A filtering technique to lower LC oscillator phase noise

Based on a physical understanding of phase-noise mechanisms, a passive LC filter is found to lower the phase-noise factor in a differential oscillator to its fundamental minimum. Three fully integrated LC voltage-controlled oscillators (VCOs) serve as a proof of concept. Two 1.1-GHz VCOs achieve -153 dBc/Hz at 3 MHz offset, biased at 3.7 mA from 2.5 V. A 2.1-GHz VCO achieves -148 dBc/Hz at 15 MHz offset, taking 4 mA from a 2.7-V supply. All oscillators use fully integrated resonators, and the first two exceed discrete transistor modules in figure of merit. Practical aspects and repercussions of the technique are discussed     

The effect of crosstalk and phase noise in multichannel coherentoptical DPSK systems with tight IF filtering

Results for two-channel differential-phase-shift-keying (DPSK) systems using finite integrator and raised cosine response IF filters are presented. The sensitivity of an optical receiver in a two-channel DPSK system is studied. The results are compared with previous work in the limit of no phase noise and it is shown that the agreement is good. The penalty due to crosstalk for different linewidths and filter shapes is computed, and it is shown that the minimum channel spacing is a few bit rates for an ideal integrator IF filter and is larger for an IF filter with a raised cosine impulse response. The penalty is increased somewhat by phase noise     

Intracavity filtering of the spectrum of a short-pulse FEL byinducing interpulse phase coherence

The number of active cavity modes in the short-pulse free-electron laser FELIX was reduced by a factor of 40 at a constant level of the saturated power. This was achieved by inducing phase-coherence between the 40 optical micropulses that are independently amplified at 1 GHz in the 25-MHz cavity. A 1-GHz Fox-Smith intracavity etalon was used to this aim. The resulting spectrum consists of a comb of frequencies that are spaced by 1 GHz. Based on a CW frequency analysis of the coupled cavities we predict that the individual frequencies have a linewidth of 170 kHz. The stability of the selected frequencies was analyzed. Mode hopping over 25 MHz was occasionally observed between macropulses of the laser, but not actually during the macropulse. Simulations by means of a simple pulse evolution model corroborated this behavior. We show that the comb of resonant frequencies can be scanned over a range of 1 GHz by scanning the length of the intracavity etalon. The work presented here gives the theoretical and experimental background of single-linewidth experiments that will be described in a separate paper. The latter experiments concern the selective transmission of a single cavity mode from the phase-locked signal by means of external etalons. This single line should be narrow, stable, and continuously tunable for high-power high-resolution experiments in the far-infrared region of the electromagnetic spectrum     

Image sequence filtering in quantum-limited noise with applications to low-dose fluoroscopy

Clinical angiography requires hundreds of X-ray images, putting the patients and particularly the medical staff at risk. Dosage reduction involves an inevitable sacrifice in image quality. In this work, the latter problem is addressed by first modeling the signal-dependent, Poisson-distributed noise that arises as a result of this dosage reduction. The commonly utilized noise model for single images is shown to be obtainable from the new model. Stochastic temporal filtering techniques are proposed to enhance clinical fluoroscopy sequences corrupted by quantum mottle. The temporal versions of these filters as developed here are more suitable for filtering image sequences, as correlations along the time axis can be utilized. For these dynamic sequences, the problem of displacement field estimation is treated in conjunction with the filtering stage to ensure that the temporal correlations are taken along the direction of motion to prevent object blur.     

Analysis of optical phase noise in fiber-optic systems employing a laser source with arbitrary coherence time

Performance of many optical circuits and systems, such as signal processing and sensing devices, is influenced by random fluctuations of the optical source emission field. This paper outlines a formalism for the analysis of laser phase noise effects on a general linear time-invariant optical system. Theoretical expressions are presented for the autocovariance function of the instantaneous output intensity which are valid for any source coherence time as long as the source intensity fluctuations are assumed to be negligible. Applications of the results to some fiber-optic systems such as Mach-Zehnder and recirculating delay line devices also are demonstrated, both in the coherent and incoherent regimes. Although the analysis is not limited just to fiber-optical systems, our attention is focused on fiber optics.     

On-line estimation of signal and noise parameters with application to adaptive Kalman filtering

This paper presents two algorithms for on-line estimation of the optimal gain of the Kalman filter applied to sensor signals when the signal-to-noise ratio is unknown. First-order spectra of a pure signal and colored measurement noise have been assumed. The proposed adaptive Kalman filtering algorithms have been tested for various spectra of the pure signal and noise, and for various signal-to-noise ratios. The effect of the length of an adaptation step and a sampling frequency on the mean square errors of the pure signal estimation has also been examined. Although the test have been performed for stationary signals, the algorithms presented can also be used successfully for time-varying sensor signals when the signal-to-noise ratios vary very slowly in comparison with the length of the adaptation step.The results are helpful for designers who synthesize optimal linear digital filters for sensor signals with first-order spectra and colored measurement noise. The estimation error curves presented enable designers to determine the noise reduction attainable for particular applications of the adaptive Kalman filtering algorithms.     

Autoassociative neural networks and noise filtering

We introduce linear autoassociative neural (AN) network filters for the removal of additive noise from one-dimensional (1-D) time series. The AN network will have a (2M+1)/spl times/L/spl times/(2M+1) architecture, and for M fixed, we show how to choose the optimal L value and output coordinate from square error estimates between the AN filter outputs and the clean series. The frequency response of AN filters are also studied, and they are shown to act as matched band filters. A noise variance estimate is also derived from this analysis. We numerically illustrate their behavior on two examples and also compare their theoretical performance with that of optimal Wiener filters.     

Adaptive robust impulse noise filtering

It is well known that when data is contaminated by non-Gaussian noise, conventional linear systems may perform poorly. The paper presents an adaptive robust filter (adaptive preprocessor) for canceling impulsive components when the nominal process (or background noise) is a correlated, possibly nonstationary, Gaussian process. The proposed preprocessor does not require iterative and/or batch processing or prior knowledge about the nominal Gaussian process; consequently, it can be implemented in real time and adapt to changes in the environment. Based on simulation results, the proposed adaptive preprocessor shows superior performances over presently available techniques for cleaning impulse noise. Using the proposed adaptive preprocessor to clean the impulsive components in received data samples, conventional linear systems based on the Gaussian assumption can work in an impulsive environment with little if any modification. The technique is applicable to a wide range of problems, such as detection, power spectral estimation, and jamming or clutter suppression in impulsive environments     

Low-frequency noise and phase noise in MESFETS with LTG-GaAs passivation

We report measurements of the low-frequency noise and phase noise of conventional unpassivated GaAs metal semiconductor field-effect transistors (MESFETs) and of MESFETs fabricated using an overlapping-gate structure and the low-temperature grown (LTG) GaAs as a passivation layer. The noise of the LTG-GaAs passivated MESFET was found to behave quite differently from that of a conventional MESFET and to be significantly reduced at low offset frequencies. These observations are explained in terms of the surface passivating effect of the LTG-GaAs. Low-frequency noise measurements seem to support the idea that the LTG-GaAs passivation reduces the number of active traps, in particular traps with large activation emergies. These results indicate that LTG-GaAs passivation can substantially reduce the near-carrier phase noise of MESFET-based oscillators.     

A lower bound on filtering error with application to phase demodulation

A new lower bound on nonlinear filtering mean square error (MSE) based on rate distortion theory is derived for message and observation models described by state space equations. Unlike previous contributions, the present bound is general in applicability: it can be used in beth the continuous and discrete time cases; it is applicable during the transient (e.g., acquisition) as well as steady state phases; vector-valued processes of arbitrary dimension can be treated; stationary as well as nonstationary processes can be considered; and essentially no restrictions are placed on the nonlinearities. Two easily implemented approaches to the evaluation of the new lower bound are given: one is analytical in nature while the other uses Monte Carlo simulation techniques. The theory is extended to filtering with distortion measures other then MSE. The MSE lower bound is applied to a phase demodulation problem and compared to other lower bounds which are based on rate distortion theory and the Cramacute{e}r-Rao inequality. Results for this problem show the new lower bound to be tighter than the others in the nonlinear, high noise-to-signal ratio region of receiver operation.     

Spatial noise filtering in magnetocardiographic measurements

A mathematical approach to construction of spatial filters for multisensor measuring systems that uses the available prior information on the behavior of the fields being measured is described. The application of spatial filtering to the inverse problem of restoration of the local dipole’s parameters in the presence of the noise field from distant sources is shown.     

数字图像中混合噪声的滤波

中值滤波法主要用来抑制椒盐噪声,均值滤波法主要用来抑制高斯噪声.当数字图像中即有椒盐噪声又有高斯噪声时,两种方法均无法达到令人满意的效果.提出了一种混合噪声的滤波方法.首先通过设定阈值,将椒盐噪声和高斯噪声加以区分.然后先对椒盐噪声使用改进了的中值滤波方法进行滤波,之后使用近邻域均值滤波法对高斯噪声进行滤波.仿真结果表明,文中提出的混合去噪声算法计算简单,对数字图像中存在的混合噪声有较好的滤波效果.     

DF-DD for channels with phase noise

In this letter, we design decision-feedback differential detection (DF-DD) schemes for channels with phase noise. If the DF-DD feedback filter is properly optimized, a performance similar to that of a more complex scheme, recently reported by Chiavaccini and Vitetta, based on expectation maximization, can be achieved. However, it is also shown that an error floor is unavoidable for channels with phase noise. If a constraint is introduced, the DF-DD receiver can be made robust against unknown frequency offsets.     

基于标记理论和相位滤波的相位解缠绕算法

相位解缠绕是磁共振成像技术应用中准确获取相位信息的关键所在,如温度监控,油水分离,测定血流速度等.然而,由于磁共振成像噪声比较严重,而且组织交界区域附近相位本身就存在突变,造成解缠绕的精度下降.本文将相位解缠绕转化为计算机视觉中的标记问题,并构造出相应的能量函数,为利用网络优化对其求解提供方便.同时,针对磁共振相位解缠绕中的噪声问题,本文提出一个新的滤波方法,可以高效地消除噪声导致的解缠绕的不一致性.通过仿真实验和磁共振图像的相位解缠绕实验,表明本文方法相位解缠绕精度更高,且计算速度快.     

全相位滤波白噪声产生有色噪声及其功率谱估计

本文提出用全相位数字滤波器这种新型滤波器滤波白噪声产生有色噪声的方法,这种方法比用传统FIR滤波器产生的有色噪声频谱泄漏少,阻带衰减功率大等优点.并且分别用Welch法和全相位法对在有色噪声中混入的双频小信号余弦波进行谱估计,实验证明全相位谱估计性能优于经典谱估计.     

Transversal and longitudinal noise and their coherence in MOST

The thermal and 1/f noise have been studied in MOSTs with side contacts along the channel. In difference biasing conditions, the distribution of the transversal noise in the channel has been explained in terms of the local values, including surface concentration, noise parameter, and mobility. Below saturation, the calculations are in good agreement with the observed experimental data. At saturation, the 1/f noise source strength seems to be nonuniform, along the channel, and the simple model deviates from experimental results. Possible reasons for this are discussed. The observed transversal noise and the coherence of longitudinal noise indicate that 1/f noise is uniformly generated through the channel in the ohmic region. However, there is no proof of the hypothesis that temperature fluctuation is the origin of the 1/f noise. A simple expression is derived to estimate the ratio of transversal to longitudinal noise in terms of channel geometric parameters. For MOSTs biased in the ohmic region at high gate voltage there is good agreement between experimental and calculated results     

一种有效的混合噪声滤波算法

以噪声特点和图像结构分析为基础,提出了一种有效的混合噪声滤除算法。算法首先通过极值判断和像素间的相容性检测,分离出脉冲噪声并以中值滤波滤除;然后对含有高斯噪声的图像以模糊滤波算法进行降噪处理。实验结果表明,本算法能有效地滤除图像中脉冲与高斯混合噪声,且较好地保护了图像细节特征。     

A multiple-crystal interface PLL with VCO realignment to reduce phase noise

An enhancement to a conventional integer-N phase-locked loop (PLL) is introduced, analyzed, and demonstrated experimentally to significantly reduce voltage-controlled oscillator (VCO) phase noise. The enhancement, which involves periodically injection locking the VCO to a buffered version of the reference, has the effect of widening the PLL bandwidth and reducing the overall phase noise. It is demonstrated in a 3-V 6.8-mW CMOS reference PLL with a ring VCO capable of converting most of the popular crystal reference frequencies to a 96-MHz RF PLL reference and baseband clock for a direct conversion Bluetooth wireless LAN. The peak in-band phase noise at an offset of 20 kHz is -102 dBc/Hz with the technique enabled and -92 dBc/Hz with the technique disabled. A theoretical analysis is presented and shown to be in close agreement with the measured results.     

In-band amplified spontaneous emission noise filtering with adispersion-imbalanced nonlinear loop mirror

This paper reports the removal of in-band amplified spontaneous emission (ASE) noise in a 10 Gb/s noise-loaded return-to-zero (RZ) signal using a dispersion-imbalanced loop mirror. The resulting receiver sensitivity is improved from -37.0 dBm to -37.7 dBm. The dependence of receiver sensitivity on the loop mirror bias is also investigated, and the optimal bias point is found to be where the loop transmissivity decreases with increasing input power