Showing papers on "Noise (electronics) published in 1977"

Statistical-Physical Models of Electromagnetic Interference

Abstract: Most man-made and natural electromagnetic interference, or "noise," are highly non-Gaussian random processes, whose degrading effects on system performance can be severe, particularly on most conventional systems, which are designed for optimal or near optimal performance against normal noise. In addition, the nature, origins, measurement, and prediction of the general EM interference environment are a major concern of any adequate spectral management program. Accordingly, this study is devoted to the development of analytically tractable, experimentally verifiable, statistical-physical models of such electromagnetic interference. Here, classification into three major types of noise is made: Class A (narrow band vis-a-vis the receiver), Class B (broad band vis-a-vis the receiver), and Class C (= Class A + Class B). First-order statistical models are constructed for the Class A and Class B cases. In particular, the APD (a posteriori probability distribution) or exceedance probability, PD, vis;P1 (? > ?o)A,B, (and the associated probability densities, pdf's w1(?)A,B,[1]) of the envelope are obtained; (the phase is shown to be uniformly distributed in (0, 2?). These results are canonical, i.e., their analytic forms are invariant of the particular noise source and its quantifying parameter values, levels, etc. Class A interference is described by a 3-parameter model, Class B noise by a 6-parameter model.

Least-square fitting a wave-front distortion estimate to an array of phase-difference measurements

Abstract: The problem of fitting a wave-front distortion estimate to a (single-instant) set of phase-difference measurements has been formulated as an unweighted least-square problem. The least-square equations have been developed as a set of simultaneous equations for a square array of phase-difference sensors, with phase estimates at the corner of each measurement element. (This corresponds to the standard Hartmann configuration and to one version of a shearing interferometer of a predetection compensation wave-front sensor.) The noise dependence in the solution of the simultaneous equations is found to be expressible in terms of the solution to a particular version of the measurement inputs to the simultaneous equation, a sort of “Green’s-function” solution. The noise version of the simultaneous equations is solved using relaxation techniques for array sizes from 4 × 4 to 40 × 40 phase estimation points, and the mean-square wave-front error calculated as a function of the mean-square phase-difference measurement error. It is found that the results can be approximated within a fraction of a percent accuracy by 〈(δΦ)2〉=0.6558[1+0.2444 ln(N2)]σpd2, where 〈(δΦ)2〉 is the mean-square error (rad2) in the estimation of the wave-front distortion, for a square array consisting of N2 square subaperture elements over which two phase-difference measurements are made—one phase difference across the x dimension and the other difference across the y dimension. Here σpd2 is the mean-square error (rad2) in each phase-difference measurement.

Noise Due to Photon Counting Statistics in Computed X-Ray Tomography

Abstract: A general expression is derived for the noise due to photon counting statistics in computed X-ray tomography. The variance is inversely proportional to the cube of the resolution distance. For scanners using a water box, the noise in the reconstructed image depends inversely on the number of detected primary photons, summed over all angles, that have passed through a resolution element. Predictions of this formula agree well with the results of computer simulations. It is shown how this formula can be used to determine such parameters as required X-ray flux, detector counting rate, and dose, with special emphasis on tradeoffs between these parameters and resolution. It is also shown that to determine the X-ray attenuation coefficient of a resolution element to a given precision, the number of photons required by computed X-ray tomography is close to a theoretical limit.

Rejection of flow noise using a coherence function method

Abstract: A method has been developed for rejecting transducer flow‐noise interference. The method makes use of coherence‐function relations between simultaneous pressure measurements at three transducers in the signal field and extracts from the flow‐noise background the power spectrum of the signal as received at each transducer. The signal in question can be periodic or stationary random. The coherence function relations are derived on the basis of a multivariate, random‐process model. The theory indicates that for a single source or for a group of completely coherent sources, the three transducers can be placed at arbitrary locations in the signal field. For a group of sources that are not completely coherent, the transducers must be located close together relative to the distance from the group. In any case, however, the flow noises at the transducers must be mutually independent. Successful results were obtained in laboratory tests of the method. In these tests a 10–15‐dB reduction in flow‐noise interference was attained. The method can also be used to reduce other types of noise such as instrument electronic noise.

Statistical-physical models of electromagnetic interference

Abstract: Most man-made and natural electromagnetic interference, or "noise," are highly non-Gaussian random processes, whose degrading effects on system performance can be severe, particularly on most conventional systems, which are designed for optimal or near optimal performance against normal noise. In addition, the nature, origins, measurement, and prediction of the general EM interference environment are a major concern of any adequate spectral management program. Accordingly, this study is devoted to the development of analytically tractable, experimentally verifiable, statistical-physical models of such electromagnetic interference. Here, classification into three major types of noise is made: Class A (narrow band vis-a-vis the receiver), Class B (broad band vis-a-vis the receiver), and Class C (= Class A + Class B). First-order statistical models are constructed for the Class A and Class B cases. In particular, the APD (a posteriori probability distribution) or exceedance probability, PD, vis;P1 (? > ?o)A,B, (and the associated probability densities, pdf's w1(?)A,B,[1]) of the envelope are obtained; (the phase is shown to be uniformly distributed in (0, 2?). These results are canonical, i.e., their analytic forms are invariant of the particular noise source and its quantifying parameter values, levels, etc. Class A interference is described by a 3-parameter model, Class B noise by a 6-parameter model.

Shape and Frequency Composition of Pulses From an Impact Pair

Abstract: With the need to improve the reliability and noise emissions from real mechanisms, an impact in the classical impact pair configuration is investigated considering the impact pulse level and its frequency composition as possible sources of high-frequency energy in articulated systems. The analog representation of the impact pair uses a nonlinear surface stiffness together with a nonlinear surface damping. Developments of the Dubowsky model carried out by Crossley are further extended to allow the surface damping coefficient, as a function of the coefficient of restitution, to be calculated assuming energy is dissipated throughout the impact for any series power law representation of the surface stiffness. The practical system used by Veluswami is simulated, and the results for Dubowsky, Crossley, and the direct solution are compared to Veluswami’s practical data.

A solution to the problem of spontaneous line splitting in maximum entropy power spectrum analysis

Abstract: Under certain conditions, Burg maximum entropy spectra of sampled sine waves, in the presence of additive Gaussian white noise, show either spontaneous line splitting (at low noise levels) or appreciable frequency shifting (at moderate noise levels). This difficulty arises because an unnecessary constraint is imposed during the minimization of the prediction error power. When the constraint is relaxed and a lighter one imposed, the error power decreases and the problem is solved. The nature of the constraint is discussed, and the mathematical details of the new method are presented. The new method is verified by using a few simple test cases in which spontaneous line splitting is healed or frequency shifting is reduced drastically (Fougere, 1975).

A Passband Data-Driven Echo Canceller for Full-Duplex Transmission on Two-Wire Circuits

Abstract: A digitally-implemented echo canceller operating at a rate greater than twice the highest passband frequency is proposed for fullduplex data transmission on a two-wire circuit. Located at each end of the communication circuit, the cancellers operate independently of the local receivers and do not require synchronization of the two data stations. Economies in storage and A/D conversion, in comparison with voice-type cancellers described in the literature, are achieved by accepting data symbols as reference input instead of samples of the transmitted line signal. Convergence of transversal filter tap weights is demonstrated under a mean-squared error criterion, and use of the real passband error rather than the complex analytic error is found to lead to the same residual error at the expense of a doubled convergence time. An operational protocol and adaptation algorithm are proposed which make possible both rapid start-up and slower adaptation during double talking. Provision is made for limiting the number and location of active taps on the transversal filter to those actually necessary for replicating the echo channel, resulting in two transversal filter sections of moderate length separated by a bulk delay. Results from a computer simulation of the proposed canceller are offered to demonstrate that convergence of mean-squared tap-weight error follows the predicted exponential characteristic, that the length of the bulk delay can be determined from a single channel sounding under typical channel noise conditions, and that the use of an averaged-gradient algorithm will allow the canceller to adapt, although slowly, to a change in the echo channel during full-duplex operation.

Heating effects in high-frequency metallic Josephson devices: Voltage limit, bolometric mixing, and noise

Abstract: In this paper, we analyze nonequilibrium effects within a simple heating approximation for the case of metallic Josephson weak links which have a favorable three‐dimensional cooling geometry. Our principal conclusions are the following: (1) The temperature in the center of the junction with voltage V applied is Tm=[Tb2+3 (eV/2πk)2]1/2, where Tb is the bath temperature, e is the electronic charge, and k is Boltzmann’s constant. This Tm can be as high as 70 K in Nb point contacts on a high Josephson step; thus the device noise temperature TN? (1/2)(Tb+Tm) can greatly exceed Tb. (2) The critical current falls approximately as exp(−P/Po), where P is the power dissipated in the junction and Po is typically 10 μW for Al, Sn, Pb, and Nb, but much smaller for Nb3Sn. This leads to a decrease in the amplitude of Josephson steps at high voltages which is in good agreement with data on the best Sn variable‐thickness microbridges and Nb point contacts. In a junction of optimized resistance level, the overall maximum v...

Analysis of electrical noise in turtle cones.

Abstract: 1. Properties of the light-sensitive voltage noise in cones in the retina of the turtle, Pseudemys scripta elegans, have been studied by intracellular recording. 2. Suppression of the noise by light was a function of the hyperpolarizing response of a cone but not of the size or pattern of illumination. 3. Power density spectra of the noise were fitted in many cones by the product of two Lorentzians with characteristic time constants τ1 and τ2 averaging 40 and 7 msec respectively. The spectra of some cells were peaked and could be fitted by a resonance curve. 4. Spectra in dim light exhibited decreased low frequency power. They could often be fitted by a product of two Lorentzians using the same value of τ2 as used in darkness but decreasing τ1 and the zero frequency asymptote. An e-fold reduction in τ1 occurred with lights which hyperpolarized by 4-7 mV. 5. Injection of hyperpolarizing currents of about 0·1-0·2 nA into weakly coupled cones reduced the noise, and also reduced the sensitivity to dim flashes. 6. The variance-voltage relation during steady illumination of different intensities differed from cone to cone. Dim lights increased the noise in some cells and decreased it in others, but moderately bright lights which gave steady responses of more than about one third maximal reduced the noise in all cells. 7. When the cell was transiently depolarized during the differentiated component following steady illumination, the noise was less than it was after prolonged darkness. 8. In the after-effect of bright light, the time course of recovery of noise was the same as that of flash sensitivity and voltage. The noise was reduced e-fold for hyperpolarizations averaging 3 mV while for sensitivity this reduction occurred for 1·3 mV. For a given hyperpolarization the noise was lower during the after-effect than during steady dim illumination. 9. When a series of dim flashes was delivered to a cone, no significant increase in variance over the dark noise was detected during the photo-response. This implies that each photoisomerization evokes no more than about 1·5 μV at the peak of the response in a coupled cone, corresponding to about 50 μV in an isolated cone. 10. The elementary shot events underlying the noise are about 100 μV in amplitude in an isolated cone, have a characteristic time constant of 16-60 msec and reflect unit conductance fluctuations of about 16 pS (S, Siemen ≡ Ω−1). 11. It is concluded that the noise source is internal to the cones. We postulate that the noise arises from opening and closing of the light-sensitive ionic channels in the outer segment, and that in darkness there is a residual concentration of the blocking substance which on average closes up to about one third of the channels. It seems likely that the unit event involves a considerable number of blocking molecules and ionic channels.

Utilization of vacuum fluctuations as an optical brightness standard

Abstract: A proposal is made to use the relationship between the spontaneous and stimulated multiphoton transitions in determination of the spectra] brightness B and the effective temperature of noncoherent light. It is shown that the signal/noise ratio M of a parametric amplifier and frequency converter operating under constant pumping conditions is equal to B expressed in units of hc2/λ5 = Bvac, i.e., to the number n of photons per mode in the converted radiation (this number is known as the degeneracy factor). The strong frequency and angular selectivity of the coherent frequency subtraction ensures a high resolution of the proposed "photon counter". Since the relationship M = n is independent of the rate of pumping, conversion efficiency, etc., such a counter may be considered as a new type of spectral brightness and radiation temperature standard. Another possible metrological application of the parametric amplifier noise arises from the simultaneity and close coupling of the directions of emission of the idler and signal photons. This feature may be used in a source emitting a known number of photons with known momenta and directions of emission; in particular, this approach should make it possible to determine the absolute quantum efficiency of a photomultiplier.

An investigation of temporal moments of stochastic waves

Abstract: It is proposed to describe the temporal characteristics of a wave propagating in a random medium in terms of its temporal moments. The first two moments are related to the mean arrival time and the mean pulse width. It is shown that the one-position two-frequency mutual coherence function enters in the formulation naturally. The form of the expression suggests expanding the mutual coherence function in a narrow-band expansion whose coefficients can be solved exactly from the parabolic equation that takes into account all multiple scattering effects except the backscattering. A brief survey of the literature shows that the irregularity spectrum, under various conditions, has a power-law dependence. In order to conform to this observation a Bessel function spectrum proposed by Shkarofsky is found convenient to use since it not only reduces to the desired power-law form in the proper range of wavenumber space, but also has all the finite moments. Exact expressions for the mean arrival time and mean square pulse width are obtained; some numerical examples are given. Finally, the effect of noise on these moments is discussed.

Conductance noise investigations with four arbitrarily shaped and placed electrodes

Abstract: A general relation is derived for the spectral noise density of the voltage between two arbitrarily shaped and placed sensor electrodes on a conductor, when a constant current or voltage is applied to another pair of arbitrarily shaped and placed driver electrodes. The general relation is based on the sensitivity calculation in linear electrical networks. The relation is elaborated for conductivity fluctuations due to 1/f noise by using empirical 1/f noise relations. The influence of spot radii of the sensor electrodes on the noise is demonstrated.

Current Developments in SAW Oscillator Stability

Abstract: The results reported were obtained with two port delay lines and resonators used in a simple feedback oscillator. The feedback oscillator employed inherently operates with the amplifier in a saturated condition and, therefore, the AM noise is suppressed. Consequently the dominant noise is FM. Generally, it can be concluded that for very narrow-band, or fixed-frequency applications, the resonator-type oscillator will give the best noise performance. For applications where tunability and linearity are important, the delay-line-type oscillator may give the best performance. There have been no significant improvements in oscillator temperature stability. The only two demonstrated materials for temperature stable SAW oscillators are ST-cut quartz and the SiO2/LiTaO3 overlay structure. Aging tests have been going on for the past two-and-a-half years and it has become obvious that the observed aging rates are largely related to cleaning and packaging.

Noise-induced error rate as limiting factory for energy per operation in digital ICs

Abstract: Due to the steady progress in computer technologies over the past years, the physical limits of the energy required per logical operation are assuming increased practical significance. Whereas earlier studies have considered only the elementary energies (e.g., kT, hv) or the power consumption during gate switching, the present work takes account of the random errors induced in an idealized gate by noise. The dependence of the minimum energy W/SUB I/ on the permissible error rate A is calculated for thermal noise. The result obtained, W/SUB I//spl ap/3.9 kT in (5A)/SUP -1/ for 10/SUP -7/>A>10/SUP -23/, is discussed regarding other limits in semiconductor circuits.

Estimate of minimum measurable voltage in the SEM

Abstract: The minimum measurable voltage in the SEM is estimated for a voltage contrast linearization scheme with restricted- and unrestricted-aperture analysers, using approximations for the shapes of the energy distribution curves and assuming that the only significant source of noise is on the collected electrons. A similar estimate is also made for Auger electron voltage measurement schemes. A numerical example shows that the hemispherical retarding-potential analyser system (unrestricted aperture) provides the lowest measurable voltage.

A Comparative Analysis of the Analytical Capabilities of Coherent Anti-Stokes Raman Spectroscopy (CARS) Relative to Raman Scattering and Absorption Spectroscopy

Abstract: The performance capabilities of coherent anti-stokes Raman spectroscopy (CARS) relative to incoherent Raman scattering and absorption spectroscopy are considered for gases. Four gases are considered for model calculations: hydrogen, nitrogen, oxygen, and carbon monoxide. The signal/noise ratio is estimated for these molecules under a number of assumed experimental conditions. The signal/noise ratios for CARS and Raman scattering scale quite differently with partial pressure and temperature. CARS offers distinct advantages when detecting a major component of a gas mixture at total pressures of considerably less than 1 atm pressure. Raman scattering offers 1 to 2 orders of magnitude greater sensitivity for a signal/noise ratio of unity when detecting a minor component of a gaseous mixture at 1 atm total pressure. These model calculations should be of use when a specific experimental method is to be chosen for spectroscopic examination.

Theory of the Signal Transfer and Noise Properties of the rf SQUID

Abstract: The forward and reverse interactions between the SQUID input and output via the weak link are investigated in detail and presented in the form of an electronic equivalent circuit of the SQUID. The input and output impedances and the gain of the SQUID are given. The noise properties for all kinds of source impedances are computed and optimal values derived.

Pile-up rejection: Limitations and corrections for residual errors in energy-dispersive spectrometers

Abstract: Pile-up rejection is commonly achieved through use of an auxiliary inspection channel with short time constants but low energy events can be missed because of electronic noise. A formula is derived to predict the detection efficiency of such a channel and the effect of adjusting the discriminator threshold or changing the time constants is demonstrated. When pulses are missed, spectrum distortion and counting losses are greater than would be predicted from the specified pulse pair resolution time so it is important to optimize the inspector adjustment. A mathematical analysis leads to formulae which can be used to correct for residual counting losses and sum peaks, and the relationship between empirical parameters obtained for these formulae and time constants in the electronics is demonstrated. The present state of the art in noise performance dictates that 1 μs is about the best resolution time that can be achieved for 1 ke V pulses. For further improvements, a fast beam switching technique could be used which would improve pulse pair resolution by a factor of three or more and thus afford adequate protection for the very low energy pulses that would be encountered with thin-window or windowless solid-state detectors.

The quantum correction of the Einstein relation for high frequencies

Abstract: For nondegenerate semiconductors the low-frequency Einstein relation qD0 = kTμ0 must be replaced by qD′(ω) = kTμ′(ω)p(ω) at high frequencies, where D′ and μ′ are the real parts of the complex diffusion coefficient D and the mobility μ, respectively, and p(ω) is the quantum correction factor for thermal noise. This is shown, first by treating thermal noise as a case of velocity-fluctuation noise, and secondly by using a simple statistical model. The results are extended to degenerate semiconductors and metals.

System for reducing the effects of power supply switching

Abstract: A power supply arrangement that substantially eliminates the effects of the sidebands and harmonics at the switching frequency of the power supply on the RF carrier of an amplifier or other unit being supplied power. The switching frequency of the power supply arrangement is continuously varied in frequency at an appropriate rate and deviation with the result that a band of frequencies replaces the fundamental switching frequency of the power supply and all harmonics so that noise created by the switching frequency signal and the RF signal are then similarly diffused. The net power present as spurious noise in the amplifier unit is constant with the maximum amplitude of the noise as viewed, for example, in a doppler filter being substantially reduced. In a radar transmitter, for example, the transmitted pulse frequency appears as PRF sidebands of the CW frequency and the extraneous modulation from the switched power supply provides sidebands of the PRF sidebands. At each of these PRF sidebands, the switching frequency modulation, in accordance with the invention, causes the sidebands resulting from the switching frequency to be spread over a relatively wide band so as to substantially decrease the amplitude of the spurious signals.

Electrical noise from lipid bilayer membranes in the presence of hydrophobic ions.

Abstract: In the presence of the hydrophobic ion dipicrylamine, lipid bilayer membranes exhibit a characteristic type of noise spectrum which is different from other forms of noise described so far. The spectral density of current noise measured in zero voltage increases in proportion to the square of frequency at low frequencies and becomes constant at high frequencies. The observed form of the noise spectrum can be interpreted on the basis of a transport model for hydrophobic ions in which it is assumed that the ions are adsorbed in potential-energy minima at either membrane surface and are able to cross the central energy barrier by thermal activation. Accordingly, current-noise results from random fluctuations in the number of ions jumping over the barrier from right to left and from left to right. On the basis of this model the rate constant ki for the translocation of the hydrophobic ion across the barrier, as well as the mean surface concentration Nt of adsorbed ions may be calculated from the observed spectral intensity of current noise. The values of ki obtained in this way closely agree with the results of previous relaxation experiments. A similar, although less quantitative, agreement is also found for the surface concentration Nt.

The probability density function of the monopulse ratio for n looks at a combination of constant and Rayleigh targets

Abstract: The exact probability density function for the "monopulse ratio, of an amplitude-comparison monopulse radar is presented in closed form. The analysis is valid for multiple looks at any combination of fixed targets, pulse-to-pulse independently fluctuating targets, and receiver noise. The average receiver noise powers in the difference and sum channels need not be equal. Arbitrary signal-to-noise ratio, arbitrary monopulse ratio versus angle characteristic, and arbitrary locations of the targets and jammers in the beam are accommodated. Plots of the density function for various signal-to-noise ratios and off-axis location of the targets are included. A linear approximation is introduced, and the exact results are compared to the linear theory when the signal-to-noise ratio is "small."

A Gaussian Sum Approach to Phase and Frequency Estimation

Abstract: In this paper, a theory of optimal nonlinear estimation from sampled data signals where the a posteriori probability densities are approximated by Gaussian sums is adapted for application to phase and frequency estimation in high noise. The nonlinear estimators (demodulators) require parallel processing of the received signal. In the limit as the number of parallel processors becomes infinite the FM demodulators become optimum in a minimum mean square error sense and the PM demodulators become optimum in some well defined sense. For the clearly suboptimal case of one processor, the demodulators can be readily simplified to the familiar phase-locked loop. On the other hand, for the intermediate case, significant extension of the phaselocked loop threshold is achieved where (say) six parallel processors are involved.

Noise considerations in stellar speckle interferometry

Abstract: A general model for the signal-to-noise ratio for stellar speckle interferometry is developed, including the effects of finite sample averaging, quantum fluctuations, and additive noise. We find that in most cases the controlling parameter is the ratio of the total spatial spectrum to the signal spatial spectrum. Provided the average number of photo events per frame is greater than one, the signal-to-noise ratio for an ideal quantum limited detector is insensitive to aperture size. If a signal-independent noise source is also present, either source can dominate under certain conditions. Large resolved objects must be considerably brighter than their equivalent point source if diffraction details are to be observed.

Theory of noise investigations on conductors with the four‐probe method

Abstract: A general relation is derived for the spectral noise density of the ac open‐circuit voltage fluctuations between two point electrodes in a homogeneous conductor with conductivity fluctuations. The conductor is assumed to extend to infinity and a noiseless direct current flows through the conductor via two current electrodes. Besides, the spectral noise density between two point electrodes is calculated for a number of conductors with specified dimensions and specified current electrodes. These theoretical results are compared with experimental results.

Low frequency noise measurements on silicon-on-sapphire (SOS) MOS transistors

Abstract: Measurements of the equivalent input noise are performed on P -channel silicon-on-saphire, metal-oxide-semiconductor (SOSMOS) transistors as a function of the silicon layer bias voltage with respect to the source. Devices are operated in the ohmic region of the characteristics so that the results are easier to analyse. The results show a high degree of dispersion in the noise characteristics for the different tested devices all originated from the same silicon on sapphire wafer. We conclude that a dispersion of the silicon-sapphire interface properties, such as SRH centers density or fixed positive charge, exists on the wafer. From one specific device, we calculate a fixed positive silicon-sapphire interface charge (about 4 × 10 11 cm −2 ) and a high density (about 5 × 10 11 cm −2 ) SRH centers localized near the silicon-sapphire interface.

On the use of active coupling networks with electrically small receiving antennas

Abstract: An analysis of the noise performance of an active receiving antenna system consisting of an antenna, active coupling network, and high-gain preamplifier is described. It is shown how this noise performance can be described by circular loci in the output-impedance plane of the coupling network. These loci are not restricted to the right-half plane. A coupling network having an output impedance in the left-half plane was built using a negative-impedance converter (NIC), and experimental results obtained by using this circuit in conjunction with a short monopole are presented.

Detectability In The Presence Of Computed Tomographic Reconstruction Noise

Abstract: If the standard filtered backprojection algorithm with a filter of the form g(f) = |f|h(f) is applied to noisy projections, all of which have a noise power spectral density (NPSD), Sproj(f), then the resulting computed tomographic (CT) reconstruction has a two dimensional NPSD of the form, S(f) ~ |f||h(f)|2 Sproj(f). For proper reconstruction, h(f) must approach a non-zero constant as f 0. Provided Sproj(f) is constant, i.e. white projection noise, the CT noise at low frequencies is supprbssbd by the |f| factor. This low frequency suppression results in a long range negative spatial correlation of the CT noise. If white noise is spatially averaged over a circle of diameter d, then the variance in the averaged values will behave as a2 ~ d-2. For CT noise the variance drops faster than d-2. Simple signal-to-noise ratio considerations suggest that the dependence of minimum detectable contrast upon the diameter of the circle to be detected could be significantly differ-ent in the presence of CT noise than in that of white noise. Simulated reconstructions of a suitable detectability pattern demonstrate these differences may not exist unless the image is spatially smoothed before observation. It is pointed out that the pixel width used in the image display should be from 1/3 to 1/2 the width of the point spread function in order to avoid discrete binning problems.

Techniques for Evaluating Charge Coupled Imagers

Abstract: Charge coupled imagers present special problems not encountered with beam-scanned sensors. This paper describes some of the instrumentation, procedures, and analyses used to evaluate charge coupled imagers. A simple analog technique for extracting signals from noise is discussed. Techniques are given for measuring and interpreting MTF (modulation transfer function), limiting resolution, temporal and spatial noise, image spreading, and image lag. Effects peculiar to CCDs (charge coupled devices) and CIDs (charge injection devices), such as aliasing and image smearing, are discussed.