Showing papers on "Bandwidth (signal processing) published in 1988"

A non conventional power converter for plasma stabilization

Abstract: The authors present the design and control of a nonconventional converter with a peak power of 3.6 MVA and a bandwidth ranging from DC values up to 10 kHz for nuclear-fusion experiments. The novel control technique is based on a comparison between the current reference and the load current, through a hysteresis comparator system. The required voltage waveforms have been obtained in two specified frequency ranges, with a strictly unified control over the whole bandwidth range. The conditions on the maximum phase shift and the maximum switching frequency required have all been fulfilled. Excellent results obtained by digital simulation confirm the validity of the proposed control scheme. The converter described here has been studied and designed for an experiment that has recently been proposed within the framework of the JET (Joint European Torus) project. >

High-capacity coherent lightwave systems

Abstract: Recent theoretical work on coherent optical detection systems is reviewed and experimental results in high-speed coherent transmission are summarized. The theoretical advantages and limitations of the various modulation and detection formats are discussed and experimental progress towards the implementation of these systems is reviewed. The most significant obstacles to the attainment of quantum-noise limited detection at higher speeds are seen to be the requirement of uniform frequency response from electronic components and the local oscillator laser power requirement, which increases as the square of the bit rate. To make full use of the single-mode fiber bandwidth, frequency-division multiplexing of many moderate-rate channels is a very promising technique for local systems. For long-distance applications, frequency multiplexing is still possible but is limited by the need for optical amplifiers or wavelength-selective multiplexers. >

Aperture-coupled patch antennas with wide-bandwidth and dual-polarization capabilities

Abstract: The authors discuss the design of a wideband, dual-proportion, C-band patch radiator with a 20% bandwidth for input voltage-standing-wave ratio >

Secondary channel for digital modems using spread spectrum subliminal induced modulation

Abstract: A system and method are provided for use of a secondary channel in high speed (greater than 2400 bits per second) wire line modems to provide low speed (around 110 bits per second) non-interfering communications for control or monitoring functions. The secondary channel is used to monitor, configure and test the modems in the network using commands sent over the secondary channel. Secondary channels are also used in low duty cycle applications such as in alarms or teletype data transmission. The invention provides a spread spectrum secondary channel which occupies the same bandwidth as the primary data channel and is immune to jamming tones that would interfere with conventional frequency shift keying secondary channels. The spectrum spreading is achieved through use of the primary channel's trellis encoder. Phase shifts of 0°, 90°, 180° or 270° are induced depending upon the trellis state. This rotation simplifies decoding which as a result can be done in the reference system of the decoded primary trellis signals instead of the baseband equalizer reference system. The primary trellis decoder automatically resolves the 90° references. There is no penalty in terms of additional circuitry or software because the transmitter spreading eliminates the need for additional rotation in the receiver which is required for non-spread signals.

Performance analysis of an adaptive notch filter with constrained poles and zeros

Abstract: A statistical analysis of the sine-wave frequency estimate obtained by the adaptive notch filter (ANF) recently proposed by A. Nehorai (ibid., vol.ASSP-33, p.983-96, Aug. 1985) is presented. Asymptotically valid expressions for the bias and mean-square error of the frequency estimate are provided. The analysis verifies the performance exhibited by the ANF, and clarifies the role of some user variables, such as the notch bandwidth and the number of data samples processed. >

Comparison between computed and measured bandwidth of quarter-wave microstrip radiators

Abstract: A simple theory based on a cavity model is used to analyze a quarter-wavelength flat element, without a dielectric substrate, which is connected to the ground plane in one of its sides by a metallic wall. The bandwidth of the element is computed from this model. Very good agreement is obtained between the computed and the measured bandwidth for elements in the S-band. >

Polarization-independent electro-optically tunable narrow-band wavelength filter

Abstract: We propose and demonstrate the first polarization‐independent electro‐optically tunable wavelengh filter with single‐mode waveguides The Ti:LiNbO3 filter utilizes narrow‐band electro‐optic TE ↔ TM conversion and employs TE/TM polarization splitters in the input and output waveguides The filter operates at 152 μm with a bandwidth of only 12 A and can be electro‐optically tuned over at least 110 A at a tuning rate of 055 A/V

Method and apparatus for combining encoding and modulation

Abstract: A method and apparatus for combining encoding and modulation creates signal sets from available amplitude and phase modulations by indexing ordered subspaces. The subspaces need not be limited to the class of subspaces known as binary subspaces. The resultant signal sets, for a preselected power and bandwidth, are widely separated and unlikely to be confused by the effects of channel noise. Such signals can be in either finite block or convolutional form, depending on the natural format of the desired transmission. Further according to the invention are basic apparatus for encoding and modulating as well as demodulating and decoding a signal in accordance with the invention. Specifically, a method is provided for decoding the incorporates a specific type of decoding/demodulation techniques which develops accurate estimates of the information from the received signal in a computationally efficient manner and which permits high speed operation using softdecision decoders.

The relationship between tapped delay-line and FFT processing in adaptive arrays

Abstract: The use of fast Fourier transform (FFT) processing behind the elements in adaptive arrays is often considered as a means of improving the nulling bandwidth of such arrays. However, it is shown that the output signal-to-interference-plus-noise ratio obtained from an adaptive array with FFTs behind the elements is identical to that of an equivalent adaptive array with tapped delay-line processing. The equivalent tapped delay-line array has the same number of taps in each delay line as the number of time samples in the FFTs, and has a delay between taps equal to the delay between samples in the FFTs. Thus, while the bandwidth performance of an adaptive array can be improved by using time-delayed samples of each element signal, no further improvement results from taking FFTs of these sampled signals. The same bandwidth performance is obtained by simply weighting and combining the time-domain samples directly. >

Using spread-spectrum in a high-capacity fiber-optic local network

Abstract: A local fiber-optic communications network capable of supporting tens of thousands of simultaneous users, each requiring on the order of 10-Mb/s continuous data rate, is proposed. The system uses coherent optical techniques to fully utilize the vast bandwidth offered by single-mode optical fibers (tens of thousands of GHz) a spread-spectrum technique is used to circumvent the problem caused by the instabilities of present-day semiconductor lasers. These include difficulty in reliably setting a laser's frequency with an accuracy better than several hundred GHz, and phase noise in the laser output, which would otherwise result in excessive amounts of interference among the various users. The method proposed is a variant of code-division multiple access (CDMA) that is called random-carrier (RC) CDMA, since the modulated carriers can be assumed to be completely randomly placed in the available optical band. >

Digital fm demodulator with a reduced sampling rate

Abstract: A method of recovering a frequency modulated signal wherein the frequency modulated information signal is sampled at a rate less than the Nyquist sampling rate, following which the under sampled signal can be demodulated to extract the original information signal.

Apparatus for matching unbalanced R. F. baseband signals to balanced signals on a twisted two-wire line

Abstract: Impedance of each termination in a wiring environment, wherein unshielded twisted pair wires with a characteristic impedance at 10 MHz of about 85 to 130 ohms are used as a communication medium for a local area network where run lengths do not exceed about 250 feet between terminations, is matched at each termination by an impedance matching termination and line conditioning apparatus, hereinafter a matching device. The matching device matches impedance of twisted pair wiring to a nominal 50 termination over a bandwidth of d.c. to above about 10 MHz. The matching device comprises a low-pass filter coupled at a first port to a fifty-ohm standard BNC connector and at a second port to a tapped autotransformer means with a d.c. blocking capacitor for impedance matching and balanced to unbalanced conversion, which in turn is coupled to a common mode inductor, which in turn is coupled to terminals for each side of a twisted pair cable. The common mode inductor is for passing differential signals over the entire frequency spectrum while rejecting all common mode signals over substantially all of the frequency spectrum.

Spectrographic analysis of animal vocalizations: implications of the “uncertainty principle”

Abstract: Many animal vocalizations are non-periodic, frequency-modulated signals. Because this type of signal varies simultaneously in two dimensions, time and frequency, spectrographic measurement is constrained by the “uncertainty principle”: to increase accuracy of measurement in one dimension we must sacrifice accuracy of measurement in the other dimension. Although this trade-off is unavoidable, inherent in the measurement of frequency, for any particular frequency-modulated, non-periodic signal, there is an intermediate, optimal setting of spectrographic bandwidth, equal to the square root of the average rate of change of the measured signal. This optimal bandwidth minimizes the time-frequency smear, and thus permits the most accurate measurement of the instantaneous frequency. Investigators analyzing the microstructure of animal vocal signals therefore should choose their analyzer bandwidths to match the signals under study.

Digital image compression and transmission system employing visually weighted transform coefficients

Abstract: A system for compressing and transmitting a digital image signal over a limited bandwidth communication channel, transform codes the image values and quantizes the transform coefficients according to a two-dimensional model of the sensitivity of the human visual system. The model of the human visual system is characterized by being less sensitive to diagonally oriented spatial frequencies than to horizontally or vertically oriented spatial frequencies, thereby achieving increased compression of the image.

Bandwidth choice for density derivatives

Abstract: An adaptation of least squares cross-validation is proposed for bandwidth choice in the kernel estimation of the derivatives of a probability density. The practicality of the method is demonstrated by an example and a simulation study. Theoretical justification is provided by an asymptotic optimality result

Noise reduction system

Abstract: In order to overcome problems in an active noise reduction system of sound buffets at low frequency and signal enhancement caused by imperfect transfer functions of a noise cancelling sound generator and a microphone, one or more high pass filters for reducing low frequency signals are provided in a feedback loop between the sound generator and microphone. A low pass filter is provided for extending the bandwidth of the system but which does not introduce unduly large phase shifts.

The application of digital signal processing to transmitter linearisation

Abstract: A description is given of three prototype transmitters, using digital signal processing in different ways to achieve linear RF amplification. The Cartesian feedback technique, the adaptive predistortion technique, and the LINC (linear amplification with nonlinear components) technique, are all shown to provide a good degree of intermodulation product suppression (>50 dB), with the LINC amplifier approach potentially the most power-efficient (>70%). Both the LINC and predistortion systems can operate in a wideband mode. The Cartesian approach, because of the inherent feedback control system, is limited to a bandwidth of about 10% of the operating frequency. The implementation costs are considerably reduced over conventional analog realizations of such systems for all three techniques. >

Measuring and controlling signal feedback between the transmit and receive antennas of a communications booster

Abstract: A cellular communication system booster has plural narrowband and wideband channels each having down mixers, local oscillators, IF filters, and amplifiers for amplifying communication signals On a narrowband channel preferably not in use, and preferably in a time-multiplexed manner, a reference signal of known signal strength is substituted at the output amplifier in lieu of a normal communications signal When this amplified reference signal is broadcast at the output antenna a portion of the signal is feed back to the input antenna due to imperfect isolation between the antennas This feedback signal is measured by a Radio Signal Strength Indicator (RSSI) circuit at the point of its receipt into the IF filter of the narrowband channel The measured signal strength serves as a measurement of signal isolation/feedback between the antennas A microprocessor receiving the measurement and knowledgeable of the reference signal strength controls the gain of the channel amplifier so as to maintain the feedback signal 10 dB below the reference signal, thereby preventing RF signal feedback oscillation In different embodiments the reference signal may be derived from an injection oscillator or, alternatively, from an actual communication signal upon another communications channel Isolation/feedback measurements are typically stepped across multiple channels spanning the entire communications frequency band, thereby providing measurement and appropriate control at all frequencies and operational conditions

H ∞ robust control synthesis for a large space structure

Abstract: In a design study involving the use of H∞ optimal control theory, an 11-state control law is generated for a 116-state model of a large flexible space structure. A combination of co-located rate feedback, modal truncation, and optimal Hankel-norm model techniques is found to lead to a vastly simplified four-state model for the structure which, by singular-value theory, is proved to be satisfactory for design of a controller whose bandwidth exceeds the natural frequencies of all of the modes of the original 116-state model. Specifications regarding disturbance attenuation, bandwidth, and stability robustness are quantitatively expressed as weighting functions in a mixed-sensitivity H∞ optimal control synthesis problem, the solution to which is computed by the authors' program LINF.

Asynchronous communication system

Abstract: Methodology, and associated circuitry, for encoding and decoding signals utilize combined time-signal path diversity techniques that trade-off bandwidth for autonomy of communications and simplified signal processing, thereby improving reliability. Transmitter (800) processes an incoming data stream by partitioning the stream into contiguous blocks. Each block is encoded into a data frame for transmission over one or more paths comprising the medium interconnecting the transmitter and receiver. Frame encoding is effected by generating an array of pulses distributed in time as well as distributed across the paths. Receiver (100) is generally arranged as a correlation detector in that the receiver only responds to the particular time-signal path pattern for which it is configured. Sensors in energy transfer relation to the one or more paths are positioned at detection points on the paths in correspondence to the time-space pattern of the receiver. The outputs of the sensors are processed to produce a correlation signal at the given frame rate whenever the coded signal corresponding to the receiver configuration is propagating along the paths.

Bandwidth considerations for binary phase-only filters

Abstract: A binary phase-only filter (BPOF) bandwidth and, correspondingly, the performance with respect to stochastic noise are introduced as filter design parameters. A BPOF figure of merit is defined which references the matched filter. Analytical bounds on the BPOF signal-to-noise ratio are derived. The noise performance is illustrated with simulation results. It is demonstrated through analysis and simulation that BPOFs can be designed to perform well with respect to stochastic noise.

Real-time alkali monitoring system

Abstract: A fiber optics based optical emission line monitoring system is provided in which selected spectral emission lines, such as the sodium emission line, may be detected in the presence of interfering background radiation. A combustion flame is fed by a diverted portion of a process stream and the common end of a bifurcated or quadfurcated fiber optic light guide is adapted to collect light from the flame. The light is guided through the branches of the fiber optic cable to bandpass filters, one of which is adapted to each of the branches of the fiber optic light guide. The bandpass filters are centered at wavelengths corresponding to the emission lines to be detected and two separate filters are required for each species being detected. The first filter has a bandwidth of about 3 nms and the second filter has a bandwidth of about 10 nms. Light detectors are located to view the light passing through the bandpass filters and amplifiers are connected to receive signals from the light detectors. The amplifier corresponding to the bandpass filter having the narrower bandwidth is preset to scale the signal by a factor equal to the ratio of the wide and narrow bandwidths of the bandpass filters. This scaling produces a scaled signal from which the difference between the scaled signal on the other signal can be calculated to produce a signal having an amplitude directly proportional to the concentration of the species of interest and independent of background radiation.

Signal processor implementation of variable digital filters

Abstract: The implementation of two recently introduced variable digital filter schemes using a TMS320-series digital signal processor is presented. One is a method for updating the coefficients of an FIR (finite-impulse response) filter in a simple manner such that the cutoff frequencies can be controlled through a single parameter. The other is a method for tuning the cutoff frequency of an IIR (infinite-impulse response) filter with one parameter using a series expansion of the low-pass-low-pass frequency transformation. The measured frequency responses compare well with the theory. >

Resonant p-i-n-FET receivers for lightwave subcarrier systems

Abstract: A theoretical and experimental analysis of narrowband resonant direct-detection p-i-n-FET receivers for subcarrier multiple-access networks is described It is shown how a small inductance can be used to optimize the coupling between the p-i-n and FET, over a range of microwave subcarrier frequencies, minimizing the frequency-dependent thermal noise and leaving shot-noise as the ultimate limitation Shot-noise then establishes a fixed ratio of the total usable bandwidth to the minimum received power per channel, which for the binary FSK system considered is 61 GHz/ mu mW A resonant p-i-n-FET receiver, designed to provide maximum sensitivity between 25 and 50 GHz, has been constructed The measured signal-to-noise ratio is in excellent agreement with that predicted by the noise analysis >

Generation of wideband linear frequency modulation signals

Abstract: A low-frequency low bandwidth Linear Frequency Modulation (LFM) waveform, nominally a 1 MHz to 10 Mz swept frequency analog sinusoid or digital square wave, is produced by direct digital synthesis. This waveform is upconverted in frequency and expanded in bandwidth, nominally to microwave frequencies with bandwidths of nominally 160-360 MHz, in a multiplying offset phase locked loop. The phase locked loop also linearly frequency modulates a X-band carrier with the microwave frequency LFM waveform to produce an output signal suitable for Synthetic Aperture Radar. The phase locked loop induces low phase error, and may be closed around the radar transmitter to remove phase errors induced by that unit.

Tvro receiver system with automatic bandwidth adjustment

Abstract: In a TVRO receiver having a tuner producing output signals having bandwidths which vary from channel to channel, the improvement comprising bandpass filtering means receiving the output signals and having a controllably adjustable pass band, bandwidth detecting means responsive to the output signals for producing a control signal in response to deviations of the bandwidth of the current output signal beyond the current pass band of the filtering means, and control means responsive to the control signal for adjusting the pass band of the filtering means to correspond to the bandwidth of the current output signal.

Dual port fm demodulation in phase locked receivers

Abstract: A problem arises in attempting to recover low frequency FM components in receivers using wide bandwidth tracking loops. Wide loop bandwidths are desirable for rapid signal acquisition but interfere with demodulation of low frequency FM components, such as those produced by digital signalling data. This invention permits recovery of the full modulation spectrum with flat frequency response. A conventional FM demodulator (15) recovers the high frequency modulation components; the frequency control signal for the local oscillator (3) provides the low frequency components. Combining (35) the components with proper weighting (31, 33) produces recovered modulation (37) with bandwidth limited only by the IF bandpass filter (7) or demodulator. The invention would find application in an FM receiver of the superheterodyne or direct conversion type that requires precise frequency control, such as one used for communications in a narrow channel spacing environment at ultra-high frequencies.

Near obstacle detection system

Abstract: In a radar system, an RF signal is transmitted at a frequency modulated by a sawtooth signal. The transmitted RF signal is mixed with the transmitted signal reflected from an object. The resulting IF signal is analyzed into a Fourier series having a DC coefficient and harmonic frequency coefficients. The range to the object is determined from the harmonic frequency coefficients in multiples of the range resolution determined by the RF bandwidth of the system when the DC coefficient is less than each of the harmonic frequency coefficients. The range to the object is determined to be within 1/2 the range resolution determined by the RF bandwidth when the DC coefficient is greater than each of the harmonic frequency coefficients.