Both techniques yield similar results, showing that the optical variations are similar for all objects and are a combination of flicker and shot noise. The radio variability is distinct, being described by shot noise (less variation at short times), with the two OVVs exhibiting the most extreme behavior. The time lag between the variations at the three radio frequencies and the optical flux is studied with the discrete cross-correlation function and the mean variance methods. It is concluded that the optical and radio emitting regions are physically related, exist on distinct time scales, and excite their synchrotron emitting plasmas differently.

Radio frequency automatic identification system

Abstract: A radio frequency automatic identification system detects targets which include a plurality of radio frequency resonators Information is attributed to the target in accordance with the radio frequency response characteristics of the target, such as the resonant frequencies of resonators present, and/or the spatial locations within the target of the resonators Preferred resonators include thin dipoles, which may be metallizations on a plastic film substrate Information may be attributed to a target by fabricating the target with resonators disposed at locations to encode information in accordance with a predetermined encoding system; alternatively, resonators may be randomly disposed, and a translation code applied to convert the radio frequency information to particular information in a particular format Readers for use in the system read the radio frequency response characteristics of a target in the near field of a radiating aperture which is illuminated by a radio frequency source Items which may be encoded with radio frequency readable information include documents, credit cards, and computer diskettes

MRI compatible pulse oximetry system

Abstract: Magnetic resonance imaging hardware (A) defines a patient receiving region (20) that is surrounded by a bore liner (22). A socket (50) is mounted in the bore liner with an appropriate receptacle for receiving a standard plug (52) of a conventional pulse oximetry system. Conventional pulse oximetry systems include a sensor unit (54) connected with a cable (56) having the plug (52) at one end thereof. A notch filter (62) attenuates currents near the resonance frequency of the imager. A preamplifier (60) amplifies signals from the sensor unit. Within the shielding (66) of the preamplifier, a low pass filter (68) is provided to remove induced radio frequency components from the preamplified sensor unit signal. A radio frequency filter (70) mounted at the shield of the shielded room (B) prevents radio frequency signals from reaching an exterior processing and display unit (E) and prevents radio frequency signals from a clock (72) of the processing and display unit from being conveyed into the shielded room (B). The processing and display unit processes the signal received from the preamplifier to generate a pulse rate display (78) and a blood oxygen concentration display (80).

Power control circuitry for a tdma radio frequency transmitter

Abstract: A transmitter (604) of a TDMA cellular telephone (600) includes power control circuitry (100) comprised of variable gain stage (104), mixer (106), bandpass filter (109), RF amplifier (110), and directional coupler (112) in a forward path, and two detectors (116 and 117), four A/D converters (118, 119, 121 and 123), digital controller (120), and D/A converter (126) in a feedback path. In operation, the variable gain stage (104) is responsive to a gain control signal AOCCNT (128) for adjusting a modulated IF signal, which is then mixed in mixer (106) with an RF reference signal to produce the transmit RF signal. The temperature and supply voltage are sampled by controller (120) at the beginning of each time slot. Adjustments in the gain control signal (128) dictated by the sampled temperature and supply voltage are made by controller (120) in each time slot prior to keying the RF amplifier (110). The transmit RF signal is amplified by the RF amplifier (110) to produce the transmit output signal which is coupled by the directional coupler (112) and transmit filter (618) to an antenna (620) for transmission The forward power and reverse power of the transmit output signal are sampled by controller (120) at the end of each time slot. The sampled forward power is used by controller (120) in calculating the value of the gain control signal for the next time slot. If the sampled reverse power or supply voltage exceed respective maximum values, the RF amplifier (110) is dekeyed by controller (120).

Apparatus and method for determining power in plasma processing

Abstract: An apparatus and method including a current sensor having a radiation emitter, such as an electrical resistor, and a radiation detector, such as an infrared detector, for sensing current flowing to a plasma generating electrode from a radio frequency (RF) power source. The resistor may include a high emissivity infrared coating to enhance efficiency of the current sensor. The infrared detector provides a highly accurate indication of the average or root-mean-square current delivered to the plasma generating electrode without introducing parasitic capacitance into the measurement, or sensing, circuit. A voltage sensor and a second current sensor, such as a torroidal current sensor, provide the voltage and phase angle of the current delivered to the plasma generating electrode to thereby permit calculation of the power delivered to the plasma generating electrode. A processor controls the RF source responsive to the sensed average current, sensed voltage, and sensed phase angle of the current.

Electrical characterization of radio‐frequency discharges in the Gaseous Electronics Conference Reference Cell

Abstract: Measurements of the electrical characteristics of radio‐frequency (rf) discharges can be subject to large errors due to limitations in the measurement instruments and the stray impedance of the discharge cell. This study reports electrical measurements of argon discharges in the GEC Reference Cell in which special care has been taken to identify and minimize these sources of error. Careful calibration of current and voltage probes was found to be essential. In addition, parasitic impedances in the cell were found to be large, sensitive to minor changes in electrical connections, and not adequately described by simple a priori models. A general technique for characterizing the stray impedance, including an analysis of the propagation of errors, is presented here. This technique assures accurate results with specified uncertainties. Error analysis demonstrated that large gains in the precision of the measurements can be obtained using an inductive shunt circuit. Together, these techniques should improve the utility of electrical measurements for gauging the reproducibility of plasma conditions among rf discharge cells, for testing theoretical results, and for monitoring plasma processing.

Plasma source and method of manufacturing

Abstract: A plasma source for generating a plasma in a chamber in conjunction with a radio frequency generator is described. The plasma source comprises a coil spiral, at least one insulator and at least one capacitor. The coil spiral conducts the radio frequency wave from the radio frequency generator and induces a plasma in the chamber. It comprises at least two segments. Each insulator and capacitor couple two adjacent segments of the coil spiral together.

Evaluation of Microwave and Radio Frequency Catheter Ablation in a Myocardium-Equivalent

Abstract: A highly localized burst of energy applied to the myocardium via a transvenous catheter-mounted power source can be used to destroy endocardial tissue regions which mediate life-threatening arrhythmias. In the past, high-voltage direct current pulses, radio-frequency (RF) current, and laser light have been used as energy sources. In this paper, the use of 2450 MHz microwave energy applied via a miniature coaxial cable- mounted helical coil antenna designed specifically for this ap- plication was investigated as a means to increase the treated volume of cardiac tissue in a controllable and efficient manner during ablation. Using an array of fiber optic temperature probes implanted in a saline-perfused, tissue-equivalent gel phantom model designed to simulate the myocardium during ablation, the heating pattern from the microwave antenna was characterized and compared to that induced by a commercial RF electrode catheter at 550 kHz. Effects of variable contact angle between the heat source and heart wall were assessed in terms of the radial penetration and overall volume of heated tissue. Heating patterns from the RF electrodes dropped off much more abruptly both radially and axially than the micro- wave antenna such that the volume of effectively heated tissue was more than ten times larger for the microwave antenna when the heat sources were well-coupled to the tissue, and more than four times larger for the microwave antenna when the sources were angled 30" away from the tissue surface.

High speed sample and hold circuit and radio constructed therewith

Abstract: A sample and hold circuit is formed within an integrated circuit and has a small, substantially linear hold capacitance. The circuit includes a sampling switch, a hold capacitor, and a buffer amplifier. The buffer amplifier includes a common drain FET and a constant current source FET. The common drain FET provides an input which couples to the hold capacitor. The constant current FET isolates the source of the common drain FET from ground. The sample and hold circuit may be used as a wide bandwidth mixer. In a radio application, a pulse generator provides a stream of pulses in which the sampling rate times an integer number equals the RF frequency minus the IF frequency. The width of the sampling pulse is less than the period of an RF signal. In an oscillator application, the sample and hold circuit operates as a mixer in a frequency multiplying phase locked loop.

Radio frequency broadcasting systems and methods using two low-cost geosynchronous satellites

Abstract: High quality audio broadcasts at radio frequencies to mobile receivers at or near the earth's surface are provided by substantially simultaneous transmission of the same signal from two geosynchronous, spatially-separated satellites (#1 and #2) on the geosynchronous orbit which virtually eliminates multipath fading and foliage attenuation and thereby permits the use of a low-cost space segment.

Super-reltron theory and experiments

Abstract: A highly efficient, high-power microwave tube called super-reltron is reported. The authors have achieved operation at >400 MW with approximately 50% efficiency at 1 GHz, and 250 MW with 40% efficiency at 3 GHz. The RF pulse durations are typically a few hundred nanoseconds. These compact lightweight tubes do not require an external magnetic field. The RF output coupling is straightforward and delivers the power directly via the fundamental TE/sub 10/ wave in a rectangular waveguide without a mode converter. The key features of the tube include (i) generation of a well-modulated electron beam by periodic virtual cathode formation, (ii) postacceleration of the modulated beam to reduce the relative electron energy spread, and (iii) a multicavity output section that efficiently extracts power without RF breakdown. Various theoretical aspects of the device are discussed and the experimental results are summarized. >

Stepped frequency ground penetrating radar

Abstract: A stepped frequency ground penetrating radar system is described comprising an RF signal generating section capable of producing stepped frequency signals in spaced and equal increments of time and frequency over a preselected bandwidth which serves as a common RF signal source for both a transmit portion and a receive portion of the system. In the transmit portion of the system the signal is processed into in-phase and quadrature signals which are then amplified and then transmitted toward a target. The reflected signals from the target are then received by a receive antenna and mixed with a reference signal from the common RF signal source in a mixer whose output is then fed through a low pass filter. The DC output, after amplification and demodulation, is digitized and converted into a frequency domain signal by a Fast Fourier Transform. A plot of the frequency domain signals from all of the stepped frequencies broadcast toward and received from the target yields information concerning the range (distance) and cross section (size) of the target.

Comparison of urban propagation models with CW-measurements

Abstract: The European research committee COST 231 has developed a propagation model for the estimation of urban transmission loss in cellular mobile radio. This model considers the diffraction loss due to equidistantly spaced buildings along the direct radio path and the diffraction and scatter losses from the rooftop down to the mobile. CW measurements in the 900- and 1800-MHz bands were performed in the cities of Mannheim and Darmstadt, Germany. Mannheim can be characterized as a homogeneously structured urban area with a rectangular street grid. The land cover in Darmstadt, on the other hand, is characterized by an inhomogeneous structure. The influences of base station antenna height, radio frequency, and street orientation are analyzed in detail. The quality of path-loss estimation and the shortcomings of the COST-231 model are discussed. >

Multiple use digital transmitter/transceiver with time multiplexing

Abstract: A multiple use radio frequency (RF) transmitter comprises means (2, 8, 22) for generating a plurality of different digital information signals and means (12) for generating respective digital carrier signals for said information signals, said carrier signals having higher frequencies than their respective information signals. Further included are means (10) for modulating said digital carrier signals with their respective digital information signals and means (24) for accumulating said modulated signals to a composite digital signal. Digital-to-analog converter (DAC) means (26) are common to each of said modulated signals for converting said composite digital signal to a composite signal in analog format. Further provided are means (30) for altering the frequency of said composite signal to a desired radio frequency (RF) band and means (32) for excluding frequencies outside said desired RF band from said composite signal.

Mapping of the radio frequency magnetic field with a MR snapshot FLASH technique.

Abstract: A fast method to measure the radio frequency magnetic field intensity distribution during magnetic resonance examinations within objects or patients is proposed. Using a snapshot FLASH sequence, several images are obtained with different transmitter amplitudes for the single prepulse prior to the imaging sequence. The signal intensity at each pixel within a selected slice is diminished by this prepulse, unless the pulse angle is 0° or a multiple of 360°. This condition can be used to measure that transmitter amplitude U max for each pixel which causes an excitation of 360° by the prepulse. The values of U max can be visualized in an image and are inversely proportional to the radio frequency magnetic field strength at each position within the image. The method can also be used for in‐vivomeasurements, where it may be difficult to realize prepulses with sufficient high flip angles. In these cases, signal corresponding to a 180° prepulse can be used for calculating U max.

New results on the radio-far-infrared relation for galaxies

Abstract: The radio-FIR relation has been decomposed into thermal bremsstrahlung-FIR and synchrotron-FIR relation for a sample of 31 galaxies. Both radio emission components are tightly correlated with the FIR emission for early and late-type spiral as well as irregular galaxies. It follows that any mixture of the radio components produces a tight universal radio-FIR relation. At high radio frequencies, thermal bremsstrahlung can dominate the radio emission over a large range of luminosities so that the slope of the radio-FIR relation approaches that of the thermal bremsstrahlung-FIR relation and is therefore close to unity (0.97 +/- 0.02). At lower frequencies, synchrotron emission dominates and the slope approaches that of the synchrotron-FIR ratio which is significantly steeper than unity (1.33 +/- 0.10). The results are consistent with a scenario in which the FIR luminosities of galaxies scale in direct proportion to the star formation rate.

Powder Dynamics in Very High-Frequency Silane Plasmas

Abstract: Particulate contamination produced during plasma‐assisted deposition of amorphous silicon devices can be responsible for reduced quality and yield. The threshold for powder formation imposes an upper limit on the radio frequency (rf) power and hence the deposition rate. In this work, the parallel‐plate capacitor discharge volume is illuminated and global, spatio‐temporal powder dynamics are recorded by CCD camera for analysis. The onset of powder production is determined visually and from observed modifications to the discharge electrical properties such as the matching condition, the direct current self‐bias and the rf power transfer efficiency. A systematic study has been made of the powder‐free operational space as a function of rf power, rf frequency (13.56–70 MHz) and substrate temperature.

Radio-frequency beam conditioner for fast-wave free-electron generators of coherent radiation.

Abstract: A method for conditioning electron beams is proposed, making use of the TM{sub 210} mode of microwave cavities, to reduce the axial velocity spread within the beam, in order to enhance gain in resonant electron beam devices, such as the free-electron laser (FEL). Effectively, a conditioner removes the restriction on beam emittance. The conditioner is analyzed using a simple model for beam transport and ideal RF cavities. Analysis of an FEL is employed to evaluate performance with reduced axial velocity spread. Examples of FELs are presented showing the distinct advantage of conditioning.

DC offset correction for direct-conversion TDMA receiver

Abstract: The direct conversion receiver is designed for demodulating signals in a TDMA system, in which the carrier frequency may vary from time slot to time slot. The received RF signal is demodulated by mixing with a local oscillator (LO) in a quadrature mixer (MIX). Due to carrier leakage, and imperfections in the mixers (CO1,CO2), the demodulated signal may have a DC offset, which must then be compensated by baseband processing (OCC). Because of the varying carrier frequency, and of the varying gain of the front end amplifier (LNA), the amount of DC offset will vary from time slot to time slot. In order to account for this, a correction loop is provided with a memory (MEM), for storing correction parameters for each carrier frequency and gain setting. This correction is updated slowly over many time slots, so that the error variance will be small. In order to correct offsets before this slow loop has converged correctly, a wide bandwidth loop is also provided, functioning within individual time slots.

Treatment of water with static and radio frequency electromagnetic fields

Abstract: A device for purifying water of mineral contaminants, bacteria, protozoa, algae, fungus, and other undesirable impurities, as well as for preventing and removing scale from conduits and containers within which water is maintained, that utilizes an electronic circuit capable of generating a plurality of interacting electromagnetic fields. The circuitry is capable of generating the combination of a first static electromagnetic field of variable offset, a second radio frequency varying electromagnetic field, and a third low frequency varying electromagnetic field with high amplitude, short pulse width, spikes. In addition, the circuitry is capable of inducing a high negative ion concentration in the water within which electrodes connected to the circuitry are immersed. The invention anticipates the use of electrodes suitable not only for the flow of water about the electrodes through a conduit, but the placement of electrodes within a pool of water to be purified.

High power switch-mode radio frequency amplifier method and apparatus

Abstract: Disclosed are both methods and a circuit to achieve powers of many kilowatts in radio frequency amplification using a switch mode amplifier in a new class of operation. In operation the invention utilizes internal switch characteristics. Methods create a substantial voltage step at the end of a response time period which allows greater output power without increasing the maximum switch voltage, reduces the maximum switch voltage for the same power, and which permits reduction of the stress on the switch element. Utilization of internal varactor capacitance avoids undesirable circulating currents and avoids the effects of lead inductance. The design allows use of less expensive components and high voltage switches not manufactured for radio frequency applications by preferring a substantial internal capacitance to establish maximum power. Other components of the network circuitry are also coordinated with the internal varactor capacitance. Adjustment of the conduction angle for optimum power and elimination of a need to bias the driver are also disclosed.

Magnetic resonance imaging device

Abstract: PURPOSE:To correct the nonlinearity of an RF amplifier which amplifies the power of RF pulses and feeds RF signals to a transmission coil CONSTITUTION:The input voltage of an RF amplifier 12 is changed by switching switches A16, B17 to a b contact and this output is measured to obtain data about the nonlinearity of an RF amplifier 11 and data about the correction of the nonlinearity is arranged into a table from this nonlinearity data and this correction table is stored in the memory of a computer 5 or the like and RF pulse waveforms(shape, amplitude) fed to the RF amplifier 11 during image pickup are corrected by using the correction table as a reference whereby the linearity of the RF amplifier 11 is maintained

Radio-frequency electromagnetic fields associated with cellular-radio cell-site antennas.

Abstract: Because of a heightened public awareness of issues pertaining to the use of electromagnetic energy, concurrent with a rapid growth of the cellular telephone industry, a study was initiated to characterize the electromagnetic environment associated with typical cellsite antennas. In particular, the radio-frequency electromagnetic (RF) fields in the vicinity of several antenna towers, ranging in height from 46–82 m, were characterized by measurement. In all cases, the antennas were omnidirectional co-linear arrays. The maximal power densities considered representative of public exposure were found to be less than 100 μW/m2 (10 nW/cm2) per radio channel. Comparison of measured values with the corresponding values that were calculated from the free-space transmission formula indicated that the analytical technique is conservative (i.e., overestimates field levels). The measured and corresponding analytical values were found to be well below accepted exposure limits even when extrapolated to simultaneous and continuous operation of the maximal number of transmitters that would be expected to be installed at a cell-site. Additional measurements were made in the near field of the same antenna type in a roofmounted configuration. At a distance of 0.7 m from the antenna, the maximal power density in the main beam was found to be less than 30 W/m2 (3 mW/cm2) when normalized to sixteen radio channels (the maximal number used on a single antenna) and less than 30 mW/m2 (3μW/m2) at 70 m. In all cases, the effective radiated power (ERP) by each radio channel was 100 W referenced to a half-wave dipole. This paper describes the instrumentation and measurement techniques used for this study and provides a summary of the results. 1992 Wiley-Liss, Inc.

Multi-spectral seeker antenna

Abstract: A multi-spectral seeker system for detecting radiation in the infrared (IR) band and a plurality of radio frequency (RF) bands suitable for use in a guided missile is disclosed. The IR and RF seekers are coaxially integrated on a single gimbaled platform and share a common aperture without significant performance degradation. In the preferred embodiment, the IR seeker uses a Cassegrainian reflector assembly with surfaces reflective to IR but transparent to RF signals. The first RF seeker uses a resonant dichroic reflector antenna which is transparent to RF signals at frequencies no closer than 0.5 octaves from the dichroic reflector system operating frequency. The second RF seeker may use any suitable RF antenna system although a planar-array of multi-mode spiral antennas or a standard flat plate slotted array are preferred. The second RF seeker may operate in an active, semi-active, or passive mode. The IR seeker means and both RF seeker means together provide coaxial seeking in a single aperture covering three frequency bands without significant loss of performance in the RF antenna and IR sensor. The material on which the parabolic reflector is formed is tapered in thickness as a function of radius to eliminate phase delays introduced by the curvature as seen by the second RF seeker.

On the use of double probes in RF discharges

Abstract: The paper deals with the interpretation of double-probe characteristics in the presence of RF voltages. The theory analyses the equations of the double probe with the insertion of electric currents modulated by the radio frequency and an exact solution for the thin-sheath approximation is derived. The case of an ion curve strongly dependent on the voltage is also considered and leads to a new method of RF compensation and to a derivation which, together with the experimental results, gives the electron temperature. Measurements which support the above theory are also presented. The electron temperature obtained from the double-probe characteristic of a thin probe is found to agree well with the electron temperature obtained from single-probe analysis. The plasma density is obtained by the comparison of the ion curves derived from the double-probe characteristic with the radial motion theory. This method does not need a return electrode inside the discharge vessel.