Narrow-band Input Research Articles

TOWARDS THE STANDARDIZATION OF A TESTING METHOD FOR THE END-STOP IMPACTS OF SUSPENSION SEATS

Standard tests of the vibration isolation efficiency of suspension seats currently use narrow-band input spectra representing the notions of specific vehicles. The motions used in these tests have near-Gaussian amplitude distributions, whereas vehicle motions include occasional extreme movements causing end-stop impacts of the seat suspension which increase the exposure of the seat occupant to vibration and shock. Tests have been conducted with various vibration magnitudes so as to quantify suspension seat isolation efficiency in terms of the ratios of the vibration dose values (VDVs) at the base of the seat, above the seat suspension and above the seat cushion. It is shown that the vibration response of a seat suspension occurs in five stages according to the magnitude of the input vibration, and that the cushion of a suspension seat may reduce the severity of severe end-stop impacts. The dynamic performance of a suspension seat when supporting a human subject and when supporting a sand bag is compared and the effect of mid-position adjustment is demonstrated. The form of an end-stop impact test which could supplement existing tests so as to evaluate the overall performance of suspension seats is discussed.

Conditional Linearization in Nonlinear Random Vibration

In this paper, an improved probabilistic linearization approach is developed to study the response of nonlinear single degree of freedom (SDOF) systems under narrow-band inputs. An integral equation for the probability density function (PDF) of the envelope is derived. This equation is solved using an iterative scheme. The technique is applied to study the hardening type Duffing's oscillator under narrow-band excitation. The results compare favorably with those obtained using numerical simulation. In particular, the bimodal nature of the PDF for the response envelope for certain parameter ranges is brought out.

Theoretical analysis of the ADPCM CCITT algorithm

The unfavorable effects of narrowband inputs on the decoder adjustment when the LMS algorithm is used are analyzed. An explanation is given for the behavior of the CCITT algorithm. Suboptimality of prediction is granted to achieve adjustment, resulting in a satisfactory tradeoff between reduction rate and adjustment. The link between adjustment and uniform stability is enhanced.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mistracking in successive PCM/ADPCM transcoders

An analysis is presented of the causes of the accumulation of quantizing noise found in the transient state for successive CCITT adaptive differential pulse-code-modulation (ADPCM) transcoders connected synchronously. By decoupling the predictor and quantizer effect it is proved that, owing to a self-stabilization phenomenon, narrowband inputs cause local instabilities in the predictor of the jointly adaptive autoregressive moving-average-prediction/quantization used in the ADPCM 32-kb/s algorithm. Despite the assured global stability, these local instabilities are not synchronized at the encoder and its preceding decoder, and a mistracking occurs which creates quantizing noise accumulation. The tracking is then shown to be very sensitive to predictor/quantizer interaction. The discontinuities introduced in the standardized adaptive quantizer extend the mistracking problem to wideband inputs. A smoothed quantizer with reduced inauspicious interaction is proposed to remedy the problem. >

Analytical model for injection-controlled excimer laser amplifiers

A semiempirical model of a pulsed, injection-controlled laser is investigated analytically and is applied primarily to extensive experimental results that have been obtained for the XeF (C to A) excimer laser. The gain medium inside an unstable cavity is represented by a folded pulsed amplifier which is seeded by a narrowband input signal. A set of coupled rate equations for the population densities of the upper laser states, the wideband absorbers, and the photon flux was numerically integrated. Measured gain and absorption of the amplifier were used as input data to evaluate the model. Excellent agreement between calculated and experimentally observed peak intensity and various other pulse characteristics such as peak shape and laser pulse delay time was obtained. This model also predicts the experimentally determined injection-control threshold for KrF excimer lasers successfully, and should be applicable to numerous other injection seeded lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stability of an Adaptive ARMA Predictor in Presence of Narrow-Band Input Signals

In presence of narrow-band input signals, adaptive ARMA prediction is studied from the viewpoint of stability. The adaptation algorithm under investigation has been promoted in digital coding of audio-signals. It is a sub-optimal, simplified version of the stochastic gradient, using no recursive filtering at all for the information vector entering the parameter increment. For narrow-band input signals, local and quasi-periodic instabilities will appear. At the time when one predictor pole crosses outwards the unit circle, the increasing output error becomes essentially periodic at the frequency of the in-question pole. This non linear effect will quickly reinforce the estimated parameters inside the stability region. This propitious behaviour of the adaptive algorithm has been called self-stabilization. It is investigated in detail in the paper.

Performance of a Discrete Spectral Peak Frequency Estimator for Doppler Wind Velocity Measurements

A frequency estimator formed from the peak of a discrete power spectrum is examined. Expressions to compute the performance of the estimator as a function of input signal bandwidth and signal-to-noise ratio are developed and used to study the estimator's capability to measure signal mean frequencies for remote Doppler measurement of atmospheric winds. Performance comparisons with the widely used complex-covariance estimator, based on simulations, show that frequency estimates formed from the spectral peak tend to have larger variance, except in the case of narrow-band input signals where the estimate standard deviation approaches that of the complex-covariance estimator.

Instantaneous Frequency Estimation Using Adaptive Linear Predictor Weights

A computationally efficient scheme for estimating the digital instantaneous frequencies of narrowband inputs is introduced. The frequency estimates are obtained by searching for minima of the inverse input power spectrum. This spectrum is estimated at each input sample from the weights of an adaptive linear predictor which uses the LMS (least mean square) algorithm to update its weights. The related minima are sought via an iterative search algorithm, referred to as the iterative frequency estimator. This algorithm is computationally more efficient than available methods, and also provides a higher resolution. Simulation results are included; these include tracking of random message sequences in FM signals, and the formant frequency estimation of speech.

The Intermodulation Due to Multicarrier Quantization

The quantization of a multicarrier signal with arbitrary carrier amplitudes is analyzed by using a simple, general approach. As opposed to the sum of signum functions used by Blachman, the quantization staircase output is represented by the sum of the input plus a sawtooth. Using an infinite Fourier series for expanding this sawtooth, series expressions are obtained for the output quantized carriers and the resulting intermodulation products of any order. The special case of a narrow-band input consisting of a strong carrier and a weak carrier is considered in detail and the results are found qualitatively in agreement with those published by Blachman.

Determination of the parameters of injection laser amplifiers based on GaAlAs heterostructures from superluminescence characteristics

A modified method for the determination of the amplifying characteristics of active media from the dependence of the amplified spontaneous radiation (superluminescence) spectrum on the geometry of the active region was applied to stripe heterolasers which had no feedback. The spectral dependences were obtained for the main parameters of laser amplifiers: the gain, sensitivity, and dynamic range of linear amplification of a narrow-band input signal. These parameters of heterojunction amplifiers were practically as good as those of homostructure amplifiers operating at cryogenic temperatures.

Frequency and reverberation response of a pipe resonator.

A frequency and reverberation response of a circular pipe resonator are studied by newly developed techniques of the signal compression method and the calculation method of reverberation response from impulse response. By these methods, reverberation decay curves for very narrow band input signals are obtained and dips in reverberation curves are observed. These dips appear when two frequency response peaks are influenced equally by narrow band input signals. These measuring methods would be useful for room acoustics when fine structure of reverberation decay curve is desired.

Steady-state characteristics of a GaAs injection quantum amplifier receiving a narrow-band input signal

An investigation was made of the amplifying characteristics of a stripe homojunction laser in which positive feedback was suppressed by orienting the axis of the active region at an angle to the cleaved end faces. Radiation from a conventional cw stripe homojunction laser emitting a single mode was used as the input signal. An analysis was made of the output power and spectral characteristics of the amplifier as a function of the rate of pumping of the amplifier and of the amplitude of the input signal. It was found that the gain exceeded 103 in the linear amplification regime and that the superradiant background was almost completely suppressed when the amplification process was saturated by a sufficiently strong input signal. An optical filter with a pass band 0.1 nm wide was placed at the amplifier exit and in this configuration the amplifier sensitivity was 0.4×10−7W.

Second-order output statistics of the adaptive line enhancer

The adaptive line enhancer (ALE) is an adaptive digital filter designed to suppress uncorrelated components of its input, while passing any narrow-band components with little attenuation. The purpose of this paper is to analyze the second-order output statistics of the ALE in steady-state operation, for input samples consisting of weak narrow-band signals in white Gaussian noise. The ALE output is shown to be the sum of two uncorrelated components, one arising from optimum finite-lag Wiener filtering of the narrow-band input components, and the other arising from the misadjustment error associated with the adaptation process. General expressions are given for the output auto-correlation function and power spectrum with arbitrary narrow-band input signals, and the case of a single sinusoid in white noise is worked out as an example. Finally, the significance of these results to practical applications of the ALE is mentioned.

Two-bit instantaneously adaptive delta modulation for p.c.m. encoding

A two-bit instantaneously adaptive delta modulator intended for application as the input stage of a code converter is presented. Its step adaptation algorithm is derived from that used in high information delta modulation (HIDM). The improvement in performance relative to the HIDM system is gained by its ability to reduce overshoots and respond faster to signal variations. A hardware model that allows the comparison between these delta modulators is described and results of measurements on narrow-band noise and sinewave input signals are presented. Two possible structures to generate p.c.m. signals by code conversion are described and computer simulation results presented for one of them.

Adaptation by direct phase-shift adjustment in narrow-band adaptive antenna systems

Voltage-controlled phase shifters are used as the adjustable elements in an adaptive phased-array antenna system for spatial filtering of narrow-band inputs. A gradient-search adaptation algorithm for adjusting phase angle is developed and applied to an adaptive antenna system which can be adapted either to enhance signals or reject interference. The transient behavior of this system during adaptation is analyzed in terms of steepest ascent/descent on a multidimensional trigonometric performance surface approximated in two regions by quadratic surfaces.

Envelope Detector Performance for a Partially Coherent-Fading Sine Wave in Noise

The performance of a matched filter square-law envelope detector is investigated for a new signal-in-noise model, that of a partially coherent-fading random-phase sine wave in additive zero-mean noise. This model is of interest when analyzing the effects of random amplitude and/or phase channels on narrow-band input signals. The amplitude and the phase of the sine wave are assumed to be independent random processes with known bandwidths. This paper obtains deflection-criteria detection-performance results for the envelope detector as a function of the bandwidths of the amplitude and phase of the signal process, the noise process, the observation time, and the second-moment statistics of the amplitude, phase, and noise processes. The results show quantitatively that the detection performance of the envelope detector is much more sensitive to loss of phase coherence than to loss of amplitude coherence.

Power spectrum of an injection-locked Josephson oscillator

Previous experiments have shown that a Josephson oscillator, exposed to a weak narrow-band input signal, exhibits behavior characteristic of an injection-locked oscillator. When in lock, Adler's theory of injection locking describes the experimental observations reasonably well. This paper extends the range of applicability of the theory to the out-of-lock regime where a spectrum of output frequencies is observed. Obtaining the theoretical output power spectrum requires solving a differential equation having the same form as the equation describing the resistively shunted junction model of Stewart and of McCumber. Experimental measurements of the output spectrum of a nearly locked Josephson oscillator are shown to be in reasonable agreement with the theory. Additional results discussed briefly include the observation of a frequency dependence of the locked Josephson oscillator output, and experiments in which a Josephson oscillator-mixer was injection locked by a weak signal at the if.

Approximate Analysis of Nonlinear Systems With Narrow Band Random Inputs

A variational averaging technique is applied to nonlinear systems with narrow band inputs. The technique requires that a form for the approximate solution be obtained by some means. The case of narrow band inputs places unique requirements on the selection of certain parameters in the approximate form. A conceptual development of the important factors is presented and applied to several specific examples. Results show that the technique is capable of producing good approximations and specific methodology is suggested for several cases.

Analysis and Compensation of Bandpass Nonlinearities for Communications

There are many instances in communication systems where bandpass signals are passed through nonlinear devices, such as traveling wave tubes, which exhibit both amplitude and phase nonlinearities. When the input signal is narrow band, the device may be characterized by measurements of its single-carrier amplitude and phase transfer functions. A sufficient model for such a device is a quadrature structure that includes two nonlinearities each of which, acting on its own, would exhibit only amplitude distortion. The outputs of the two halves of this model are linearly independent for arbitrary narrow-band input signals so that their power spectra add. Consequently, almost all previously published results for amplitude nonlinearities can be readily applied to the analysis of the general device. Emphasis is laid on practical procedures for analysis based directly on measured device characteristics rather than analytic approximations and accuracy is checked by comparison of certain intermodulation results with previous results and with measurements. A new result is the performance of an Intelsat IV tube for a large number of independent equal-power-density signals. A heuristically optimal saturating nonlinearity is introduced and analyzed and two methods of compensating an arbitrary saturating device to obtain this optimal characteristic are presented. Two methods of inverting the Chebyshev transform are used in this paper and the choice of basis functions for obtaining series representations of the measured device characteristics is discussed.

Detectors, bandpass nonlinearities, and their optimization: Inversion of the Chebyshev transform

From the voltage-response characteristic of a memoryless nonlinearity the output amplitude in any harmonic zone is easily found as a function of the amplitude of a narrow-band input, but no general method has been known for inverting this (Chebyshev) transformation. The inversion is of interest because the best detector, bandpass non-linearity, or harmonic generator for various purposes, e.g., maximization of the output signal-to-noise ratio, is most readily described in terms of its amplitude-response characteristic for the desired harmonic zone. A simple integration over the amplitude-response characteristic (or describing function) is found to yield the required voltage-response curve.