Showing papers on "Bandwidth (signal processing) published in 1977"

Bandwidth specification for adaptive optics systems

Abstract: A simplified expression for the bandwidth of an adaptive optics system is found to depend on a weighted path integral of the turbulence strength, where the weighting is transverse wind velocity to the 5/3 power. The wave-front corrector is conservatively assumed to match the phase perfectly, at least spatially, if not temporally. For the case of astronomical imaging from a mountaintop observatory, the necessary bandwidth is found to be less than 200 Hz.

Operational Amplifiers and Linear Integrated Circuits

Abstract: 1. Introduction to Op-Amps. 2. First Experience with an Op-Amp. 3. Inverting and Noninverting Amps. 4. Comparators and Controls. 5. Selected Applications of Op-Amps. 6. Signal Generators. 7. Op-Amps with Diodes. 8. Differential, Instrumentation, and Bridge Amplifiers. 9. DC Performance: Bias, Offsets, and Drift. 10. AC Performance: Bandwidth, Slew Rate, Noise, and Frequency Compensation. 11. Active Filters. 12. Modulating and Frequency Changing with the Multiplier. 13. Integrated Circuit Timers. 14. D to A and A to D Converters. 15. Power Supplies. Answers. Index.

A Nonlinear Model for the Spatial Characteristics of the Human Visual System

Abstract: Several recent papers have presented data from experimental investigations of the human ivsual system (HVS) which support the general hypothesis that the HVS is composed of spatial frequency channels. It has been pointed out, however, that a linear systems analysis of the entire system is not valid. Furthermore, a nonlinear model consisting of a log-bandpass filter produced some experimental results with deviations at high spatial frequencies. A new structure for a nonlinear mathematical model which is easily quantifiable, produces results which compare with experimental data, and has a physiological correlate is presented. The significance of this model is that the bandwidth of the visual system decreases as contrast increases. Thus the system appears to maximize the signal to noise ratio while attempting to maintain a constant " perceptual" spatial-frequency fidelity.

Nine digital filters for decimation and interpolation

Abstract: Filtering is necessary in decimation (decreasing the sampling rate of) or interpolation (increasing the sampling rate of a digital signal. If the rate change is substantial, the process is more efficient when the decimation or interpolation occurs in stages rather than in one step. Half-band filters are particularly efficient for effecting octave changes in sampling rate and nine digital filters are presented, eight of them half-band filters, to be used as components of multistage interpolators and decimators. Also presented is a procedure for combining the filters to produce multistage designs that meet a very wide range of accuracy requirements (stopband attenuation to 77 dB, passband ripple as low as 0.00014). The nine filters admit changes between sampling rates above 4W, where W is the nominal bandwidth of the signal. Established design techniques may be used to obtain efficient filters for conversion between 4W Hz sampling and 2W Hz, the "baseband sampling rate." With these multistage filters, the possible interpolation and decimation ratios are all integer multiples of powers of two. To overcome this restriction we present a simple resampling technique that extends the range of designs to conversions between any two rates. The interpolation or decimation ratio need not be an integer or even rational. In fact, it can vary slightly as in a practical situation where the input signal and output signal are under the control of autonomous clocks. We demonstrate the approach by means of several design examples and compare its results with those obtained from the optimization scheme of Crochiere and Rabiner.

Optimum Performance of Suppressed Carrier Receivers with Costas Loop Tracking

Abstract: The performance of suppressed carrier receivers with Costas loop tracking is optimized by proper choice of loop arm filter bandwidth. In particular, it is shown that for a variety of passive arm filter types, there exists, for a given data rate and data signal-signal-to-noise ratio, an optimum filter bandwidth in the sense of minimizing the loop's squaring loss. For the linear theory case, this is equivalent to minimizing the loop's tracking jitter. When symbol synchronization is known, it is shown that by replacing the passive arm filters with active filters, i.e., integrate-and-dump circuits, one can achieve an improvement in carrier-to-noise ratio of as much as 4 to 6 dB depending on the passive arm filter type used for comparison and the value of data signal-to-noise ratio (SNR).

Noncoherent Pseudonoise Code Tracking Performance of Spread Spectrum Receivers

Abstract: The optimum design and performance of two noncoherent PN tracking loop configurations, namely, the delay-locked loop and tau-dither loop, are described. In particular, the bandlimiting effects of the bandpass arm filters are considered by demonstrating that for a fixed data rate and data signal-to-noise ratio, there exists an optimum filter bandwidth in the sense of minimizing the loop's tracking jitter. Both the linear and nonlinear loop analyses are presented and the region of validity of the former relative to the latter is indicated. In addition, numerical results are given for several filter types. For example, assuming ideal bandpass arm filters, it is shown that the tau-dither loop requires approximately 1 dB more signal-to-noise ratio than the delay-locked loop for equal rms tracking jitters.

Improving Frequency Acquisition of a Costas Loop

Abstract: A Costas loop for tracking a BPSK signal does not acquire when the initial frequency error is comparable to the loop bandwidth. This paper describes methods for deriving control proportional to frequency error so as to improve the frequency acquisition capability by an AFC augmentation. A composite AFC/Costas loop is realized by combining the individual loops. Pull-in from a frequency error much greater than the Costas loop bandwidth is now feasible. The increase in phase error due to the AFC capability is evaluated by a linearized analysis. If the bandwidth of the AFC portion is sufficiently narrow, the degradation is negligible. Improved frequency acquisition can also be realized by a simple modification of the Costas loop to remove the low-pass filter on the quadrature phase detector channel, with negligible degradation to phase tracking.

Software Implementation of a PN Spread Spectrum Receiver to Accommodate Dynamics

Abstract: To optimize the threshold of a pseudonoise (PN) spread spectrum modem for use over an aircraft/satellite communications link at SHF, the effects of Doppler must be taken into account. Reconstitution of carrier phase by a Costas loop to coherently demodulate the PSK data and also the delay-lock error voltage has typically been the practice in PN modems intended for ground applications. To accommodate the platform dynamics, the Costas loop must have a relatively wide bandwidth, and this implies a significant threshold degradation. An alternate implementation employs a noncoherent carrier tracking loop which maintains frequency lock rather than phase lock. Now, the delay-lock error voltage is noncoherently demodulated. For the airborne application, analysis and simulations show this implementation will extend the receiver's tracking threshold significantly (up to 6 dB) for the worst case dynamics profile. An experimental project was undertaken to modify an existing ground PN modem (AN/USC-28, ADM version) for flight test. A software implementation of the digital tracking algorithms was selected where a HP-2100A minicomputer controls carrier frequency and PN code phase via digital phase shifters. The Costas demodulator for extracting PSK data resides entirely in software, and is completely segregated from PN tracking. In laboratory testing of the receiver with simulated dynamics and in actual flight tests, the demonstrated performance was found to approach closely the goals established by the analyses and simulations.

Transmitting signals over alternating current power networks

Abstract: This invention relates to communication systems of the type particularly useful in power line communications. Signals to be transmitted, which can be in the form of short, fixed frequency signals, are expanded in time and/or bandwidth to provide a longer signal with a variable frequency. At the receiver, power line noise, which is typically either short wideband noise or long narrowband noise, is removed and the remaining expanded signal is detected. The expanded signal can then be compressed in time and/or bandwidth to reproduce the original signal.

Growth of masking and the auditory filter.

Abstract: The threshold for a sinusoidal signal (1, 2, and 4 kHz) centered in the ’’notch’’ of a broadband masker was determined as a function of notch width for five noise spectrum levels (10, 20, 30, 40, and 50 dB SPL). For narrow notch widths the signal‐to‐noise ratio at threshold remains constant as a function of level, which according to the critical‐ratio hypothesis implies an auditory filter of constant bandwidth. For wide notch widths the signal‐to‐noise ratio at threshold increases as a function of level and this implies an auditory filter of increasing bandwidth. If the estimates of the filter bandwidth obtained for wide notch widths are used to predict thresholds for broadband noise, the corresponding signal‐to‐noise ratios will increase as a function of noise spectrum level. The predicted increase in signal‐to‐noise ratio is very small, however, and provides a good description of most available data. In fact, the predicted increase equals that observed by Reed and Bilger [J. Acoust. Soc. Am. 53, 1039–1044 (1973)]. The increase in filter bandwidth has significant consequences only when the signal and masker are widely separated in frequency; for other conditions, the assumption of a constant filter permits accurate predictions of performance.

Filter and demodulation arrangement

Abstract: A digital filter and demodulation arrangement for passband signals, whose corresponding baseband signal has a bandwidth limited to a given maximum frequency The passband signals are filtered according to two bandpass characteristics which, apart from their asymmetrical distortion relative to their central frequency, are versions from one another shifted 90° in phase The filtered passband signals are demodulated with an in-phase carrier and a quadrature carrier and the demodulated signals are combined to a baseband signal In the digital filtering process the sampling frequency is reduced from a value higher than twice the highest frequency in the passband signals to a value which is not higher than twice said maximum frequency in the baseband signal and in the digital demodulation and combination processes the reduced sampling frequency is also used so that a considerable reduction in the internal processing speed is obtained

A Review of Acousto-Optic Signal Correlators

Abstract: Many techniques have been proposed in the past for using acousto-optic devices to perform signal correlation. This paper reviews these techniques and discusses the design tradeoffs involved in choosing a particular configuration for a given application. Configurations are classed as two major types. The first type, spatial integrating correlators, perform correlation by integrating light diffracted by all parts of the signal(s) which are simultaneously present in the acoustic device. This type of correlator has a large range window, but the time bandwidth product is limited by device parameters. The second type, time integrating correlators, use a detector array or vidicon to perform an integration in time for each point within the cell. This provides a limited range window but a large time bandwidth product. Results obtained with both types of systems are shown, along with pictures of fabricated hardware. Acousto-optic device parameters are also summarized for comparison with competitive technologies.

The effect of a memoryless nonlinearity on the spectrum of a random process

Abstract: It is shown that, for a Gaussian process and for some non-Gaussian processes, any memoryless nonlinearity has a whitening effect in the sense that the output spectrum is smoother and occupies a greater bandwidth than the input spectrum. The ratio between input and output bandwidths is investigated by using several measures of bandwidth. Also, it is shown that classes of nonlinearities exist that are equivalent in the sense of producing the same spectral transformations.

Correlative coding: A bandwidth-efficient signaling scheme

Abstract: In most transmission channels, bandwidth is at a premium and an important attribute of any good digital signaling scheme is its ability to make efficient use of the bandwidth. Conventional Nyquist-type pulse amplitude modulation signaling schemes, which are designed to eliminate intersymbol interference, achieve high data rates only at the expense of a large number of signal levels. In many applications, correlative coding or partial response signaling, which introduces intersymbol interference in a controlled way, is able to achieve high data rates with fewer levels and hence with better error rate performance. In addition to higher data rates, correlative schemes achieve convenient spectral shapes and have error-detecting capabilities without introducing redundancy into the data stream. This paper explains how correlative schemes work and why they are advantageous.

Sampled data positioning system employing a model of the physical system for time optimal control

Abstract: A sampled data positioning system for moving a member between positions in a minimum time employs a model of the physical system to overcome the low bandwidth constraints of fed back position and velocity samples. The model system is an approximate analog of the physical system and provides continuous pseudo position and velocity signals of high bandwidth. These are applied to a seek controller which generates an error signal between a desired reference velocity trajectory and the pseudo velocity signal. The error signal is fed back to the model system input and is also applied to control the physical system. The model velocity and position outputs are reset at sampling times to the values of the sampled velocity and position in the physical system.

High speed, wide dynamic range analog-to-digital conversion

Abstract: A wide dynamic range, wide bandwidth, analog-to-digital conversion system and method. A plurality of overlapped analog-to-digital converters are utilized in conjunction with scaling amplifiers to provide a plurality of output ranges. Means for selecting the set of output bits which provides a magnitude representation of the input signal are provided along with means for outputting a digital representation of the appropriate range.

Automatic channel equalization with time base expansion

Abstract: An automatic channel equalization system for use with a television receiver includes a control section for automatically operating a transversal equalizer according to the distortion exhibited by a reference signal transmitted through the channel. The television line including the transmitted reference signal is supplied to the control section through a time base expander enabling the implementation of a relatively fast sampling rate for achieving full bandwidth equalization of the channel with relatively slow-speed signal processing technology.

Method and means for improving the spectrum utilization of communications channels

Abstract: A system and method for improving the spectrum utilization of communications channels. In certain embodiments of the invention a plurality of narrowband communications channels are used to transmit a multiplicity of speech signals. In another embodiment of this invention, a multiplicity of speech signals jointly utilize a common transmission facility and circuitry shifts the amount of bandwidth assigned to each channel as a function of the short term characteristics of the individual speech signal. Special circuitry is provided to reduce the loss of consonant sounds at the initiation of speech bursts as well as the end of the bursts.

Clutter free communications radar

Abstract: A clutter immune radar where the interrogating unit transmits a modulated interrogation signal to a remote unit, which radiates an information-bearing reply signal, in order to enable the interrogating unit to obtain remote unit information and range by the use of three bandpass filter channels where one bandpass filter bandwidth is equal to the sum of the other two bandwidths.

Detection of Digital FSK and PSK Using a First-Order Phase-Locked Loop

Abstract: A classical problem in digital frequency-shifted keyed (FSK) demodulation is the evaluation of the bit error probability performance when an estimator-correlator that incorporates a phaselocked loop (PLL) is employed. Although some attention has been devoted to this problem in the past, an accurate account of the mechanism which produces decision errors has not yet been advanced. This paper examines a special case, viz., a first-order PLL preceded by a wideband IF filter, of the above problem using a new approach which is based upon the renewal Markov process theory and the Meyr distribution. In particular, the ad hoc approach of invoking the Gaussian assumption on the decision variable and patching it with a correction term based on Rice's click theory is not used. Rather, the effective noise is properly characterized by unfolding the renewal Markov process associated with the loop phase error. As a slight extension of the results, the performance of the above PLL detector operating on low data rate PSK is given and demonstrated to be approximately 3 dB superior to that of FSK reception. The theory and analysis presented herein apply to the special case where the bandwidth of the IF filter preceding the first-order PLL is required to be several times the data rate because of frequency uncertainties due to channel Doppler and oscillator instabilities, but the frequency deviation to data rate ratio may be chosen small (if desired) to optimize system error probability performance. In addition to presenting results for the case where the oscillator instabilities are assumed absent and channel Doppler is prefectly tuned out at the receiver oscillator, the effects of small residual Doppler on bit error probability performance is considered. In all cases tested, excellent agreement was obtained between theory and computer simulation results.

PSK-Type Modulation with Overlapping Baseband Pulses

Abstract: One of the most interesting challenges in the field of digital radio is to generate and transmit signals which meet the spectral requirements and which have high detection efficiencies. The spectral efficiency of PSK-type modulation with overlapping baseband pulses is known to be better than that with nonoverlapping pulses of the same form; but, to our knowledge, no analysis is presently available to determine their detection efficiency. In this paper a method has been given to evaluate the error probability performance of PSK-type modulation with baseband pulses of finite overlap and mutually independent samples. It is shown that the error probability can be computed with any desired accuracy. For BPSK and for some typical transmit and receive filters and for a transparent channel corrupted by additive Gaussian noise we compare the detection efficiencies when the baseband modulation pulses have zero and nonzero overlap and when the transmitted signal satisfies the same spectral requirement. The nonoverlapping rectangular signaling is shown to be superior to overlapping raised-cosine signaling, but the differential degradation is less than 0.8 dB if the twosided 99-percent bandwidth W 99 satisfies 1.1/T \leq W_{99}\leq 1.6/T . The analytical results presented here for a binary system can be extended with minor modifications to any M -ary PSK system, M > 2 .

A Unified Approach to Pulse Design in Data Transmission

Abstract: The problem of designing optimum pulseshapes for data transmission over randomly selected channels is examined using a meansquared error criterion. Earlier work has focused principally on SSB or VSB systems in the presence of timing and carrier phase jitter, but we extend the results here to any two-dimensional signaling scheme (including as special cases SSB, VSB, combined AM-PM, QAM, and staggered QAM) and to any type of channel dispersion. By imposing certain mild constraints on the transmitter and receiver filters, it is seen that one can solve for optimum pulses more easily than with earlier approaches, and yet the resulting system is still essentially optimum. These constraints leave design freedom only in the rolloff bands of the pulse spectra, and a major thurst of the work is to design for optimal utilization of whatever excess bandwidth is available. The approach used involves immunity to channel distortions: only the type of channel distortion to be encountered is known, and series expansions of mean-squared error are used to find sensitivity coefficients. These coefficients are then minimized by proper signal design. Closed form expressions are found for the optimum pulses, and these results are compared with previous work. A second approach, which uses as its criterion the mean-squared error averaged over all possible channel characteristics, is also discussed.

Maximization of lineal recording density

Abstract: A brief outline is given for design and analysis of the analog signal processing portion of peak sensing digital magnetic recording systems for which lineal recording density is maximized. This is a linear system analysis in which superposition is applicable to saturation recording if the media is indeed fully saturated at the termination of each transition. Results are stated in terms of parameters of run bounded codes because of their practical importance. However, the concept applies in general to the maximization of the information rate through a channel of finite bandwidth under pulse-position modulation.

The Effect of Bandwidth on Filter Instrument Total Ozone Accuracy

Abstract: The effect of the width and shape of the New Zealand filter instrument's passbands on measured total-ozone accuracy is determined using a numerical model of the spectral measurement process. The model enables the calculation of corrections for the 'bandwidth-effect' error and shows that highly attenuating passband skirts and well-suppressed leakage bands are at least as important as narrow half-bandwidths. Over typical ranges of airmass and total ozone, the range in the bandwidth-effect correction is about 2% in total ozone for the filter instrument, compared with about 1% for the Dobson instrument.

Design Procedure for Wide Bandwidth Acousto-Optic Modulator

Abstract: In this paper, we give the design equations and a procedure for the complete specification of the design parameters for a wide bandwidth acousto-optic modulator (AOM). Although the modulating medium, transducer material, and bonding agents determine the modulator performance, we concentrate here on the question of how to design an AOM, given specifications on its minimum modulation bandwidth, diffraction efficiency, and the maximum ellipticity of the output beam.

Accurate Measurements of Spectral Density of Phase Noise in Devices

Abstract: Systematic errors larger than 10 dB can occur in the measurement of the spectral density of phase unless considerable caution is exercised. Some potential problems due to the shape of the analyzer passband and the Fourier frequency dependence of mixers are discussed. Three measurement systems are analyzed to determine the conditions under which they may be used to make spectral density measurements with an accuracy of 0.2 dB.

Statistical model for the prediction of the bandwidth of an optical route

Abstract: The behaviour of the bandwidth along an optical-fibre route is examined, and a simple statistical theory is developed and confirmed by measurements.

Signal strength responsive sound trap

Abstract: A television receiver signal translating system includes a tuner frequency-converting a received broadcast signal to an intermediate frequency signal. An acoustic surface wave filter and gain controlled amplifier are coupled to the tuner and drive a synchronous detecting system. An automatic gain control (AGC) maintains the output of recovered signal constant while a variable bandwidth stage changes the system frequency response as a function of signal strength. A variable influence sound carrier trap responds to the AGC providing increased sound carrier attenuation when strong signals are received and reduced attenuation during weak signal reception.

Acousto-Optic Signal Processors

Abstract: Acousto-optic cells have found a major application in providing compact real-time electrical to optical transducers for optical signal processing systems(1). Such acousto-optic devices with bandwidths up to 1.0 GHz(2) and time bandwidth products exceeding 1000 may be configured with other optical components and devices to perform signal spectrum analysis, correlation, and signal sorting. This paper will present a conceptual overview of the principal acousto-optic techniques available for processing of wideband electronic signals.