Broadband Test Signal Research Articles

Dynamic Testing of A/D Converters Using the Multiple Coherence Function

The multiple coherence function is utilized to dynamically test an arbitrary number of A/D converters that all sample the same signal. Based on the multiple-channel coherent removal method that has been popularized by Bendat and Piersol, the residual spectrum for each A/D converter under test is determined by removing the portion of the signal that can linearly be predicted by the remaining channels. Following accepted practice, the multiple coherence function is calculated from cross-spectral densities by using the Welch periodogram method. The acquisition channel of each A/D converter is modeled using an equivalent noise source model. The residual spectrum of any particular channel, which is expressed in terms of the equivalent noise sources, is determined to be the converter noise of the particular channel plus the parallel combination of the converter noise of the remaining channels. The resulting set of equations can iteratively be solved, yielding the converter noise of every channel under test. Broadband test signals are recommended so that close-to-unity coherence is achieved throughout the frequency band. When sine waves are used, the requirement for a sine wave of high spectral purity is eliminated, similar to the requirement for sidelobe suppression that uses data windowing or coherent sampling (acquiring an integer number of oscillations). Since the method confirms both synchronization and resolution, it is well suited to simultaneously test sampled multiple-channel data acquisition cards. Testing results with white noise and sine-wave signals that use an eight-channel 24-bit data acquisition card, which is synchronized with a four-channel, 16-bit data acquisition card, are presented and compared to the signal-to-noise-and-distortion ratio (SINAD) testing procedure. This procedure estimates converter noise by removing all harmonic content that can linearly be predicted by the remaining channels. With sine waves of sufficient amplitude, the removed harmonic content includes harmonic distortion that was created in the A/D converter process. For broadband test signals and small-amplitude sine waves, harmonic distortion that is created in the A/D converter process is negligible compared to the converter noise.

Statistical Method for Analog-to-Digital System Testing in Time and Spectral Domain

The article deals with an improved method for testing analog-digital system (ADS) as the system objects with the dynamic nonlinear and stochastic properties. A proposed technique makes it possible to utilize the statistical testing method to estimate the dynamic of ADS as in time, as in spectral domain, without the use of additional hardware or the need for additional measurements. The implementation of the statistical testing method that is described is based on a broadband test signal that simulates actual operating conditions. The technique makes it possible to avoid the standard problems associated with precise estimation of the spectrum of a broadband signal. It has been validated in tests of the dynamic parameters of 6 up to 1-bit high-speed.

Generating broadband test signals with resonant piezoelectrictransducers

A new method is proposed for generating broadband (BB) test signalsfor use in impulse response (IR) system identification applications. Thismethod uses resonant reciprocal transducers with low losses along withsuitable excitation in order to increase the effective bandwidth of thetransducer. The approach is general and can be extended to any resonant lowpass or band pass linear structure with an autoregressive transient response.The new approach is justified in a brief review of the shortcomings of impulseresponse measurements and deconvolution in real/imperfect physical systems.The resulting mean square error (MSE) of a system's impulse response obtainedis related to the performance of the optimized Wiener filter and noiseconditions. The idealized description is followed by an experimentalevaluation of small size resonant piezoelectric actuators in air, using thebandwidth broadening excitation.

MAGNETOINDUCTIVE DIFFUSION LAYER THICKNESS GAUGING

Abstract Measuring the properties of thermo-chemically produced surface layers, in particular gauging the thickness of such layers, presents a nondestructive testing problem hitherto not satisfactorily solved. To date, magnetoinductive or eddy current methods have failed to eliminate the influence of the smelting charge and the method of treatment on the resultant signal. Smelting charges differ in conductivity and magnetic permeability and, hence, the calibration curve obtained for a particular charge cannot be translated to any other charge. Thus, application of eddy current methods to this problem area on a broad scale has not been possible. The idea of eliminating the influence of charge is to avoid measuring the properties of the layer in absolute terms and, instead, quantify the differences between the properties of the bulk material and those of the layer. To this end, information for various depths of the material can be obtained by both discrete multifrequency and broad-band test signals. This pa...

Coherence measurements in hearing instruments, using different broad-band signals.

In this investigation the effect of different broad-band test signals on non-linear distortion in hearing instruments was examined using coherence measurements, and the interaction between different test signals and types of automatic signal processing is explored. It is concluded that the use of coherence measurements to quantify non-linear distortion in hearing instruments is only valid for instruments without automatic signal processing. This limitation arises because interaction between type of test signal and the automatic signal processing used causes system time-variations, which influences the measured coherence function in an uncontrollable way.

Minimum-free-energy method of spectral estimation: autocorrelation-sequence approach

A new method of parametric spectral estimation, which is called minimum-free-energy (MFE) estimation, is introduced. The MFE method produces a generic theoretic estimation model that is particularly relevant to signal-analysis problems that suffer from incomplete and/or noisy data. In the general MFE formulation, the objective function is defined as a linear combination of a mean-square-error-energy expression and a signal entropy expression. This objective function form is analogous to a free-energy function in statistical thermodynamics. The negative coefficient of the entropy term is represented by an effective signal-processing temperature that drives noise-induced fluctuations in the statistical model. The model parameters that characterize the spectrum are determined commensurate with a minimum of the objective function. The mathematical details and solution methods are developed for a specific embodiment of the MFE method, called the MFE-ACS method, in which the error energy is defined as the window-weighted sum of the absolute square of the difference between the initial and final estimated values of the autocorrelation sequence. The order of the autocorrelation sequence used corresponds to the parametric model order for the spectral estimation procedure. Simulations for a variety of narrow-band and broadband test signals and combinations thereof are presented. These simulations are performed for a variety of signal-to-noise-ratio (SNR) scenarios. The MFE algorithms have a broad application domain because they are not restricted to narrow-band sources as are the signal–noise-subspace algorithms. The MFE-ACS algorithm is shown to compare quite favorably with the signal-subspace Tufts–Kumaresan noise-reduced modified-covariance algorithm for closely separated narrow-band sources in the low-SNR regime (∼10 dB).

Transcranial conduction of acoustic-reflex-eliciting signals.

Conduction of a broad-band signal from a supra-aural earphone to an acoustic-immittance instrument coupled to the contralateral ear canal was studied using three commercial immittance instruments. Results indicated that portions of the broad-band signal reached the contralateral ear canal at presentation levels of 100-110 dB SPL. At levels as low as 100 dB SPL, the signal was detected by at least one of the acoustic-immittance instruments. At higher sound pressure levels the transmitted signal caused deflections of the acoustic-immittance meter similar to those associated with an acoustic-reflex response. However, these artifacts could be differentiated from acoustic-reflex responses in several respects and could be prevented by high-pass filtering of the broad-band test signal.