Showing papers on "Frequency band published in 1990"

A frequency-dependent finite-difference time-domain formulation for dispersive materials

Abstract: The traditional finite-difference time-domain (FDTD) formulation is extended to include a discrete time-domain convolution, which is efficiently evaluated using recursion. The accuracy of the extension is demonstrated by computing the reflection coefficient at an air-water interface over a wide frequency band including the effects of the frequency-dependent permittivity of water. Extension to frequency-dependent permeability and to three dimensions is straightforward. The frequency dependent FDTD formulation allows computation of electromagnetic interaction with virtually any material and geometry (subject only to computer resource limitations) with pulse excitation. Materials that are highly dispersive, such as snow, ice, plasma, and radar-absorbing material, can be considered efficiently by using this formulation. >

Phasor transformation and its application to the DC/AC analyses of frequency phase-controlled series resonant converters (SRC)

Abstract: A novel modeling technique based on phasor transformation that provides a unified model of series resonant converters (SRCs) is proposed. The approach gives explicit and simple equations that provide fruitful physical insight. When the switching frequency deviates from the resonant frequency, a first-order SRC model is obtained, and in the case of resonance a second-order model is obtained. It is shown that the frequency band of the second-order model is very narrow in practice. The time constant, small-signal gains, and system order are highly dependent on the switching frequency, load resistor, and output capacitor. >

Radio frequency receiver for a NMR instrument

Abstract: A receiver processes an NMR signal to produce a baseband image information signal from which two quadrature component signals are derived. An intermediate frequency section mixes the received NMR signal with two reference signals to shift the image information into a frequency band having a bandwidth BW and centered at a frequency that is 1.5 times the bandwidth BW. The resultant signal is filtered to remove extraneous signals outside the image information band. An analog to digital converter samples the filtered signal at a rate that is twice the bandwidth BW and digitizes the samples into a digital signal. A quadrature detector derives I and Q output signals from the digital signal by alternately selecting digital samples and negating every other sample selected for each of the I and Q output signals. The quadature detector also digitally filters the I and Q signals which are then used to construct an NMR image.

Frequency selective surface (FSS)

Abstract: A frequency selective surface (FSS) for incorporation into the outer skin of an aircraft, for transmitting electromagnetic energy in a predetermined frequency band. The FSS includes three layers sandwiched together with a dielectric material. Arrays of apertures are formed in the two outer layers, which are conductive. The inner layer consists of patches of conductive material. The apertures and patches are in substantial alignment with one another. The apertures and patches can have the shapes of crossed-dipoles, circles, squares, tripoles and Jerusalem crosses. In a preferred embodiment, the shapes of the apertures and patches are geometrically congruent. A dual-band FSS, having apertures and corresponding patches in two different sizes and spacings, can transmit two separate frequency bands.

A photonic variable RF delay line for phased array antennas

Abstract: It is shown that by applying spatial frequency-dependent phase compensation in an optical heterodyne process a variable RF delay line can be synthesized over a prescribed frequency band. Experimental results which demonstrate the performance of the delay line with regard to both maximum delay and resolution over a broad bandwidth are presented. A spatially integrated optical system is proposed for control of phased array antennas. The integrated system provides mechanical stability essentially eliminates the drift problems associated with freespace optical systems, and can provide high-packing density. The approach uses a class of spatial light modulator known as a deformable mirror device and leads to a steerable arbitrary antenna radiation pattern of the true time-delay type. >

Relationships between activity of sympathetic nerve pairs: phase and coherence

Abstract: The coherence function and phase spectrum were used to study the relationships between the discharges of sets of two postganglionic or preganglionic sympathetic nerves in baroreceptor-denervated cats anesthetized with either 5,5-diallylbarbiturate-urethan or alpha-chloralose. Most of the power in sympathetic nerve discharge was contained between 2 and 6 Hz. The coherence values relating the activity of two nerves were significantly different from zero within this frequency band. The phase spectrum was either linear or complex (i.e., showed changes in slope) within the coherent frequency band. We observed three patterns of relationship. The first pattern was characterized by a constant interval between activity in different sympathetic nerves within the coherent frequency band. The second pattern was characterized by a frequency-dependent interval. The third pattern was characterized by uncoupling of the 2- to 6-Hz rhythms in the discharges of different nerves. Switching between these patterns was observed. We suggest that the three patterns reflect different functional states of the central system responsible for the 2- to 6-Hz rhythm. Two models of this system are entertained. The first model is one of a system of coupled oscillators while filtering circuits that receive common inputs are the elements of the second model.

Digital signal encoding apparatus

Abstract: A digital signal encoding apparatus in which input digital signals are divided into a plurality of frequency bands so that the bandwidth of each frequency band will be greater the higher the frequency band, the allowable noise level is set for each frequency band on the basis of the energy value of each frequency band, and components of each frequency band are quantized with the number of bits consistent with the level difference between the energy of each frequency band and the preset allowable noise level. The output information volume following the quantization is detected and the number of bits of allocation for quantization is corrected in dependence upon the error between the detection output and the target value to render the information volume constant over a predetermined time period to enable bit rate adjustment or bit packing with lesser signal deterioration by a simplified construction.

Multifunctional aperture coupled stack patch antenna

Abstract: A multifunctional aperture coupled stacked patch antenna is presented that operates at dual frequency bands. At the lower frequency band, the antenna exhibits a 2:1 VSWR bandwidth of 20%, and at the higher frequency band, it behaves like a conventional narrowband patch antenna with a 2:1 VSWR bandwidth of 3%. Measured VSWR and far-field radiation patterns as well as numerical predictions are presented. A proper choice of the patch dimensions has been found critical to achieve this multifunctional performance.

Disc player for playing different types of discs which automatically identifies the type of a currently loaded disc

Abstract: A disc player capable of playing discs of different types such as: discs where only a digital audio signal is contained in a first frequency band; discs where only an analog audio signal is contained in a frequency band other than the first frequency band; and, discs where digital and audio signals are contained in the first frequency band and the other frequency band. Processing arrangements are further included to determine (according to disc size and/or presence of a table of contents) whether a currently loaded disc is recorded in the same format as a previous disc which was loaded immediately prior thereto, and if the same, playing the currently loaded disc according to the same reproduction settings as those for the previous disc.

Estimation of Directional Wave Spectra from Multicomponent Observations

Abstract: An improved method for estimating the directional spectrum of linear surface gravity waves from in Situ observations is presented. The technique, a refinement and extension of the inverse method of Long and Hasselmann, is applicable to multicomponent wave measurements at fixed locations in constant or slowly varying depth water. On a frequency band by frequency band basis, an estimate of the directional distribution of wave energy S(θ) is obtained by minimizing a roughness measure of the form ∫dθ[d2S(θ)/dθ2]2 subject to the constraints: (i) S(θ) is nonnegative with unit integral, (ii) S(θ) fits the data within a chosen statistical confidence level, and (iii) S(θ) is zero on any directional sectors where energy levels are always relatively low because of the influence of geographic surroundings. The solution to this inverse problem is derived through a variational formulation with Lagrange multipliers. A series of simulations using the new estimator show the fundamental limitations of sparse array...

Broadband microwave slot antennas, and antenna arrays including same

Abstract: A broadband microwave antenna exhibiting high radiation efficiency over a broad frequency band in which the VSWR is less than 2.5:1 over at least 15% of the frequency band, includes a ground plane at one side of a dielectric substrate and formed with at least one slot, and a feed strip at the other side of the substrate. The feed strip is of uniform width for substantially its complete length, but includes a change in width at the feed end of the slot to produce a first impedance matching network effective to bring the slot impedance to the level of the feed line over the broad frequency band, and another change in width at the load end of the slot to produce a second impedance matching network which reduces the slot reactance to match the reactive impedance of the load to the reactive part of the slot impedance over the broad frequency band.

Activation in unipolar cardiac electrograms: a frequency analysis

Abstract: Several detectors of local activations in unipolar cardiac electrograms are discussed. The detectors are based on the frequency content of the waveforms. For this study, myocardial regions with no local electrical activity were created with cryoablation in canine ventricles, so that the characteristics of electrograms reflecting local activation could be compared with those with only distant electrical activity. For each electrogram, representations, of the original signal were created using the output of bandpass filters; for each representation, the value of the maximum amplitude was taken as a measurement of the frequency content of the electrogram in that frequency band. The content of each frequency band and the first derivative of the signal were tested as discriminators between local and distant electrical activity. Combinations of frequency bands were also tested using a logistic regression technique. It was found that a detector based on multivariate analysis of different frequency components of a signal may be more effective than single-band filtering in discriminating between local and distant electrical activity in the heart, especially when those components have very different magnitudes. >

On adaptive multiband signal detection with the SMI algorithm

Abstract: In a nonstationary and/or nonhomogeneous interference environment, an adaptive system for target detection may suffer a severe performance degradation due to the lack of a sufficient amount of data from which the system can learn (estimate) the statistics of the environment. The detection performance of an adaptive system, which employs a frequency diversity (multiband) signaling waveform and a multiband sample matrix inversion algorithm (SMI), is analyzed. By comparison with the corresponding single-band system under the chosen system constraint, it is shown that the multiband system can significantly outperform the single band when the amount of data available from a single frequency band is severely limited by the environment. >

Sparse codebooks for the quantization of nondominant sub-bands in image coding

Abstract: A full-motion video coder based on a threedimensional subband framework is developed. The original image data is decomposed into different spatial-temporal frequency bands and based on the characteristics of the data in each band, different quantization strategies are applied to each band. The lowest spatial-temporal frequency band is coded using a three dimensional switched predictor and optimum scalar quantizer. The nondominant subbands which contain perceptually significant data are coded using a vector quantization approach in which the sparse, highly structured nature of the data in the higher-frequency subbands is exploited in the design of the codebook. >

Evidence for wind-driven current fluctuations in the eastern North Atlantic

Abstract: Evidence is presented for local and remote wind-forcing of deep ocean currents in three frequency bands (corresponding to 3.7–8 day, 8–22 day, and 23–82 day periods) at a two-year mooring in the eastern North Atlantic. Coherence between current meter velocities and estimated wind stress curl is largest for the deepest instrument (3000 m) and decreases toward the surface. The strongest evidence is found at 3000 m depth in the high frequency (3.7–8 day) band, where both components of velocity are significantly coherent with wind stress curl. Meridional velocity at 3000 m is also significantly coherent with wind stress curl in the low frequency (23–82 day) band, while zonal velocity in the low frequency band and both velocity components in the intermediate frequency (8–22 day) band are only marginally coherent. The results are compared with a stochastically-forced linear quasi-geostrophic model that includes a simple representation of the mid-Atlantic Ridge. Good agreement between predicted and observed coherences is found for the 3000 m currents in the high frequency band, but only partial agreement is found between predicted and observed coherences in the low frequency band, and little agreement in the intermediate band. Kinetic energy levels at 3000 m are well predicted in all three bands. For the shallower instruments (1100 m and 500 m), observed coherences are at most marginally significant and energy levels are considerably higher than the theory predicts. Including an upper-layer mean flow in the model improves the prediction of energy levels at the shallower instruments.

Polarization independent frequency selective surface for diplexing two closely spaced frequency bands

Abstract: A frequency selective surface suitable for separating two closely adjacent frequency band microwave signals. The frequency selective surface comprises a symmetrical array of discreet open center outer conductor elements and an open center conductor element centered in spaced relationship within each of the outer conductor elements. Both square loop and circular loop conductor elements may be employed. The spacing between corresponding parts of the conductor elements is periodic. The spacing between adjacent edges of the outer conductor elements and the spacing between adjacent edges of the outer and inner conductor elements are maintained within predetermined ratios of the wavelength of a microwave signal to be reflected by the surface providing a frequency selective surface in which the ratio of transmitted microwave signals to the frequency of reflected microwave signals is 1.15.

Electrical properties of unidirectional carbon-epoxy composites in wide frequency band

Abstract: The square-array model with infinite chains of contact is proposed to account for the dependence of electrical properties on frequency and fibre volume fraction in unidirectional carbon-fibre/epoxy-resin composites. The longitudinal AC conductivity, E vector parallel to fibre axis, decreases with increasing frequency due to magnetic flux change from neighbouring fibres and increases linearly with fibre volume fraction due to increment of specific fibre cross section within a volume element. A longitudinal permittivity of nearly unity for the frequency range of 1.7 GHz to 10 GHz, irrespective of fibre volume fraction, suggests that the composites can be considered as good conductors even at higher frequency. The three stages of increment and decrement with increasing frequency for transverse conductivity and permittivity, are ascribed to space charge polarisation. The increment of transverse AC conductivity with fibre volume fraction is ascribed to the increase of conducting channels.

Transmission system for still-picture TV telephone

Abstract: A still-picture TV telephone system for transmitting and receiving, via a telephone circuit, a still picture taken by a camera; a picture signal stored in a memory is modulated in phase and amplitude digitally and is offered or supplied to the telephone circuit during talking in voice. To superposing the talk in voice also during offering of the picture signal, the voice and picture signals are divided by frequency band. The frequency band of the voice signal is partially restricted, and the picture signal is inserted in the omitted band, during which time the picture signal is restricted in band by setting the band rate frequency to be lower than the carrier signal frequency when modulating the phase and amplitude. The picture signal is further restricted in band by a low-pass filter so that the voice and picture signals are separated by band so as not to overlap each other at the same frequency band.

Method and apparatus for locating an abnormality in a gas-insulated electric device

Abstract: Disclosed is a method and apparatus for locating an abnormality in a gas-insulated electric device, operative to locate the spot of an insulation abnormality, which emerges inside a metallic container (3), from outside of said metallic container. A plurality of detectors (S₀-Sn) disposed at certain positions in the metallic container produce detected signals, from which spectrum strengths (YH) in a high frequency band of 500 MHz or above are evaluated, and a position which renders a maximum spectrum strength is calculated from the relation between the spectrum strengths and the installation positions of the detectors and is determined to be the spot (x) of partial discharge. The apparatus comprises a plurality of detectors (S₀-Sn) disposed at certain positions in the metallic container, means (21) for analyzing the frequency spectrum of detected signals provided by the detectors thereby to evaluate the spectrum strength, and means (22) for calculating a position, which renders the maximum spectrum strength, from the installation positions of detectors and the spectrum strengths and determining the position to be the spot of partial discharge. The detectors detect the propaga­ tion of an electromagnetic wave caused by the partial discharge, and the spectrum strengths are evaluated from the detected signals. Since the electromagnetic wave is attenuated under the influence of the wall resistance, etc. of the metallic container, different spectrum strengths result from detector signals depend­ ing on the distance from the spot of partial discharge to each detector. Accordingly, a position which renders the maximum spectrum strength can readily be calculated from the relation between the magnitudes of spectrum strength of detectors and the installation positions of detectors, and the spot (x) of partial discharge can easily and promptly be located.

Infrared sensor suitable for fire fighting applications

Abstract: An infrared detector particularly well suited for the detection of heat sources, specifically from fires in an outdoor environment. The infrared detector comprises an infrared sensor receiving infrared radiation from a focused refractive optical unit. The infrared radiation from the optical unit is appropriately filtered so as to optimize the reception of infrared radiation by the detector in a frequency band of within about 2.5 to 5 microns. The sensor is configured utilizing a linear matrix of individual infrared sensing elements which may be flexibly applied to modify the field of view of the sensor. The detector contains appropriate power and signal amplification circuitry so as to provide an electrical output signal corresponding to infrared radiation received in the desired frequency band. The entire detector may be housed in a hermetically sealed unit appropriate for outdoor use and mounted on a movable pedestal for inclusion in an overall fire protection system such as, for example, a forest fire detection and warning system.

Target identification using optimization techniques

Abstract: A scheme for synthesizing bandlimited radar waveforms for detection and discrimination of targets is presented. The procedure, which involves a maximization of the ratio of the energy within a specified time interval to the total energy of the received waveform, can be used to synthesize a bandlimited generator waveform that produces a target response with the majority of its energy concentrated in the specified time duration. For an appropriate time interval, which depends on the bandwidth, such response exhibits energy concentration comparable to that produced by the K-pulse and E-pulse waveforms. The analysis does not require a numerical or empirical determination of the target's complex poles, since the desired waveform is obtained directly from the knowledge of the system transfer function over the frequency band of interest. The procedure is based on an analytical optimization, so no iterations or nonlinear programming techniques are required. The method is illustrated using simple thin-wire models as targets and antennas. Numerical examples are given, and the effects of bandwidth, duration of optimization, and aspect angle are discussed. >

Monaural envelope correlation perception, revisited: Effects of bandwidth, frequency separation, duration, and relative level of the noise bands

Abstract: This article presents the results of two experiments investigating performance on a monaural envelope correlation discrimination task. Subjects were asked to discriminate pairs of noise bands that had identical envelopes (referred to as correlated stimuli) from pairs of noise bands that had envelopes which were independent (uncorrelated stimuli). In the first experiment, a number of stimulus parameters were varied: the center frequency of the lower frequency noise band in a pair, f1; the frequency separation between component noise bands; the duration of the stimuli; and the bandwidth of the component noise bands. For a long stimulus duration (500 ms) and a relatively wide bandwidth (100 Hz), subjects could easily discriminate correlated from uncorrelated stimuli for a wide range of frequency separations between the component noise bands. This was true both when f1 was 350 Hz, and when f1 was 2500 Hz. In each case, narrowing the bandwidth to 25 Hz, or shortening the duration to 100 ms, or both, made the task more difficult, but not impossible. In the second experiment, the level of the higher frequency noise band in a pair was varied. Performance did not decrease monotonically as the level of this band was decreased below the level of the other band, and only showed marked impairment when the level of the higher frequency band was at least 60 dB below that of the lower frequency band. The pattern of results in these two experiments is different from that which is obtained when the same stimulus parameters are varied in experiments investigating comodulation masking release (CMR). This suggests that the mechanisms underlying CMR and those underlying the discrimination of envelope correlation are not identical.

Design of a small-aperture steering mirror for high-bandwidth acquisition and tracking

Abstract: The high bandwidth steering mirror (HBSM) prototype is the product of a research program established to develop a high-bandwidth, low angular range, two-dimensional beam steerer frequently found in optomechanical pointing, acquisition, and tracking systems. This research centered around the optimization of a beam-steering mechanism composed of mirror, restoring flexure, actuators, position sensors, and encompassing housing. Various design trade-offs and manufacturing issues involved in building the prototype are discussed, and the performance data are presented. The resulting HBSM design allows integration with a simple closed-loop control scheme. The mirror/controller has a closedloop bandwidth of 10 kHz and 10 peak-to-peak stroke (mirror normal) at low frequencies. This increased bandwidth yields excellent disturbance rejection in the 10 to 1000 Hz frequency band and enables the generation of faster scan patterns.

Mixed mode regulation controller for a resonant power converter

Abstract: Frequency variation in a resonant zero voltage switching (ZVS) power converter is combined with supplemental duty cycle modulation to obtain voltage regulation and to narrow the required frequency band needed for regulation of a given output load. These two regulation processes are applied simultaneously to the power switch control in order to accommodate a wider range of regulated load. Hence in the alternative, for a given load range a smaller frequency range is needed than would be the case if frequency variation alone is used. The regulation control circuit includes both a duty cycle modulator and a frequency modulator each being responsive to an error signal responsive to a differential between an output voltage of the converter and a reference voltage. Each individual modulator has its own transfer function; one transfer function being kHz/volt the other transfer function being % duty cycle/volt. The regulation control circuit output is a rectangular pulse type signal that contains both elements of frequency modulation and duty cycle modulation.

Estimation of structured covariance matrices and multiple window spectrum analysis

Abstract: A close relationship between low rank modeling and multiple window spectrum estimation is demonstrated by using maximum likelihood estimates of structured covariance matrices. The power in a narrow spectral band is estimated by estimating the variances in a low rank signal plus noise covariance model. This model is swept through the entire frequency band to obtain an estimate of power as a function of frequency. The resulting spectrum estimates are given by weighted combinations of eigenspectra. Each eigenspectrum results from projecting the data onto an orthogonal component of the signal subspace and squaring. The multiple window spectrum estimates of Thomson (1982) correspond to a particular choice for the low rank signal model. The low rank modeling and structured covariance matrix framework is also used to derive the maximum likelihood estimate for the center frequency of a signal in noise. This estimate is also obtained from a weighted combination of eigenspectra. >

Temperature measuring method using infrared sensors and processor

Abstract: A method of measuring the temperature of a remote body which comprises determining the frequency range to be covered and the portion of that range or frequency band to be covered by each detector of a detector array or by a single detector via a filter for applying different frequency bands to the single detector. The required detector or detectors are then provided and sense the energy radiated by the remote body whose temperature is to be measured. The detector or detectors then provide an output of the amount of energy measured in each unique frequency band and sends these measured outputs to a processor. The processor receives the measured outputs from the detector(s) and determines therefrom in accordance with an algorithm thereat the measured temperature. The processor then provides an output indicative of the measured temperature.

Microwave detectors based on granular high-T/sub c/ thin films

Abstract: The detecting and mixing properties of microstrip superconducting Y-Ba-Cu-O and Bi-Ca-Sr-Cu-O thin-film structures deposited on various substrates have been investigated. The device performance was tested in the 22-, 55-, and 110-GHz frequency bands at temperatures ranging from 100 K to about 50 K. The sensitivity obtained at 110 GHz was comparable to that of crystalline detectors. Mixing experiments were performed in the 25-GHz frequency band and indicated that the detector response time is less than 40 ps. The intermediate frequency was varied from 50 MHz to 5 GHz without any decrease in the mixer output up to 3 GHz. Auxiliary emission measurements performed at 12 GHz and down to 4.2 K revealed that the detector low-temperature performance limit is associated with microwave radiation from clusters of intergrain weak links arranged in multiloop quantum interferometers. >

Apparatus for detecting sonar signals embedded in noise

Abstract: Apparatus for detecting sonar signals embedded in noise includes a neural network trained to detect signals in response to the slope of amplitude rank ordered noise corrected powers. A detector detects an analog waveform. Means samples and digitizes the analog waveform to obtain digital samples which in turn are passed through a cosine window. The digital samples are Fourier transformed into conjugate sets of complex numbers representing amplitude and phase. One conjugate set of the complex numbers are discarded, and the remaining complex numbers ranked according to frequency. The sum of the square of the real and imaginary component of each of the remaining complex numbers in a frequency band are provided to obtain a corresponding series of representing estimated power ranked by frequency over the band. The noise contained in subbands of the band is estimated. Each estimated power is then divided by the estimated noise of the subband containing the estimated power to obtain corresponding noise corrected powers, which are ranked ordered according to amplitude. The amplitude rank ordered noise powers are provided to corresponding inputs of the neural network.