Showing papers on "Noise (electronics) published in 2003"

How spike generation mechanisms determine the neuronal response to fluctuating inputs

Abstract: This study examines the ability of neurons to track temporally varying inputs, namely by investigating how the instantaneous firing rate of a neuron is modulated by a noisy input with a small sinusoidal component with frequency ( f ). Using numerical simulations of conductance-based neurons and analytical calculations of one-variable nonlinear integrate-and-fire neurons, we characterized the dependence of this modulation on f . For sufficiently high noise, the neuron acts as a low-pass filter. The modulation amplitude is approximately constant for frequencies up to a cutoff frequency, f c , after which it decays. The cutoff frequency increases almost linearly with the firing rate. For higher frequencies, the modulation amplitude decays as C / f α , where the power α depends on the spike initiation mechanism. For conductance-based models, α = 1, and the prefactor C depends solely on the average firing rate and a spike “slope factor,” which determines the sharpness of the spike initiation. These results are attributable to the fact that near threshold, the sodium activation variable can be approximated by an exponential function. Using this feature, we propose a simplified one-variable model, the “exponential integrate-and-fire neuron,” as an approximation of a conductance-based model. We show that this model reproduces the dynamics of a simple conductance-based model extremely well. Our study shows how an intrinsic neuronal property (the characteristics of fast sodium channels) determines the speed with which neurons can track changes in input.

1/f Noise Sources

Abstract: This survey deals with 1/f noise in homogeneous semiconductor samples. A distinction is made between mobility noise and number noise. It is shown that there always is mobility noise with an /spl alpha/ value with a magnitude in the order of 10/sup -4/. Damaging the crystal has a strong influence on /spl alpha/, /spl alpha/ may increase by orders of magnitude. Some theoretical models are briefly discussed none of them can explain all experimental results. The /spl alpha/ values of several semiconductors are given. These values can be used in calculations of 1/f noise in devices. >

Fast Evaluation of Fluctuations in Biochemical Networks With the Linear Noise Approximation

Abstract: Biochemical networks in single cells can display large fluctuations in molecule numbers, making mesoscopic approaches necessary for correct system descriptions. We present a general method that allows rapid characterization of the stochastic properties of intracellular networks. The starting point is a macroscopic description that identifies the system's elementary reactions in terms of rate laws and stoichiometries. From this formulation follows directly the stationary solution of the linear noise approximation (LNA) of the Master equation for all the components in the network. The method complements bifurcation studies of the system's parameter dependence by providing estimates of sizes, correlations, and time scales of stochastic fluctuations. We describe how the LNA can give precise system descriptions also near macroscopic instabilities by suitable variable changes and elimination of fast variables.

A high-speed oscillator-based truly random number source for cryptographic applications on a smart card IC

Abstract: The design of a high-speed IC random number source macro-cell, suitable for integration in a smart card microcontroller, is presented. The oscillator sampling technique is exploited and a jittered oscillator which features an amplified thermal noise source has been designed in order to increase the output throughput and the statistical quality of the generated bit sequences. The oscillator feedback loop acts as an offset compensation for the noise amplifier, thus solving one of the major issues in this kind of circuit. A numerical model for the proposed system has been developed which allows us to carry out an analytical expression for the transition probability between successive bits in the output stream. A prototype chip has been fabricated in a standard digital 0.18 /spl mu/m n-well CMOS process which features a 10 Mbps throughput and fulfills the NIST FIPS and correlation-based tests for randomness. The macro-cell area, excluding pads, is 0.0016 mm/sup 2/ (184 /spl mu/m /spl times/ 86 /spl mu/m) and a 2.3 mW power consumption has been measured.

Fundamental noise limitations to supercontinuum generation in microstructure fiber.

Abstract: Broadband noise on supercontinuum spectra generated in microstructure fiber is shown to lead to amplitude fluctuations as large as 50% for certain input laser pulse parameters. We study this noise using both experimental measurements and numerical simulations with a generalized stochastic nonlinear Schrodinger equation, finding good quantitative agreement over a range of input-pulse energies and chirp values. This noise is shown to arise from nonlinear amplification of two quantum noise inputs: the input-pulse shot noise and the spontaneous Raman scattering down the fiber.

Noise modeling for RF CMOS circuit simulation

Abstract: The RF noise in 0.18-/spl mu/m CMOS technology has been measured and modeled. In contrast to some other groups, we find only a moderate enhancement of the drain current noise for short-channel MOSFETs. The gate current noise on the other hand is more significantly enhanced, which is explained by the effects of the gate resistance. The experimental results are modeled with a nonquasi-static RF model, based on channel segmentation, which is capable of predicting both drain and gate current noise accurately. Experimental evidence is shown for two additional noise mechanisms: 1) avalanche noise associated with the avalanche current from drain to bulk and 2) shot noise in the direct-tunneling gate leakage current. Additionally, we show low-frequency noise measurements, which strongly point toward an explanation of the 1/f noise based on carrier trapping, not only in n-channel MOSFETs, but also in p-channel MOSFETs.

Metallo-dielectric electromagnetic bandgap structures for suppression and isolation of the parallel-plate noise in high-speed circuits

Abstract: A novel approach for the suppression of the parallel-plate waveguide (PPW) noise in high-speed printed circuit boards is presented. In this approach, one of the two conductors forming the PPW is replaced by an electromagnetic bandgap (EBG) surface. The main advantage of the proposed approach over the commonly practiced methods is the omnidirectional noise suppression it provides. For this purpose, two EBG structures are initially designed by utilizing an approximate circuit model. Subsequently, the corresponding band structures are characterized by analytical solutions using the transverse resonance method, as well as full-wave finite-element simulations. The designed EBG surfaces were fabricated and employed in a number of PPW test boards. The corresponding frequency-domain measurements exhibited bandgaps of approximately 2.21 and 3.35 GHz in the frequency range below 6 GHz. More importantly, suppression of the PPW noise by 53% was achieved based on time-domain reflectometry experiments, while maintaining the signal transmission quality within the required specifications for common signaling standards.

A spatially continuous mean field theory of electrocortical activity

Abstract: A set of nonlinear continuum field equations is presented which describes the dynamics of neural activity in cortex. These take into account the most pertinent anatomical and physiological features found in cortex with all parameter values obtainable from independent experiment. Derivation of a white noise fluctuation spectrum from a linearized set of equations shows the presence of strong resonances that correspond to electroencephalographically observed 0.3–4 Hz (mammalian delta), 4–8 Hz (mammalian theta), 8–13 Hz (mammalian alpha) and >13 Hz (mammalian beta) activity. Numerical solutions of a full set of one-dimensional nonlinear equations include properties analogous to cortical evoked potentials, travelling waves at experimentally observed velocities, threshold type spike activity and limit cycle, chaotic and noise driven oscillations at the frequency of the mammalian alpha rhythm. All these types of behaviour are generated with parameters that are within ranges reported experimentally. The strong dependence of the phenomena observed on inhibitory–inhibitory interactions is demonstrated. These results suggest that the classically described alpha may be instantiated in a number of qualitatively distinct dynamical regimes, all of which depend on the integrity of inhibitory– inhibitory population interactions.

In‐building power lines as high‐speed communication channels: channel characterization and a test channel ensemble

Abstract: SUMMARY In-building power lines have often been considered as attractive media for high-speed data transmission, particularly for applications like home networking. In this paper, we develop models for power line channels based both on theoretical considerations and practical measurements. We consider power line channel frequency response and noise models in the 1–30 MHz band and propose a number of power line test channels in which to measure the performance of power line modems. Copyright # 2003 John Wiley & Sons, Ltd.

Varactor characteristics, oscillator tuning curves, and AM-FM conversion

Abstract: A simple analysis relates the small-signal specification of a varactor's capacitance to an oscillator's tuning curve. The notion of an effective capacitance across the amplitude of oscillation is introduced. The analysis also explains how the varactor converts AM noise on the oscillation into FM, which is phase noise. The analysis is experimentally validated.

Experimental investigation of the performance limitation of DPSK systems due to nonlinear phase noise

Abstract: It is known that amplitude fluctuations caused by amplified spontaneous emission noise can be converted into phase noise by Kerr nonlinearity, and this nonlinear phase noise can limit the performance of phase-shift-keying transmission systems. In this letter, we experimentally observe and study the performance degradation of differential phase-shift-keying transmission systems due to nonlinear phase noise. In order to clearly observe the effect, we intentionally add ASE noise to the DPSK signal at the transmitter, and then transmit the signal over a 600-km nonzero dispersion-shifted fiber (NZDSF) link. The results show that the probability density function of nonlinear phase noise deviates from the Gaussian distribution, and this characteristic negates the benefit of a balanced receiver.

Modeling and analysis of high-speed links

Abstract: Very low bit error rate (BER) requirements for the operation of a high-speed link system require a very precise analysis of the link performance in order to prevent unrealistic specifications on both IC design and communication algorithm development. This paper presents the analysis of the noise and distortion sources in a high-speed link system, and their impact on the choice and effectiveness of different communication techniques. Phase-locked loop and clock-and-data recovery loop modeling is also described. It is shown that the most dominant noise and distortion sources are colored and bounded, as opposed to standard unbounded Gaussian white noise assumptions, which yield large errors in the estimation of the link performance and comparison of different signaling techniques. With very low BER requirements, shape of probability distribution of noise and distortion sources and their correlations, are much more important than just their total power, which contrasts the standard analysis in communication systems.

On the design and characterization of femtoampere current-mode circuits

Abstract: In this paper, we show and validate a reliable circuit design technique based on source voltage shifting for current-mode signal processing down to femtoamperes. The technique involves specific-current extractors and logarithmic current splitters for obtaining on-chip subpicoampere currents. It also uses a special on-chip sawtooth oscillator to monitor and measure currents down to a few femtoamperes. This way, subpicoampere currents are characterized without driving them off chip and requiring expensive instrumentation with complicated low leakage setups. A special current mirror is also introduced for reliably replicating such low currents. As an example, a simple log-domain first-order low-pass filter is implemented that uses a 100-fF capacitor and a 3.5-fA bias current to achieve a cutoff frequency of 0.5 Hz. A technique for characterizing noise at these currents is also described and verified. Finally, transistor mismatch measurements are provided and discussed. Experimental measurements are shown throughout the paper, obtained from prototypes fabricated in the AMS 0.35-/spl mu/m three-metal two-poly standard CMOS process.

Self-mixing laser diode vibrometer

Abstract: The principle and the experimental realization of a new type of laser vibrometer based on the self-mixing interference effect in a laser diode are presented. The self-mixing configuration allows for a practical set-up that is simpler by far than conventional laser vibrometer schemes. The vibrometer relies on locking of the system to half the interferometric fringe, and on active phase-nulling by wavelength modulation. This allows an extended dynamic range to be achieved, whilst retaining a good sensitivity to sub-wavelength vibrations. We have designed and built a prototype of the vibrometer that can operate on nearly any kind of rough surface, covering the 0.1 Hz–70 kHz frequency range of vibration. The noise floor is less than 100 pm Hz−1/2, and the maximum measurable vibration amplitude is 180 µm peak to peak. The proposed method can find application in modal analysis and noise and vibration measurements in industrial and scientific environments.

Amorphous silicon active pixel sensor readout circuit for digital imaging

Abstract: The most widely used architecture in large-area amorphous silicon (a-Si) flat panel imagers is a passive pixel sensor (PPS), which consists of a detector and a readout switch. While the PPS has the advantage of being compact and amenable toward high-resolution imaging, reading small PPS output signals requires external column charge amplifiers that produce additional noise and reduce the minimum readable sensor input signal. This work presents a current-mediated amorphous silicon active pixel readout circuit that performs on-pixel amplification of noise-vulnerable sensor input signals to minimize the effect of external readout noise sources associated with "off-chip" charge amplifiers. Results indicate excellent small-signal linearity along with a high, and programmable, charge gain. In addition, the active pixel circuit shows immunity to shift in threshold voltage that is characteristic of a-Si devices. Preliminary circuit noise results and analysis appear promising for its use in noise-sensitive, large-area, medical diagnostic imaging applications such as digital fluoroscopy.

Thermal noise and correlations in photon detection.

Abstract: The standard expressions for the noise that is due to photon fluctuations in thermal background radiation typically apply only for a single detector and are often strictly valid only for single-mode illumination. I describe a technique for rigorously calculating thermal photon noise, which allows for arbitrary numbers of optical inputs and detectors, multiple-mode illumination, and both internal and external noise sources. Several simple examples are given, and a general result is obtained for multimode detectors. The formalism uses scattering matrices, noise correlation matrices, and some fundamentals of quantum optics. The covariance matrix of the photon noise at the detector outputs is calculated and includes the Hanbury Brown and Twiss photon-bunching correlations. These correlations can be of crucial importance, and they explain why instruments such as autocorrelation spectrometers and pairwise-combined interferometers are competitive (and indeed common) at radio wavelengths but have a sensitivity disadvantage at optical wavelengths. The case of autocorrelation spectrometers is studied in detail.

Firing rate of the noisy quadratic integrate-and-fire neuron

Abstract: We calculate the firing rate of the quadratic integrate-and-fire neuron in response to a colored noise input current. Such an input current is a good approximation to the noise due to the random bombardment of spikes, with the correlation time of the noise corresponding to the decay time of the synapses. The key parameter that determines the firing rate is the ratio of the correlation time of the colored noise, τs, to the neuronal time constant, τm. We calculate the firing rate exactly in two limits: when the ratio, τs/τm, goes to zero (white noise) and when it goes to infinity. The correction to the short correlation time limit is O(τs/τm), which is qualitatively different from that of the leaky integrate-and-fire neuron, where the correction is O(√τs/τm). The difference is due to the different boundary conditions of the probability density function of the membrane potential of the neuron at firing threshold. The correction to the long correlation time limit is O(τm/τs). By combining the short and long correlation time limits, we derive an expression that provides a good approximation to the firing rate over the whole range of τs/τm in the suprathreshold regime-- that is, in a regime in which the average current is sufficient to make the cell fire. In the subthreshold regime, the expression breaks down somewhat when τs becomes large compared to τm.

Quantum cryptography using pulsed homodyne detection

Abstract: We report an experimental quantum key distribution that utilizes pulsed homodyne detection, instead of photon counting, to detect weak pulses of coherent light. Although our scheme inherently has a finite error rate, homodyne detection allows high-efficiency detection and quantum state measurement of the transmitted light using only conventional devices at room temperature. Our prototype system works at $1.55\ensuremath{\mu}\mathrm{m}$ wavelength and the quantum channel is a 1-km standard optical fiber. The probability distribution of the measured electric-field amplitude has a Gaussian shape. The effect of experimental imperfections such as optical loss and detector noise can be parametrized by the variance and the mean value of the Gaussian distribution.

Photon counting strategies with low-light-level CCDs

Abstract: Low light level charge-coupled devices (L3CCDs) have recently been developed, incorporating on-chip gain. They may be operated to give an effective readout noise of much less than one electron by implementing an on-chip gain process allowing the detection of individual photons. However, the gain mechanism is stochastic and so introduces significant extra noise into the system. In this paper we examine how best to process the output signal from an L3CCD so as to minimize the contribution of stochastic noise, while still maintaining photometric accuracy. We achieve this by optimizing a transfer function that translates the digitized output signal levels from the L3CCD into a value approximating the photon input as closely as possible by applying thresholding techniques. We identify several thresholding strategies and quantify their impact on the photon counting accuracy and the effective signal-to-noise ratio. We find that it is possible to eliminate the noise introduced by the gain process at the lowest light levels. Reduced improvements are achieved as the light level increases up to about 20 photon pixel−1 and above this there is negligible improvement. Operating L3CCDs at very high speeds will keep the photon flux low, giving the best improvements in signal-to-noise ratio.

Photon counting strategies with low light level CCDs

Abstract: Low light level charge coupled devices (L3CCDs) have recently been developed, incorporating on-chip gain. They may be operated to give an effective readout noise much less than one electron by implementing an on-chip gain process allowing the detection of individual photons. However, the gain mechanism is stochastic and so introduces significant extra noise into the system. In this paper we examine how best to process the output signal from an L3CCD so as to minimize the contribution of stochastic noise, while still maintaining photometric accuracy. We achieve this by optimising a transfer function which translates the digitised output signal levels from the L3CCD into a value approximating the photon input as closely as possible by applying thresholding techniques. We identify several thresholding strategies and quantify their impact on photon counting accuracy and effective signal-to-noise. We find that it is possible to eliminate the noise introduced by the gain process at the lowest light levels. Reduced improvements are achieved as the light level increases up to about twenty photons per pixel and above this there is negligible improvement. Operating L3CCDs at very high speeds will keep the photon flux low, giving the best improvements in signal-to-noise ratio.

Detection of quantum noise from an electrically driven two-level system.

Abstract: The electrical noise of mesoscopic devices can be strongly influenced by the quantum motion of electrons. To probe this effect, we have measured the current fluctuations at high frequency (5 to 90 gigahertz) using a superconductor-insulator-superconductor tunnel junction as an on-chip spectrum analyzer. By coupling this frequency-resolved noise detector to a quantum device, we can measure the high-frequency, nonsymmetrized noise as demonstrated for a Josephson junction. The same scheme is used to detect the current fluctuations arising from coherent charge oscillations in a two-level system, a superconducting charge qubit. A narrow band peak is observed in the spectral noise density at the frequency of the coherent charge oscillations.

Dynamics of an ensemble of noisy bistable elements with global time delayed coupling.

Abstract: The dynamics of an ensemble of bistable elements with global time-delayed coupling under the influence of noise is studied analytically and numerically. Depending on the noise level, the system undergoes ordering transitions and demonstrates multistability. That is, for a strong enough positive feedback it exhibits a nonzero stationary mean-field ,and a variety of stable oscillatory mean-field states are accessible for positive and negative feedback. The regularity of the oscillatory states is maximal for a certain noise level; i.e., the system demonstrates coherence resonance. While away from the transition points the system dynamics is well described by a Gaussian approximation, near the bifurcation points a description in terms of a dichotomous theory is more adequate.

Pilot-assisted estimation of MIMO fading channel response and achievable data rates

Abstract: We analyze the effects of pilot-assisted channel estimation on achievable data rates (lower bound on information capacity) over a frequency flat time-varying channel. Under a block-fading channel model, the effects of the estimation error are evaluated in the case of the estimates being available at the receiver only (open loop) and in the case when the estimates are fed back to the transmitter allowing water pouring transmitter optimization (closed loop). Using a characterization of the effective noise due to estimation error, we analyze the achievable rates as a function of the power allocated to the pilot, the channel coherence time, the background noise level, as well as the number of transmit and receive antennas. The analysis presented here can be used to optimally allocate pilot power for various system and channel operating conditions, and to also determine the effectiveness of closed loop feedback.

Distribution of residence times of time-delayed bistable systems driven by noise.

Abstract: I study bistable time-delayed feedback systems driven by noise. Based on a two-state model with transition rates depending on the earlier state of the system I calculate analytically the residence-time distribution function. I show that the distribution function has a detailed structure, reflective of the effect of the feedback. By using an adequate indicator I give evidence of resonant behavior in dependence on the noise level. I also predict that this feedback-induced effect might be observed in two well-known optical bistable systems.

A 16b 400MS/s DAC with <-80dBc IMD to 300MHz and <-160dBm/Hz noise power spectral density

Abstract: A current-output DAC with on-board calibration engine guarantees 16b monotonicity and achieves better than -160dBm/Hz noise power spectral density. Well bootstrapping, local bias generation and constant data activity techniques are combined to achieve better than -80dBc IMD to 300MHz at 400MS/s.

A low-cost uncooled infrared microbolometer detector in standard CMOS technology

Abstract: This paper reports the development of a low-cost uncooled infrared microbolometer detector using a commercial 08 /spl mu/m CMOS process, where the CMOS n-well layer is used as the infrared sensitive material The n-well is suspended by front-end bulk-micromachining of the fabricated CMOS dies using electrochemical etch-stop technique in TMAH Since this approach does not require any lithography or infrared sensitive material deposition after CMOS fabrication, the detector cost is almost equal to the CMOS chip cost The n-well has a TCR of 05-07%/K, relatively low compared to state-of-the-art microbolometer materials; however, it has negligible 1/f noise due to its single crystal structure The use of polysilicon interconnects on the support arms instead of metal reduces the overall pixel TCR to 034%/K, but provides a better performance due to improved thermal isolation Based on this pixel, a 16 /spl times/ 16 prototype focal plane array (FPA) with 80 /spl mu/m /spl times/ 80 /spl mu/m pixel size and 13% fill factor has been implemented, where built-in diodes are used to simplify array scanning, at the expense of reduced overall pixel TCR of 024%/K The n-well microbolometer array with a simple readout scheme provides a responsivity of 2000 V/W, a detectivity of 26 /spl times/ 10/sup 8/ cmHz/sup 1/2//W, and an estimated NETD of 200 mK at 05 Hz frame rate Considering that this performance can be further improved with low noise readout circuits, the CMOS n-well microbolometer is a cost-effective approach to implement very low-cost uncooled infrared detector arrays with reasonable performance

Influence of automatic level control on micromechanical resonator oscillator phase noise

Abstract: Clear differences in the phase noise performance of a 10 MHz MEMS-based micromechanical resonator oscillator have been measured using sustaining circuits with and without automatic-level control (ALC), and with differing mechanisms for ALC. In particular, low output power oscillators referenced to high-Q clamped-clamped beam /spl mu/mechanical resonators exhibit an unexpected 1/f/sup 3/ phase noise component without ALC, a 1/f/sup 5/ phase noise component when an ALC circuit based on resonator dc-bias adjustment is used, and finally, removal of these components when an ALC circuit based on sustaining amplifier gain control is used, in which case the expected 1/f/sup 2/ phase noise component is all that remains. That ALC is able to remove the 1/f/sup 3/ phase noise seen in non-ALC'ed oscillators suggests that this noise component emanates primarily from nonlinearity in the voltage-to-force capacitive transducer, either through direct aliasing of amplifier 1/f noise, or through instabilities introduced by spring softening (i.e., Duffing) phenomena.

Firing-rate resonance in a generalized integrate-and-fire neuron with subthreshold resonance

Abstract: Neurons that exhibit a peak at finite frequency in their membrane potential response to oscillatory inputs are widespread in the nervous system. However, the influence of this subthreshold resonance on spiking properties has not yet been thoroughly analyzed. To this end, generalized integrate-and-fire models are introduced that reproduce at the linear level the subthreshold behavior of any given conductance-based model. A detailed analysis is presented of the simplest resonant model of this kind that has two variables: the membrane potential and a supplementary voltage-gated resonant variable. The firing-rate modulation created by a noisy weak oscillatory drive, mimicking an in vivo environment, is computed numerically and analytically when the dynamics of the resonant variable is slow compared to that of the membrane potential. The results show that the firing-rate modulation is shaped by the subthreshold resonance. For weak noise, the firing-rate modulation has a minimum near the preferred subthreshold frequency. For higher noise, such as that prevailing in vivo, the firing-rate modulation peaks near the preferred subthreshold frequency.

Analytic expressions for rate and CV of a type I Neuron driven by white Gaussian noise

Abstract: We study the one-dimensional normal form of a saddle-node system under the influence of additive gaussian white noise and a static "bias current" input parameter, a model that can be looked upon as the simplest version of a type I neuron with stochastic input. This is in contrast with the numerous studies devoted to the noise-driven leaky integrate-and-fire neuron. We focus on the firing rate and coefficient of variation (CV) of the interspike interval density, for which scaling relations with respect to the input parameter and noise intensity are derived. Quadrature formulas for rate and CV are numerically evaluated and compared to numerical simulations of the system and to various approximation formulas obtained in different limiting cases of the model. We also show that caution must be used to extend these results to the Θ neuron model with multiplicative gaussian white noise. The correspondence between the first passage time statistics for the saddle-node model and the Θ neuron model is obtained only in the Stratonovich interpretation of the stochastic Θ neuron model, while previous results have focused only on the Ito interpretation. The correct Stratonovich interpretation yields CVs that are still relatively high, although smaller than in the Ito interpretation; it also produces certain qualitative differences, especially at larger noise intensities. Our analysis provides useful relations for assessing the distance to threshold and the level of synaptic noise in real type I neurons from their firing statistics. We also briefly discuss the effect of finite boundaries (finite values of threshold and reset) on the firing statistics.