Showing papers on "Frequency band published in 1987"

Incorporation of attenuation into time-domain computations of seismic wave fields

Abstract: The only numerically tractable way yet found to incorporate attenuation into numerical time-domain computations of seismic wave fields is to approximate the viscoelastic modulus by a low-order rational function of frequency. The coefficients of this function can be determined by the Pade approximation. Our test computations show, however, that this approximation generally is of poor quality. Therefore, we suggest a new approach which is based on the rheological model of the generalized Maxwell body, which has a modulus of the desired rational form. We choose the relaxation frequencies logarithmically equidistant in the frequency band of interest, and determine the weight factors by simple numerical curve fitting to an arbitrary Q law. This approach is superior to the method above both in accuracy and in computational efficiency. For most practical applications, approximations of orders 2 or 3 are sufficient. The computing time and memory requirements for a finite-difference calculation are then approximately twice those of a purely elastic calculation. As a first application of the method, we compute SH channel waves in discontinuous coal seams with Q = 50 within the coal. The results show that the high-frequency Airy phase is strongly attenuated. This indicates that care has to be taken in comparing the results of purely elastic model calculations of the propagation of seam waves with experimental data.

Frequency dependent polarization analysis of high-frequency seismograms

Abstract: We present a multitaper algorithm to estimate the polarization of particle motion as a function of frequency from three-component seismic data. This algorithm is based on a singular value decomposition of a matrix of eigenspectra at a given frequency. The right complex eigenvector zˆ corresonding to the largest singular value of the matrix has the same direction as the dominant polarization of seismic motion at that frequency. The elements of the polarization vector zˆ specify the relative amplitudes and phases of motion measured along the recorded components within a chosen frequency band. The width of this frequency band is determined by the time-bandwidth product of the prolate spheroidal tapers used in the analysis. We manipulate the components of zˆ to determine the apparent azimuth and angle of incidence of seismic motion as a function of frequency. The orthogonality of the eigentapers allows one to calculate easily uncertainties in the estimated azimuth and angle of incidence. We apply this algorithm to data from the Anza Seismic Telemetered Array in the frequency band 0 ≤ ƒ ≤ 30 Hz. The polarization is not always a smooth function of frequency and can exhibit sharp jumps, suggesting the existence of scattered modes within the crustal waveguide and/or receiver site resonances.

System for monitoring fluent material within a container

Abstract: The acoustical energy output of a source is varied in frequency between limits to sweep a frequency band encompassing cavity resonance conditions which depend on the quantity of a fluent material within a tank excited by such acoustical energy. The volume of the fluent material is calculated from the excitation frequency registered during verified detection of resonance conditions, based on abrupt changes in signal characteristics and stored data relating to the geometry of the acoustic sensor arrangement through which the tank interior is monitored.

The Matrix Amplifier: A High-Gain Module for Multioctave Frequency Bands

Abstract: The characteristics of a new type of amplifier that makes simultaneous use of the additive and the multiplicative amplification process in one and the same module is discussed. The device, which achieves high-gain performance over multioctave bands, is a relative of the distributed amplifier. Initial experimental results demonstrated a small-signal gain of G = 13.8+-0.8 dB with -11.4 dB of maximum return loss between 2.0 and 21.5 GHz when using MESFET's manufactured on ion-implanted substrate material and G = 16.8+-0.9 dB gain over the 2.3-20.3-GHz frequency band in the case of vapor-phase-epitaxiaI material. The principle, the theory, and the experimental results are discussed in detail.

Geoacoustic parameters in a stratified sea bottom from shallow‐water acoustic propagation

Abstract: Due to the difficulty of direct measurement, there is a need to develop inverse techniques for remote sensing bottom geoacoustic parameters in the lowan mode measurements are extended to extract acoustic attenuation and speed in a horizontally stratified bottom in shallow water as a function of frequency and depth. The computational and experimental results show that, for a limited frequency band, we can find an equivalent depth profile of sea‐bottom acoustic attenuation with a linear frequency dependence that simulates the effect of nonlinear frequency dependence (without depth structure) on some field characteristics, such as the attenuation rate of individual mode, the frequency response of long‐range sound propagation, and the amplitude ratio of mode 2 to mode 1. However, the resultant equivalent negative gradient for the sea‐bottom attenuation is too strong to be accepted in light of available data. The conclusion is that nonlinear frequency dependence of the acoustic attenuation in the upper sedimentary layer is required to explain many aspects of shallow‐water sound propagation.

New surface wave launchers for sustaining plasma columns at submicrowave frequencies (1–300 MHz)

Abstract: We describe new surface wave launchers intended to sustain plasma columns with high‐frequency (hf) power in the range of 1–300 MHz. This frequency range complements that of the existing surface wave plasma generators (surfatron and surfaguide launchers), which are best suited to operate in the microwave domain (≳300 MHz). With the addition of this new type of launchers, called Ro‐box, it is now feasible to achieve a surface wave produced plasma column at applied frequencies ranging from 1 MHz to 10 GHz. The possibility of using the same physical hf coupling mechanism (surface wave propagation) over such a broad frequency band is an unprecedented situation with hf produced plasmas. This is of interest for both fundamental and applied physics, as it allows frequency effect investigations and frequency optimization of plasma processes. The new devices can be made so that the wave‐launching and the impedance‐matching functions are physically separated, leading to a modular design, a feature that facilitates c...

Satellite communications system having frequency addressable high gain downlink beams

Abstract: A satellite communications system employs separate subsystems for providing broadcast and point-to-point two-way communications using the same assigned frequency band. The broadcast and point-to-point subsystems employ an integrated satellite antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones in a manner which permits multiple reuse of the entire band of assigned frequencies. A single pool of solid state transmitters allows rain disadvantaged users to be assigned higher than normal power at minimum cost. The intermodulation products of the transmitters are geographically dispersed.

Long distance fiber-optic transmission of C-band microwave signals to and from a satellite antenna

Abstract: We demonstrate here the use of high-speed semiconductor lasers and detectors with low loss optical fiber for the transmission of the 500-MHz C-band microwave signal spectrum to and from a satellite antenna. The optical system has low attenuation and large band, width, so the microwave signals can be transmitted directly at the microwave frequency (4 or 6 GHz) over 20 km of fiber without regeneration or qualization. The optical system introduces ≈ 1 dB or less of degradation for both low spectral density QPSK signals and high spectral density FM video signals present in typical satellite transmission systems. For the video signals, the signal to noise after AM conversion was reduced from 56 to 55 dB. The microwave drive level to the laser must be significantly larger (typically > -10 dBm) than the intensity noise of the laser, yet small enough (typically < 10 dBm) to reduce the intermodulation distortion signals to an acceptable level. The laser bias current must be several times threshold so that the resonance frequency is beyond the frequency band of interest (4-6 GHz in this case). A flat modulation response is then obtained, and the intensity noise and intermodulation levels are low.

Reducing residual vibration in systems with time-varying resonances

Abstract: Robots which perform rapid motions tend to excite system resonant frequencies. In order to perform a sequence of tasks more quickly, both move time and vibration settling time must be reduced. In addition, this must be accomplished in the presence of time-varying resonant frequencies. In this paper, functions are derived which accomplish open-loop moves rapidly with greatly reduced residual vibration amplitude, as resonant frequencies are allowed to vary by ±10%. First, a direct relationship is established between the frequency spectrum of the input functions and the resulting residual acceleration amplitude. This establishes a specification on spectral magnitude at resonance which must be satisfied in order to bring residual vibration within acceptable bounds. Then functions are derived as a series expansion of ramped sinusoid functions with coefficients chosen so as to minimize spectral magnitude in a frequency band surrounding resonance. Finally, some simulations are performed to indicate that these functions can reduce residual vibration considerably even when the resonant frequency changes by 10%.

Broadband local area network

Abstract: A broadband local area network transmits outbound signals in an outbound frequency band and inbound signals in an inbound frequency band. In one configuration, the network comprises a central hub and a plurality of nodes connected to the hub. Each of the nodes comprises a bidirectional amplifier having a fixed gain across the entire inbound and outbound bandwidth and a line balancer. The line balancer includes a variable cable simulator circuit for simulating the loss of a variable amount of coaxial cable and an equalizer circuit for equalizing the loss of a fixed amount of coaxial cable. The network also includes a plurality of remote outlet clusters. Each remote outlet cluster is connected to one of the nodes by a coaxial transmission path and comprises a fixed attenuation and a RF splitter having a plurality of outlets for connection to user devices. The number of outlets of each remote outlet cluster depends on the length of the transmission path to the node to which it is attached.

Voice coding process and device for implementing said process

Abstract: The voice signal is analyzed to derive therefrom a low frequency base band signal, linear prediction coefficients and high frequency (HF) descriptors. Said HF descriptors include HF energy indications as well as indications relative to the phase shift between the low frequency and the high frequency band. Said HF descriptors are used during the voice synthesis operation to provide an inphase HF bandwidth component to be added to the base band prior to be used for driving a linear prediction synthesis filter tuned using said linear prediction parameters.

Sub-band coding of images using predictive vector quantization

Abstract: This paper presents a form of sub-band coding of images in which the advantages of both DPCM and vector quantization are combined in a single subband coder. A 16 band decomposition of the image frequency band is employed using a tree structure of two-dimensional separable quadrature mirror filters and decimators. The 16 parallel sub-bands created in this way are regarded as a 16-dimensional vector process and coded as such using a vector equivalent of DPCM. Here each vector is predicted as a single quantity and its residual is coded using vector quantization, this vector quantizer being nested within the vector-DPCM loop. As will be shown this technique yields a good SNR performance in the bit rate region between 0.4 and 0.7 bits per pixel, both for images inside and outside a large training set.

Multidriver loudspeaker apparatus with improved crossover filter circuits

Abstract: A loudspeaker system includes at least two loudspeaker drivers, together with an electrical crossover network having filter circuits with at least two separate mutually exclusive frequency passbands. The filter circuits comprising the crossover network each possess brick-wall amplitude responses, i.e., passbands with very high band-edge amplitude vs. frequency response slops, on the order of 100 dB/octave in the better embodiments. The high passband band-edge slopes, which are realized by the inclusion of transmission zeros in the separate crossover filter transfer functions, takes together with further appropriate crossover filter transfer function synthesis causes the separate loudspeaker drivers comprising the loudspeaker system to function independently of one another in their contribution to total system acoustic output. It is shown that the loudspeaker system permits an accurate approximation to the ideal delay function in acoustic space, while minimizing acoustic wave interference among drivers operating in adjacent frequency band, and also reducing overall system nonlinear distortion.

Satellite communications system having multiple downlink beams powered by pooled transmitters

Abstract: A satellite communications system employs separate subsystems for providing broadcast and point-to-point two-way communications using the same assigned frequency band. The broadcast and point-to-point subsystems employ an integrated satellite antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones in a manner which permits multiple reuse of the entire band of assigned frequencies. A single pool of solid state transmitters allows rain disadvantaged users to be assigned higher than normal power at minimum cost. The intermodulation products of the transmitters are geographically dispersed.

Confidential communication system

Abstract: The respective feature parameters extracted from a speech signal is converted into the corresponding line spectrum data in a first frequency band obtained by dividing the speech signal frequency band. Each of the line spectrum data is allocated previously to each one of the feature parameters. The extracted feature parameters are further converted into the corresponding line spectrum data in the other divided frequency bands other than the first frequency band. The converted line spectrum data are multiplexed for transmission. The corresponding line spectrum data in the divided frequency bands allocated to the same feature parameter are logically added to restore the feature parameters. Thus higher confidential communication is realized.

Antenna system for hybrid communications satellite

Abstract: A satellite communications system employs separate subsystems for providing broadcast and point-to-point two-way communications using the same assigned frequency band. The broadcast and point-to-point subsystems employ an integrated satellite antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones in a manner which permits multiple reuse of the entire band of assigned frequencies. A single pool of solid state transmitters allows rain disadvantaged users to be assigned higher than normal power at minimum cost. The intermodulation products of the transmitters are geographically dispersed.

Optimum time domain signal transmission and source location in a waveguide: Experiments in an ideal wedge waveguide

Abstract: Source location experiments in a wedge waveguide were conducted to test a single‐channel time domain technique for a known source. The laboratory measurements were made in the air wedge having rigid sheet rock boundaries. The wedge angle was 6.6°. The source transmitted a short oscillatory signal having a frequency band of 3–13 kHz. The distances of the source and receiver from the wedge apex were 1.85 and 3.13 m. The Biot–Tolstoy wedge solution was used to calculate the source‐to‐receiver impulse response. For the geometry, all of the arrivals are due to images. The waveguide matched filter was the time reverse of the theoretical impulse response. The output of the matched filter was cross correlated with the known input signal. Maps of possible source locations were made by (i) assuming a trial source location, (ii) computing a theoretical impulse response and the corresponding matched filter, (iii) filtering the signal, and (iv) mapping the peak absolute maximum of the matched filter output. The actual source location was correctly mapped. There were several lower amplitude maxima that gave false source locations. The Appendix includes a tutorial description of the Laplace generating function (z transforms) and their applications.

Wide-Band Directional Couplers in Dielectric Waveguide

Abstract: In this work, the H guide is proposed as an alternative in designing proximity directional couplers using dielectric guides in order to obtain coupling factors constant with the frequency, thus increasing the bandwidth of these devices. The propagation constants of the even and odd modes of the coupling zones are determined by means of Schelkunoff's method and the effective dielectric constant method (EDCM). Two directional couplers, 10 dB and 3 dB, made of polystyrene and Teflon respectively, were designed and measured to work in the millimetric frequency band (32-40 GHz). The inclusion of metallic walls in the curved zones avoids additional couplings and results in flat coupling. Furthermore, the metallic walls reduce the radiation losses and allow the coupling factor to be finely adjusted. The results obtained show a maximum coupling variation of +-0.5 dB for 20-percent bandwidth.

Self-compensating equalizer

Abstract: A multi-band graphic equalizer is provided that includes a plurality of controllable frequency response determining elements, each element having a primary effect in its primary frequency band and a secondary effect in at least one adjacent frequency band, a graphic input device for entering an input that graphically represents a desired frequency response characteristic, and a controller connected between the input means and the frequency response determining elements for calculating the necessary frequency response characteristic for each controllable frequency response element based on the in band and out of band effects of each element to produce the desired overall frequency response characteristic and for generating a plurality of sets of control signals, one set of control signals for each frequency response determining element, so that the overall frequency response characteristic produced by the plurality of frequency response determining elements is as close as possible to the desired frequency response characteristic entered at the graphic input device.

Seismic signal transmission system using relay radios

Abstract: A system is provided for radio transmission between a central control and recording laboratory and spaced apart acquisition boxes in which relay transmission units are used for communicating with boxes which are directly inaccessible. It includes a centralization unit adapted for managing the exchanges of orders or data from the central laboratory with directly accessible boxes by means of a radio link in a first frequency band (f2), on the one hand and with the relay units by means of a radio link in another frequency band (f1). These latter manage the radio links with the inaccessible boxes by means of radio links at a frequency (f3) distinct from (f2). The relay units and the centralization unit are programmed for automatically managing the links between the central laboratory and any one of the boxes, whether they are direct or indirect, and, on the other hand, for reserving a part of the interrogation time of each box for the transmission of service data indicative of their correct operation.

Signal recording conditioning device

Abstract: In a signal recording circuit, a dither signal in a frequency band of from a frequency higher than 1/2 of the sampling frequency of a recording signal to a frequency lower than (n-1/2) of the sampling frequency (where n is an integer larger than 1) is added to the recording signal in an addition circuit, and the output of the addition circuit is subjected to analog-to-digital conversion with a frequency n times the sampling frequency.

Beam-forming network

Abstract: A satellite communications system employs separate subsystems for providing broadcast and point-to-point two-way communications using the same assigned frequency band. The broadcast and point-to-point subsystems employ an integrated satellite antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones in a manner which permits multiple reuse of the entire band of assigned frequencies. A single pool of solid state transmitters allows rain disadvantaged users to be assigned higher than normal power at minimum cost. The intermodulation products of the transmitters are geographically dispersed.

Filter interconnection matrix

Abstract: A satellite communications system employs separate subsystems for providing broadcast and point-to-point two-way communications using the same assigned frequency band. The broadcast and point-to-point subsystems employ an integrated satellite antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones in a manner which permits multiple reuse of the entire band of assigned frequencies. A single pool of solid state transmitters allows rain disadvantaged users to be assigned higher than normal power at minimum cost. The intermodulation products of the transmitters are geographically dispersed.

Automobile remote-control system

Abstract: An automobile remote-control system is disclosed. This system is characterized such that: a transmitter includes a carrier oscillating circuit for oscillating a carrier with a frequency higher than a radio broadcasting frequency band or a telephone transmitting-and-receiving frequency band; an operation voltage of this carrier oscillating circuit is turned ON/OFF by a drive control circuit; the carrier oscillating circuit is turned ON for a preset period prior to the transmission of the code thereby to transmit a leader pulse signal; the carrier oscillating circuit is subsequently turned ON in a pulse-like configuration at an interval corresponding to the code thereby to transmit a code pulse signal; a receiver receives the signal from the transmitter through a radio receiving antenna or a telephone transmitting-and-receiving antenna; the receiver is kept in an operation-state by a periodic operation control circuit at a cycle shorter than a continuance time of the leader pulse signal; when the leader pulse signal is detected by a detecting circuit, the receiver is held in the operation-state by an operation holding circuit; and the code pulse signal is converted into a code in accordance with a pulse interval by a code converting circuit.

Audio frequency signal identification apparatus

Abstract: An audio frequency signal identification apparatus of the present invention has a correlator for obtaining the full band power component and autocorrelation coefficients of a digital audio frequency signal to be identified for each analysis time, a plurality of band pass filters for dividing and filtering the audio frequency signal into a plurality of bands, a plurality of integrators for full wave rectifying and integrating the outputs from the plurality of band pass filters for each analysis time so as to obtain individual power components for the respective bands, a first identification parameter calculator for calculating a maximum power component, a minimum power component, a frequency band yielding the maximum power component, a bandwidth, and a difference between the individual band power components in the respective analysis times, from the individual band power components as the outputs from the plurality of integrators, a second identification parameter calculator for calculating a difference between the autocorrelation coefficients as the output of the correlator and a difference between the autocorrelation coefficients in the respective analysis times, an identification and judgement processor for judging the signal type of the audio frequency signal to be identified in accordance with which of a plurality of predetermined judgement conditions the identification parameters obtained from the first and second identification parameter calculator and the full band power component correspond and if they satisfy a condition of continuity.

Dual band feed system

Abstract: A dual frequency band microwave antenna feed for a parabolic reflector has a circular waveguide for a low frequency band and a smaller circular waveguide for a higher frequency band disposed in and concentric with the low band guide. A twisted conductive baffle is disposed in each guide to permit 90° rotation of linearly cross-polarization signals and a pair of in-line ports is attached to each guide for output of such pairs of cross-polarization signals.

Decentralized cautious adaptive control system

Abstract: An output vector produced by a plant in response to an input vector is filtered by a plurality of bandpass filters, each filter having a different pass band. A filtered output vector from each filter is provided as input to a separate adaptive feedback controller, and feedback vectors produced by the separate controllers are summed to provide the input vector to the plant. Each adaptive feedback controller continuously identifies an open-loop transfer function of the plant characterizing a particular frequency band of plant operation, and continuously adjusts its own open-loop transfer function so that the poles of the closed-loop transfer function of the plant for that particular frequency mode of operation are assigned to constant stable values despite changes in the open-loop transfer function of the plant.

High frequency band-pass filter

Abstract: A high frequency band-pass filter which includes a single resonator or a plurality of resonators adapted to pass a high frequency signal of a predetermined frequency band region, and an active element device electrically coupled with one or the plurality of the resonators so as to present a negative resistance when the resonator is in a resonant state.