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Transient dynamics of tapping mode atomic force microscope (AFM) for critical dimension measurement are analyzed. A simplified nonlinear model of AFM is presented to describe the forced vibration of the micro cantilever-tip system with consideration of both contact and non-contact transient behavior for critical dimension measurement. The governing motion equations of the AFM cantilever system are derived from the developed model. Based on the established dynamic model, motion state of the AFM cantilever system is calculated utilizing the method of averaging with the form of slow flow equations. Further analytical solutions are obtained to reveal the effects of critical parameters on the system dynamic performance. In addition, features of dynamic response of tapping mode AFM in critical dimension measurement are studied, where the effects of equivalent contact stiffness, quality factor and resonance frequency of cantilever on the system dynamic behavior are investigated. Contact behavior between the tip and sample is also analyzed and the frequency drift in contact phase is further explored. Influence of the interaction between the tip and sample on the subsequent non-contact phase is studied with regard to different parameters. The dependence of the minimum amplitude of tip displacement and maximum phase difference on the equivalent contact stiffness, quality factor and resonance frequency are investigated. This study brings further insights into the dynamic characteristics of tapping mode AFM for critical dimension measurement, and thus provides guidelines for the high fidelity tapping mode AFM scanning.

This paper examines the frequency drift of sub-synchronous oscillations (SSOs) caused by the DFIG wind farms when the wind speed varies. The examination is carried out by applying the analysis about the open-loop modal coupling, a newly appeared concept to investigate power system oscillations. Analysis is presented in the paper to reveal that when the wind speed varies, the open-loop electromechanical oscillation mode of the DFIG moves on the complex plane. When it moves close to an open-loop SSO mode of the rest of power system, the open-loop modal coupling occurs to lead to the damping degradation of the SSOs caused by the DFIG. Thus, the variation of wind speed may possibly cause the open-loop modal coupling between the DFIG’s electromechanical oscillation mode and different open-loop SSO modes of the power system. Consequently, poorly or even negatively damped SSOs happen at different SSO frequency. Therefore, the paper provides a better understanding on and deeper insight into the mechanism about how the change of wind speed causes the frequency drift of the SSOs from the standpoint of the open-loop modal coupling.

Grid-tied inverters are widely used in the modern power network. The span of their applications varies from power quality enhancement, uninterruptible power supply, grid interface to distributed energy sources, etc. A major aspect of controlling the grid-tied power converter is the synchronization with the main grid. A robust synchronization method is required to maintain the power injection during grid fault and highly distorted grid voltage. This paper introduces a novel Pseudo-Open Loop Synchronization Technique (POLS) based on a fixed reference frame estimator: the Fundamental Positive Components Estimator (PFCE). The algorithm will generate three unitary sinus waveform synchronized with the fundamental positive component of a three-phase grid voltage. Open Loop Synchronization (OLS) technique differs from the closed loop PLL in that it has a faster dynamics response. Two problems may face the OLS: a low-quality main voltage and frequency drift. To face the voltage quality problem, the PFCE is used. This choice is advantageous over the use of a low-pass filter due to the absence of the time delay and filtering performances. For the frequency drift, an optional simple frequency estimator is used. No trigonometric computation is required for the proposed algorithm, making it easy to implement on a low-cost microcontroller. The performances and effectiveness of the proposed method are demonstrated through simulation and experimental results.

This paper studies a new dual-band CMOS class-C voltage-controlled oscillator (VCO). The oscillator consists of a dual-resonance LC resonator in shunt with two pairs of capacitive cross-coupled nMOSFETs. The proposed oscillator has been implemented with the TSMC 0.18 μm CMOS technology, and it shows a frequency tuning range with two frequency bands and a small tuning hysteresis is measured. The oscillator can generate differential signals at 2.4 GHz and 6.9 GHz and it also can generate concurrent frequency oscillation while the circuit is biased around the bias with frequency tuning hysteresis. With the supply voltage of VDD = 1.1 V, the VCO-core current and power consumption of the oscillator are 2.90 mA and 3.19 mW, respectively. The die area of the class-C oscillator is 0.9 × 0.97 mm2. Overvoltage stress is applied to the oscillator, measurement indicates the concurrent oscillation is sensitive to overvoltage stress.

Sandia frequency shift (SFS) is one of the active anti-islanding detection methods that depend on frequency drift to detect an islanding condition for inverter-based distributed generation. The non-detection zone (NDZ) of the SFS method depends to a great extent on its parameters. Improper adjusting of these parameters may result in failure of the method. This paper presents a proposed artificial immune system (AIS)-based technique to obtain optimal parameters of SFS anti-islanding detection method. The immune system is highly distributed, highly adaptive, and self-organizing in nature, maintains a memory of past encounters, and has the ability to continually learn about new encounters. The proposed method generates less total harmonic distortion (THD) than the conventional SFS, which results in faster island detection and better non-detection zone. The performance of the proposed method is derived analytically and simulated using Matlab/Simulink. Two case studies are used to verify the proposed method. The first case includes a photovoltaic (PV) connected to grid and the second includes a wind turbine connected to grid. The deduced optimized parameter setting helps to achieve the “non-islanding inverter” as well as least potential adverse impact on power quality. Keywords: Anti-islanding detection, Sandia frequency shift (SFS), Non-detection zone (NDZ), Total harmonic distortion (THD), Artificial immune system (AIS), Clonal selection algorithm

This paper investigates the effects of material and dimension parameters on the frequency splitting, frequency drift, and quality factor (Q) of aluminium nitride (AlN)-on-n-doped/pure silicon (Si) microelectromechanical systems (MEMS) disk resonators through analysis and simulation. These parameters include the crystallographic orientation, dopant, substrate thickness, and temperature. The resonators operate in the elliptical, higher order, and flexural modes. The simulation results show that i) the turnover points of the resonators exist at 55 °C, –50 °C, 40 °C, and –10 °C for n-doped silicon with the doping concentration of 2 × 1019 cm–3 and the Si thickness of 3.5 μm, and these points are shifted with the substrate thickness and mode variations; ii) compared with pure Si, the modal-frequency splitting for n-doped Si is higher and increases from 5% to 10% for all studied modes; iii) Q of the resonators depends> on the temperature and dopant. Therefore, the turnover, modal-frequency splitting, and Q of the resonators depend on the thickness and material of the substrate and the temperature. This work offers an analysis and design platform for high-performance MEMS gyroscopes as well as oscillators in terms of the temperature compensation by n-doped Si.

In the master clock system, the local standard time UTC(k) with a better short-term stability will be generated, if the hydrogen maser is set as a frequency source of the master clock. But the hydrogen maser always exhibits an apparent frequency drift, thus its long-term stability gets poor with the time, therefore the stability and accuracy of UTC(k) become worse. To solve this problem, the performance of hydrogen maser is compared with cesium clocks, and the time scale algorithm is modified when the hydrogen maser is involved, a new steering strategy is proposed when a hydrogen maser is used as the frequency source of the master clock. An experimental system is set up with the programs compiled, and finally the new steering strategy is testified with the laboratory data. The results show that when the hydrogen maser is involved in the atomic time scale calculation, the short-term frequency stability of the reference time scale will be improved. Meanwhile, the local time UTC(k) has a better short-term frequency stability when the frequency source of the master clock uses a hydrogen maser instead of a cesium clock.

One-chip measurements without modifying the physical structure of packaged integrated circuits such as field-programmable gate arrays (FPGA) is challenging. This paper proposes a sensor for detecting the radio frequency interference (RFI) on the supply inside the FPGA chip. The core of the sensor is a ring oscillator built with FPGA look-up tables. The paper proposes a model to predict the response of the ring oscillator to power supply RFI, and shows that the normalized frequency shift of the ring oscillator resulting from the interference is determined by the amplitude of the interference. This relationship is independent of the interference frequency and the size of the ring oscillator. To verify the model, simulations on transistor-level look-up tables of 130-nm and 40-nm technologies were performed. The simulation results matched well with the model. In addition to simulation, an FPGA test board was fabricated. Measurements of FPGA RFI response were performed and the results were consistent with the theoretical model. The effect of the interference on the ring oscillator provided a mechanism to detect the amplitude of the supply interference on the FPGA chip. The frequency of the ring oscillator was monitored through the supply pin by measuring the spectrum of the supply noise. The properties of the sensor, such as constant response in a wide frequency range, insensitiveness to the oscillator size, ease of implementation, and minimal modification requirement of the physical structure, made it suitable for performing on-chip FPGA measurements.

Use of distributed generators in distribution networks is proved highly reliable and economical. In utility practice at the present time, accidental islanding is an undesirable mode of operation as it can harm the personnel and other connected systems. Therefore, islanding must be detected and the islanded distributed generators must be disconnected from the rest of the system. This paper proposes a new islanding detection method which provides a reliable detection in two levels. At first level wavelet energy entropy is used as an indicator to suspect islanding and activate second level method. Second level method is an active frequency drift method with positive feedback and pulsating chopping fraction that vary the output voltage frequency of distributed generators to realize the anti islanding protections. The performance of the proposed method is simulated in MATLAB environment. The proposed algorithm provides fast islanding detection with least disturbance in inverter’s grid connected operation.

The response of a Duffing oscillator subject to a periodic forcing with slowly and stochastically modulated frequency is analyzed numerically. The results of both moment and cumulant-based stochastic reductions are compared to Monte Carlo simulations. It is shown how the explicit characterization of higher-order central moments of the (Gaussian) noise source and the periodic nature of the forcing enable a reliable reduction strategy providing a faithful description of the mean behavior of stochastic solutions. The reduced model is then used to illustrate how a large noise level and fast frequency drift may combine to sustain a strong response that is normally associated to resonance in the noiseless static case.

We report on a multifunctional active demultiplexer for optical frequency combs. The proposed technique, based on injection locking, combines the functionality of a tunable demultiplexer, gain equalizer and an optical amplifier, all in one device. Validation of the concept is experimentally demonstrated through simultaneous demultiplexing and amplification of combs with various free spectral ranges (2.5 to 15 GHz), achieving an adjacent channel suppression ratio > 35 dB and an output power of 7.5 dBm. The functionality of the demultiplexer is extended to that of a gain equalizer, by filtering comb tones with powers 10 dB below the spectral peak, whilst maintaining a suppression ratio > 30 dB and a constant output power. This feature increases the number of comb lines suited for data modulation (i.e. usable lines), by a factor of ~1.7. Finally, we test the stability of such a demultiplexer, by measuring the frequency drift, beat tone power variations and phase correlation between demultiplexed tones.

In this study, to measure the frequency drift characteristics of the narrow-linewidth laser, we provide a method with high accuracy and adjustable measuring range that can be applied in a resonant optical gyro (ROG) system. In an ROG system, the laser frequency drift characteristics, such as fluctuation range and frequency, influence the performance of the closed-loop frequency-locked module. To measure the above performance of the laser, we proposed a novel measurement system based on the Fabry–Perot cavity. We proved the feasibility of the scheme through theory and simulation. Then, the calibration of the experimental device and the measurement of the tested laser were completed. The measurement accuracy of this method is 1.2 kHz, and the measurement range reaches ±60 MHz, which can be adjusted by the modulation index. The results show that the laser has a frequency fluctuation of 30 MHz in the stabilization stage, and there is mainly a low-frequency component under 5 Hz in the frequency domain of the laser frequency fluctuation. Furthermore, this measurement scheme can be applied in other fiber sensor systems to evaluate the effect of laser frequency fluctuation on system output performance.

Ion capture from an external nano-electrospray ionization source in a Fourier transform electrostatic linear ion trap has been effected by mirror-switching. This capture method can suffer from time-dependent frequency shifts in the measured ion motion, which compromises mass resolution when using Fourier transform techniques for mass determination. This phenomenon was determined to be a result of the transient voltage recovery of the power supplies used for mirror-switching in response to a pulsed capacitive load, for which several examples are shown. A circuit, based on the AD210AN isolation amplifier, was fabricated to compensate for the voltage perturbation induced by mirror-switching by superimposing the inverted perturbation to the electric field of the opposing reflectron. In doing so, the dependence of the ions path length and frequency on the power supply output was greatly reduced throughout data acquisition. With this circuit enabled, no frequency shifts were observed in the mass spectrum when using mirror-switching, and thus pressure-limited theoretical resolutions were achieved. For example, an absorption mode resolving power of greater than 50,000 M/ΔM FWHM was observed for iodide (m/z 126.9) at a transient length of 300 milliseconds. The use of mirror-switching led to a much greater m/z range than in-trap potential lift for a single ion injection which is demonstrated via simulation and experimental results. This correction method is simple to implement and does not require user intervention once properly tuned.

In order to carry out the spectral observation with the TM65m radio telescope, the frequency calibration and test of DIBAS (Digital Backend System) are performed, it is found that it has a good performance. First, by injecting the PCAL signals, the frequency resolution, frequency drift and the stability of frequency interval between two spectral lines of the DIBAS backend are measured. It is found that in one hour, the maximum frequency drift of a single spike is 0.03 channel, the maximum fluctuation of spike interval is 0.05 channel. Then, by the observations on the H 2 CO maser and absorbtion lines of massive star formation regions, and the comparison with the results observed by the GBT (Robert C. Byrd Green Bank Telescope), it is shown that the results of frequency calibration are correct. Finally, by the OH maser observations in more than one hour toward W3(OH), and the methanol maser observations in more than 5 hours, it is found that the spectral profiles keep consistent, and the observational noise is consistent with the theoretical value, indicating the stability and reliability of the frequency calibration program.

This paper proposes a voltage controlled oscillator (VCO) which is based on a pseudo-differential ring oscillator suitable for multi-Gbps serial interfaces. The delay element employed in the ring oscillator topology consists of two CMOS inverters loaded by a simple negative resistance realized with pMOS transistors. The proposed topology is designed and simulated in a 65nm CMOS process with 1.2V supply voltage. As a realistic application, the VCO performance is optimized to be compliant with the MIPI Alliance M-PHY standard which is the most updated high-speed serial interface technology. The frequency tuning range covers the three frequency bands of the MPHY standard which are 0.624 and 0.7288GHz, 1.248 and 1.4576GHz and 2.496 and 2.9152GHz and corresponds to high-speed gear 1,2 and 3, respectively. In each gear an almost constant KVCO is achieved while the current consumption of the whole system is 2.87, 5.19 and 10.06mA for the gears 1,2 and 3, respectively. The phase noise is about -94dBc/Hz at 1MHz frequency offset for 2.9152GHz.

Optically pumped terahertz (THz) lasers as a reliable THz radiation sources have been widely used in THz application area. Considering the Doppler-broadened effects and the two-photon light shift effects, the physical model for the THz output power and the THz output frequency drift of optically pumped THz lasers has been established based on the rate equations. The main factors affecting THz laser output have been analyzed quantitatively. The results indicate that the THz output power increases with the increasing of the pump power, while decreases with the increasing of the pump frequency offset from the operating gas absorption centre. The THz output frequency drift is mainly caused by two-photon light shift when the pump offset is small, whereas Doppler-broadened becomes main factor if the pump frequency offset is relatively larger. Furthermore, the THz output frequency drift increases in proportion to the pump power. The stability of the THz output frequency can be enhanced and the THz output power can be improved by choosing pressure in the cavity reasonably, and the optimal working gas pressure range is 15–20 Pa. Stabilizing the pump laser frequency in the range of gas absorption centre, choosing reasonable working gas pressure in the THz cavity and the pump power can efficiently improve the performance of the THz laser output.

In this paper we report on the design of a frequency compensation system for AlN-on-Si MEMS reference oscillator to replace temperature compensated crystal oscillators (TCXOs) in cellular handsets. A 76.8MHz, 105ppm Temperature Stable, AlN-on-Si MEMS Oscillator is designed to demonstrate the frequency compensation scheme. Double layers of SiO 2 are used for passive temperature compensation. The oscillator consumes 850µA, with phase noise of -127dBc/Hz at 1kHz frequency offset. Temperature drift errors and initial frequency offset of ź 8000ppm are combined and further tackled electronically. A simple digital compensation circuitry generates a compensation word to a 21-bit MASH 1-1-1 ΔΣ modulator included in LTE fractional N-PLL for frequency compensation. Temperature is sensed using 4.6µA, 11.5 bit temperature to digital converter TDC with resolution of 0.1°C in 100ms conversion time. The paper presents the first AlN-on-Si oscillator platform with ź 0.5ppm frequency stability over temperature ranges from -40°C to 85°C. The system runs on 1.8V supply in 32nm CMOS.

A low frequency relaxation oscillator is designed using a super-capacitor. An accurate analytical expression for the oscillation frequency is derived based on a fractional-order super-capacitor model composed of a resistance in series with a Constant Phase Element (CPE) whose pseudo-capacitance and dispersion coefficient are determined using impedance spectroscopy measurements. Experimental results confirm our theoretical analysis.

Ion capture from an external nano-electrospray ionization source in an electrostatic linear ion trap has been effected by in-trap potential lift so as to avoid a time-dependent frequency drift of trapped ions. This phenomenon was observed when using mirror switching for capturing ions and compromised the mass resolution when using Fourier transform techniques for mass determination. A dual image charge detection approach was also implemented to compensate for losses in mass resolution associated with increasing the length of the electrostatic trap to accommodate the lift region. The potential lift approach for ion capture led to no detectable frequency shifts, thereby enabling the achievement of pressure-limited theoretical resolutions. For example, a resolution of roughly 11,000 M/ΔM FWHM was observed for a carborane anion population of average m / z = 520 at a transient length of 125 ms. The use of the dual detector approach led to an increase (∼12%) in the ion frequencies used for mass analysis, which more than compensated for the effect of increasing the length of the electrostatic trap. Although the implementation of the potential lift approach and dual detectors were successful in their objectives, these changes resulted in a narrower m / z range for a single ion injection event relative to mirror switching. Furthermore, the dual detector approach resulted in a higher noise floor and a more complicated frequency spectrum due to asymmetries in the electric fields of the ion trap.

International audience; In this paper, we investigate the optimal operating conditions of electrostatically-actuated MEMS oscillators in terms of frequency stability. Through a simple model, the influence of the fluctuation of several design parameters (e.g. feedback phase, feedback amplitude, bias voltage) on oscillator performance is studied. It is shown that, in the nonlinear regime, the excitation waveform (e.g. square, sinusoid, impulsive, etc.) plays a critical role on the long-term frequency stability of electrostatic MEMS oscillators, as opposed to the well-known but idealized case of a Duffing oscillator.

This paper proposes a nonlinear whole-machine dynamic model for a typical bimodal standing wave ultrasonic motor considering nonlinear electroelasticity of piezoelectric vibrator under high excitation voltage. To capture the inherent nonlinearities of the piezoelectric element, nonlinear constitutive relations of piezoelectric element are adopted to derive the nonlinear electromechanical coupling model of the stator. The contact mechanism between the stator and the mover including stick-slip-separation dynamics and asperity contact mechanics is modeled based on a physical-based equivalent spring model and a modified Coulomb friction model. The measured nonlinear relationships between the excitation voltage and the amplitude response of the stator near resonant frequency are used along with analytical approximations for the nonlinear vibration responses by harmonic balance method to identify the critical nonlinear parameters. Finally, experiments are conducted to validate the proposed model, and the results show that the simulations agree well with the experimental results. In particular, nonlinear speed saturation phenomenon and the resonant frequency drift of the motor can be predicted and analyzed by the developed model when the motor is driven at a higher voltage, which are useful for the performance evaluation and working range choice of such motors under high-power input conditions.

The resonant frequency of coils in a four-coil magnetic coupling resonance wireless power transfer (MCR-WPT) system has a direct impact on transfer coefficient. For efficient power transfer, most of the systems operate at high-Q resonant state, causing a narrow bandwidth and reducing the system tolerance to resonant frequency deviation. To deal with the problem, a four-coil MCR-WPT system characterized with broadband performance is investigated. Based on the equivalent circuit, the transfer coefficient and the input impedance are firstly calculated. The transfer characteristic at different resonant frequency under different transfer distances is discussed. By comparing the number and variance of the extreme values of transfer coefficient under different resonant frequencies, the resonant frequency for excellent broadband transmission is determined. To validate the broadband performance, frequency drift caused by capacitance variations is investigated at the resonant frequency of 19 MHz under the transfer distance of 45 mm. The results indicate that the MRC-WPT system generally maintains broadband transmission and the transfer coefficient at the excitation frequency of 19 MHz remains above 0.9 when the resonant frequency is deviated. Experiments have been performed to validate the excellent broadband performance. The results are generally consistent with the simulation analysis.

The nonlinear coupled vibrations widely exist in coupled resonant structures, which can lead to complex dynamic bifurcation behavior and expand the research scope of fundamental physics. A new micro-mass detection method is proposed by using bifurcation jumping phenomenon in nonlinear electrostatically coupled resonators in this article. Considering the fundamental frequency excitation, the one-to-one internal resonance equations to describe electrostatically coupled resonant sensor are obtained by using Hamilton’s principle and Galerkin method. Then, the perturbation analysis method is introduced to study the response and stability of the system for small amplitude vibration. Through bifurcation analysis, it is found that the isolated response branches appear in nonlinear electrostatically coupled resonators and present the physical conditions of this phenomenon. Typically, we demonstrate the exploitation of the bifurcation jump phenomena of two electrostatically coupled microbeam resonators to realize the mass quantitative detection and threshold detection, which overcomes the detection inaccuracy caused by frequency drift in the nonlinear vibration. Finally, the numerical experiments verify the validity of the method. The results of this paper can be potentially useful in micro-mass detection.

Many recent planetary entry vehicles have maintained radio links to receivers on either Earth or a nearby spacecraft during their risky atmospheric entries. The trajectory and the associated atmospheric structure of a descending entry vehicle are traditionally reconstructed from onboard accelerometer data, but they can in principle be reconstructed from the Doppler shift in a radio link instead. However, such methods have only been explored at the proof-of-concept level. Here we develop an improved radio-based reconstruction method and use it to simulate reconstruction of the trajectories and atmospheric structures of Pathfinder, Spirit, Opportunity, Phoenix, and Curiosity, validating the simulated results against those obtained by traditional means. Due to a singularity in the underlying equations, the radio-based reconstruction method may fail badly if the velocity vector of the transmitting entry vehicle with respect to the atmosphere is perpendicular to the line of sight between the transmitter and the receiver. A simple plot of the rate of change of the received frequency is also helpful for indicating the likely applicability of this method. Agreement between simulated and actual trajectories and atmospheric structures is very good for Spirit and Opportunity and for receivers on Earth and an orbiter, which establishes the validity and potential value of this method. The root-mean-square temperature error is no greater than 5 K at radial distances between 3410 km and 3470 km, a range of six scale heights. Agreement is less good for Pathfinder due to limitations in the available data and for Phoenix due to geometrical reasons. The method assumes a lift-free trajectory, which is a valid assumption for the first four landers, but not for Curiosity. Nevertheless, the results of this method are reasonable for Curiosity before its period of horizontal flight, which was not reconstructed adequately by this method at all. Sensitivity studies found that results are satisfactory for thermal noise of less than 1 part in 10 8 and frequency drift of less than 1 part in 10 9 per second. A frequency offset has no impact on the results. The radio-based reconstruction method is ready to be applied to the upcoming atmospheric entries of several Mars landers.

A novel location scheme based on the wavelet packet transform (WPT) for the phase-sensitive optical time-domain reflectometry (OTDR) is proposed. The wavelet packet energy of the backscattered signal at a time interval over the sensing fiber is provided to discriminate the disturbances regions from other regions. It is insensitive to the frequency drift of the laser. The simultaneous measurement of two disturbances at 5 km and 7 km positions is achieved by the location technique based on the WPT over a 9 km monitored length with a 50 m spatial resolution even when a laser with a frequency drift rate of 230 MHz/min is utilized. During 100 times experiments the false alarm rate is only 2% and the maximum location error is 150 m.

Tunable single-mode injection-locked optoelectronic oscillator (OEO) with low phase-noise is presented in this article. A master oscillator contributed by amplifier and tunable band-pass filter (TBPF) is embedded in the general single loop OEO. The oscillation frequency is determined by the center frequency of the TBPF. A portion of master oscillation signal is injected into the slave OEO loop, then it is locked by OEO signal after it passes through a long optical delay line with high Q value. With the RF isolator, the master oscillation signal with poor phase-noise is avoided directly mixing in the OEO signal. With the configuration a tunable range from 1.93 to 17.69 GHz with 68 dB suppression of the nearest-neighbor spur is realized. The single-sideband phase noise is below −112 dBc/Hz at an offset of 10 kHz within the whole tunable range. The frequency drift of the OEO system is

In this paper, the operation of rotary traveling wave oscillators is analyzed, the general oscillation condition is derived, and analytical formula for the oscillator loss is presented. Based on this analysis, switched transmission line is employed to extend the output frequency tuning range. Post-layout simulation shows a frequency tuning range of 3.1 GHz in the vicinity of 30 GHz. The proposed half-quadrature VCO exhibits a phase noise better than −102.2 dBc/Hz at 1 MHz offset frequency. The VCO provides an output power level ranging from −6 to −2.5 dBm with drawing 15.2 mA of dc current from a 1.8 V power supply.

Clock error estimation has been the focus of a great deal of research because of the extensive usage of clocks in GPS positioning applications. The receiver clock error in the spacecraft orbit determination is commonly estimated on an epoch-by-epoch basis, along with the spacecraft’s position. However, due to the high correlation between the spacecraft orbit altitude and the receiver clock parameters, estimates of the radial component are degraded in the kinematic approach. Using clocks with high stability, the predictable behaviour of the receiver oscillator can be exploited to improve the positioning accuracy, especially for the radial component. This paper introduces two GPS receiver clock models to describe the deterministic and stochastic property of the receiver clock, both of which can improve the accuracy of kinematic orbit determination for spacecraft in low earth orbit. In particular, the clock parameters are estimated as time offset and frequency offset in the two-state model. The frequency drift is also estimated as an unknown parameter in the three-state model. Additionally, residual non-deterministic random errors such as frequency white noise, frequency random walk noise and frequency random run noise are modelled. Test results indicate that the positioning accuracy could be improved significantly using one day of GRACE flight data. In particular, the error of the radial component was reduced by over 40.0% in the real-time scenario.

A low power frequency synthesizer for WLAN applications is proposed in this paper. The NMOS transistor-feedback voltage controlled oscillator (VCO) is designed for the purpose of decreasing phase noise. TSPC frequency divider is designed for widening the frequency range with keeping low the power consumption. The phase frequency detector (PFD) with XOR delay cell is designed to have the low blind and dead zone, also for neutralizing the charge pump (CP) output currents; the high gain operational amplifier and miller capacitors are applied to the circuit. The frequency synthesizer is simulated in 0.18 µm CMOS technology while it works at 1.8 V supply voltage. The VCO has a phase noise of −136 dBc/Hz at 1 MHz offset. It has 10.2% tuning range. With existence of a frequency divider in the frequency synthesizer loop the output frequency of the VCO can be divided into the maximum ratio of 18. It is considered that the power consumption of the frequency synthesizer is 4 mW and the chip area is 10,400 µm 2 .

Background Previous studies described long term effects of transcranial alternating current stimulation (tACS) on alpha ( α ) frequency (Helfrich et al., 2014; Muller et al., 2015; Varlet et al., 2017).tACS was hence proposed as a tool to synchronize and modulate regional biological neuronal oscillations. Objective Our objective was to validate tACS as a suitable method effecting neuronal oscillations over a stimulated areal.This study is part of a project that investigates the clinical relevance of the stimulation as a treatment option for impulse control disorders. Methods We applied tACS over the occipital cortex with an α frequency of 10 Hz as well as an appropriate sham stimulation.We studied 17 participants (18–30 yrs) in two separate sessions using the neuroConn DC-stimulator plus.Before and after the stimulation three minute EEG-intervals were analysed with respect to their α peak frequency and power spectral density.We segmented these intervals in 1 s epochs and applied an FFT followed by averaging using BrainVision Analyzer and MatLab.With R statistics we computed the individual α peak frequency and the corresponding power spectral density of every EEG-channel before and after stimulation.For comparison of pre and post stimulation differences we used the Wilcoxon signed-rank test as we cannot assume our data do be normally distributed. Results Concerning all EEG-channels, we found a significant α peak frequency shift towards the stimulated frequency in the post compared to the pre stimulation condition (Wilcoxon signed-rank test W = 18436, p = .002).When focusing on the electrodes in vicinity of the occipital stimulation we found that the averaged individual α peak frequencies (Electrode O1) over all participants slightly approximated the stimulation frequency.Comparing the segments before and after stimulation, peak frequency changed from 9.7 ± .8 Hz pre-stim to 10.1 ± 1.1 Hz post-stim.When analysing the individual frequency shifts to the stimulation frequency of 10 Hz with the Wilcoxon signed-rank test, we did not find any significant effect induced by tACS (W = 20.5, p = 0.859) ( Fig.1 ).Even analyzing α peak frequency shifts direction, no trend could be detected – out of 17 subjects only four had the expected α peak frequency drift towards the stimulation frequency ( Table1 ). Conclusions In contrast to previous work, the results of this study show a tACS-induced effect on individual α peak frequency only across all electrode positions, but we did not find any significant effect on an individual electrode near the stimulation site. This challenges the robustness of tAC stimulation effects on neuronal oscillations. In a next step, we will evaluate how tACS effects depend on the brain state (e.g. wakefulness).

A tunable optoelectronic oscillator (OEO) based on polarization controlled frequency response shifting of a dispersion-induced microwave photonic filter (MPF) is proposed and experimentally demonstrated. The MPF consists of a phase modulator (PM), a linear chirped fiber Bragg grating (LCFBG), and polarization multiplexed dual loops. Thanks to the polarization properties of the LiNbO3 crystal, the center frequency of the MPF can be tuned by simply adjusting the polarization state of the optical signal, and a tunable optoelectronic oscillator is established. Compared to the previous structure, the expensive wavelength-tunable laser is not needed in the proposed structure. In addition, the instability of oscillation frequency caused by the bias drifts of the modulator is avoided. A pure oscillation signal with a tunable frequency range from 3.3 to 7.3 GHz is generated. The single-sideband phase noise of the oscillation signal is about −110 dBc/Hz at 10-kHz offset. During more than 30 min observation, the oscillation frequency drift is about 7 kHz.

We use the specific scintillations of jovian decametric radio sources (modulation lanes), which are produced by plasma inhomogeneities in the vicinity of that planet, to probe the inner magnetosphere of Jupiter. The positions and frequency drift of 1762 lanes have been measured on the DAM spectra from archives. A special 3D algorithm is used for space localization of field-aligned magnetospheric inhomogeneities by the frequency drift of modulation lanes. As a result, the main regions of the lane formation are found: the Io plasma torus; the magnetic shell of the Gossamer Ring at Thebe and Amalthea orbits; and the region above the magnetic anomaly in the northern magnetosphere. It is shown that modulation lanes reveal the depleted magnetic tubes in practically unvisited, innermost regions of the jovian magnetosphere. The local and probably temporal plasma enhancement is found at the magnetic shell of Thebe satellite. Hence, the modulation lanes are a valuable instrument for remote sensing of those parts of jovian magnetosphere, which are not studied yet in situ.

This paper presents a chemical microsystem based on the integration of a silicon-based resonant microsensor and a CMOS ASIC for portable sensing applications in air. Two types of resonant microstructures are used as mass-sensitive microsensors: cantilever-based and disk-shape microresonators. Based on the characteristics of the microsensors, CMOS-integrated interface and control electronics have been designed and implemented. The CMOS ASIC utilizes the self-oscillation method in which a main feedback loop starts and sustains oscillation at or close to the desired resonance frequency of the microresonators. For stable oscillation, an automatic gain control loop regulates the oscillation amplitude by controlling the gain of the main feedback loop. In addition, an automatic phase control loop has been included to adjust the phase of the main feedback loop to ensure an operating point as close as possible to the resonance frequency, which results in improved frequency stability. To improve the long-term stability, a method to compensate for frequency drift has been proposed and implemented on the CMOS chip. The performance of the implemented microsystem as a chemical sensor has been evaluated experimentally by detecting different concentrations of volatile organic compounds such as benzene, toluene and m-xylene with microresonators coated with chemically sensitive polymer films.

Frequency drift has considerable adverse effects on the inductively coupled power transfer (ICPT) system such as decrease of transmission power and distance, EMC interference, system detuning and performance deterioration, and deviated operation from system design requirements endangering the stability so that robust control of ICPT system remains an open applied research problem. This article presents a relevant uncertain model for the LCL resonant ICPT system first via the generalized state-space averaging and the LFT methods, and then a solution to H∞ controller synthesis by making use of the LMI method. The control design is first tested by computer simulation and thereafter verified by means of physical experiments. The results show that by applying the proposed H∞ synthesis controller the overall system has achieved both predetermined robustness with respect to the norm-bounded load and frequency uncertainties and dynamically tracking the voltage references according to the actual power demands.

A statistical analysis of the type-III bursts observed by the spectrographs in the ranges of 625∼1500 MHz, 2600-3800 MHz, and 5200-7600 MHz during the 23rd solar cycle (from 2000 July to 2004 September) is carried out. The distribution of the type-III bursts, and their durations, frequency drift rates, polarization degrees and frequency bandwidths are given in this paper. The results indicate that the average values of the frequency drift rates and frequency bandwidths increase with the frequency. The average values of the durations and polarization degrees are neither constant nor uniformly varied over a broad frequency range. Most of type-III bursts are distributed in the range from 625 to 3800 MHz, and decrease with the frequency in number. This analysis shows that the places of electron acceleration and energy release are mainly in the decimetric range, and the characteristic of this frequency range is possibly related with the magnetic configuration at the decimeter wavelengths, as well as the electron acceleration in the magnetic reconnection site close to the main flare. However, there are also a considerable number of type-III bursts in the range of 5200-7600 MHz, it means that the sites of electron acceleration are widely distributed in the coronal region. The radiation mechanisms of type-III bursts at the centimeter-decimeter wavelengths include most probably the coherent plasma radiation and the emission process of electron cyclotron maser.

A new differential delay cell with complementary current control to extend the control voltage range as well as the operation frequency is proposed for low voltage and wide tuning range voltage-controlled ring oscillator (VCRO). The complementary current control can get rid of the restriction that control voltage is unable to cover the full range of power supply voltage in a conventional VCRO. A three-stage VCRO chip working with 1V power supply voltage is constructed using 0.18@mm 1P6M CMOS process for verifying the efficacy of the proposed differential delay cell. Measured results of the VCRO chip show that a wide range of operation frequency from 4.09GHz to 479MHz, a tuning range of 88%, is achieved for the full range of control voltage from 0 to 1V. The power consumptions of the chip are 13 and 4mW for oscillation outputs of 4.09GHz and 479MHz, respectively. The measured phase noise is -93.3dBc/Hz at 1MHz offset from 4.09GHz center frequency. The core area of the chip is 106@mmx76.2@mm.

We experimentally investigate the spectral dynamics of a phase-section tuned sampled-grating distributed Bragg reflector (SG-DBR) laser by using optical heterodyne method. The measured time-resolved spectra of the beat signal illustrate that the beat signal suffers from both frequency drift and instantaneous spectrum broadening, which indicates that the wavelengths of the lightwaves experience drift and jitter. We study the variation of frequency drift and instantaneous linewidth broadening with applied voltages. It is helpful in selection of optimum operating conditions and design of experimental setup. Experimental results demonstrate that the pulse-waveform can be used to reduce the frequency drift by shorting the beating time.

The key step in Q-based design for T or Pi network impedance matching is selecting an appropriate Q-factor. The harmonic rejection performance of impedance matching networks is the main concern of Q selection. However, for applications wherein the impedance to be matched is variable or the frequency is drifty, it is not the harmonic rejection but the stability, which is the main performance consideration. This paper analyzes T network impedance matching. The relationship between reflection coefficient, load variation, and frequency drift is determined, which provides a reference for Q selection. The Q-based design process presented in this paper can strike a balance between stability and harmonic rejection, which can then be applied in other cases involving load variation or frequency drift.

A new type of oscillator is presented in paper which integrates the interdigitated electrode (IDE) into conventional piezoceramic oscillator. Some IDE piezoelectric oscillators are prepared from PZT-5 with length=56 mm, width=15 mm and thickness=1 mm. The dimensions of IDE are 0.75 mm space and 0.5 mm. The conventional piezoelectric oscillators are also prepared with the same material and dimensions for comparison. The HP4294A High-frequency Impedance Analyzer and DU-20 Temperature-controlled Oil Bath are applied to explore the impedance of oscillator. The effect of temperature on the frequency of oscillator is researched. The results show that IDE piezoelectric oscillators have more resonant frequencies including overtone frequency and anti-resonance frequency. The resonant frequency of oscillators decreases as the temperature increase. The temperature coefficient of resonance frequency of conventional piezoelectric oscillators is -14%. The temperature coefficient of resonance frequency of IDE piezoelectric oscillators is -21%. The temperature stability of conventional piezoelectric oscillators is a little high.

The problem of identification of linear quasi-periodically varying systems is considered. This problem can be solved using generalized adaptive notch filtering (GANF) algorithms. It is shown that accuracy of system parameter estimation can be increased if the results obtained from GANF are further processed using a cascade of appropriately designed filters. The resulting generalized adaptive notch smoothing (GANS) algorithms can be employed in offline applications where causality constraints do not apply. When the instantaneous frequency of parameter changes varies in a sufficiently smooth manner, the proposed GANS algorithm, based on a new, quasi-linear model of frequency drift, outperforms the existing solutions.

Resonator integrated optic gyro (RIOG), which employs narrow linewidth laser, is a high-accuracy inertial rotation sensor based on the Sagnac effect. The performance of RIOG is greatly affected by the frequency drift of narrow linewidth laser. A simple, effective method to measure the relative frequency drift of narrow linewidth laser based on delayed self-heterodyne technique is proposed in this paper. The measurement range and sensitivity can easily be satisfied by setting the length difference of the fiber segments between two interferometer arms. The relationship between the length difference and the frequency drift is derived based on the given principle of measuring the relative frequency drift. Then the laser frequency drift measuring setup is established and the experiment results demonstrate that a center frequency drift rate is less than 2 MHz/6.7 s under the room-temperature. Moreover, the measuring setup is applied to test the modulation coefficient of piezoelectric-transducer (PZT), and the modulation coefficient of 9.62 MHz/V is obtained, which satisfies the requirements of RIOG closed-loop operation.

Radio bursts with fine structures in decimetric–centimetric wave range are generally believed to manifest the primary energy release process during flare/CME events. By spectropolarimeters in 1–2 GHz, 2.6–3.8 GHz, and 5.2–7.6 GHz at NAOC/Huairou with very high temporal (1.25–8 ms) and spectral (4–20 MHz) resolutions, the zebra patterns, spikes, and new types of radio fine structures with mixed frequency drift features are observed during several significant flare/CME events. In this paper we will discuss the occurrence of radio fine structures during the impulsive phase of flares and/or CME initiations, which may be connected to the magnetic reconnection processes.

The local narrow-band signal (LNBS) decomposition algorithm is introduced in non-dispersive infrared (NDIR) gas detection. In order to meet the requirement of real-time detection, the original LNBS algorithm is implemented in Fourier domain to be computationally efficient. The Fourier domain LNBS decomposition algorithm is both time and space saving, so it is suitable for an embedded system. The effectiveness of the technique is evaluated by the measurement of C2H5OH gas concentration. Compared with a digital lock-in amplifier, the Fourier domain LNBS extraction has an advantage of little sensitivity to the frequency drift. Additionally, it has no lagging effect and hence the changes of the gas concentration can be measured in real time.

We report on the development of a distributed fibre optic monitoring system for detection of strain and temperature along single-mode fibres. The system is based on the Brillouin optical-fibre frequency-domain analysis (BOFDA). In order to bring down costs of the measuring device, the sideband technique has been implemented for the BOFDA analysis. The use of only one low-cost DFB laser has eliminated the effects of any frequency drift and therefore no tunable laser source has been demanded. We present a BOFDA setup offers digital filtering for enhancement of the dynamic range [1].

In this paper we describe the design and fabrication of a mechanical autonomous impact oscillator with a MEMS resonator as the frequency control element. The design has been developed with scalability to large 2-D arrays of coupled oscillators in mind. The dynamic behaviour of the impact oscillator was numerically studied and it was found that the geometry nonlinearity has an effect on the static pull-in voltage and equilibrium position. The external driving power can alter the frequency of the impact oscillator. The autonomous nature of the oscillator simplifies the complexity of the drive circuitry and is essential for large 2-D arrays.

Orthogonal frequency division multiplexing (OFDM) is a very important modulation technique in wideband wireless communication and multimedia communication systems. While, it can effectively deal with multipath delay spread produced by frequency fading channels, its main drawback is the effect of frequency offset (FO) produced by the receiver local oscillator or by motion-induced Doppler. The FO breaks the orthogonality among the subcarriers and hence causes intercarrier interference (ICI). In this paper, ICI caused by frequency drift is eliminated by equalizing the complex weighting coefficients of interference. In most of the commonly used ICI cancellation schemes, bandwidth efficiency suffers because of the requirement of redundancy in the transmission. In the proposed scheme, repetition of data symbols or transmission of training sequence is not required. Thus, the bandwidth efficiency of normal OFDM system is maintained. The improved performance of the present scheme is confirmed through extensive simulations.

A gated tunnel diode has been introduced into a wave-guide oscillator circuit and the gate is used to tune the oscillation frequency and to turn the oscillator on and off. Oscillators with oscillation frequencies in the range of 9-22 GHz with a typical oscillator output power about -20 dBm have been fabricated. We found that the output power remains essentially constant over a gate bias range (about -260 to 200 mV at V-c = 1.0 V in one oscillator), while it rapidly drops to the noise level over a gate bias range of less than 60 mV above or below the threshold, respectively. In the region with constant power, a total frequency tuning of about 30% is achieved. The maximum oscillation frequency. f(max) (osc), of the gated tunnel diode is set by the tunnel diode and the gate-collector capacitance, and does not depend on the gate-emitter capacitance. This oscillator implementation, in particular, eliminates the need for a separate switch in the realisation of oscillator-based ultra-wide band impulse radios. (C) 2009 Elsevier Ltd. All rights reserved.

A thinning algorithm is proposed for real-time unbiased finite impulse response (FIR) estimation of the local clock time interval error (TIE) model (time error, fractional frequency offset, linear frequency drift rate, etc.) employing GPS-based sawtooth measurements. We show that the approach allows obtaining practically optimal estimates of the clock states, by large horizons (number of the points in the average). The algorithm is applied to the TIE measurements allowing for different time steps and averaging horizons for each of the clock states and compared to the three state Kalman filter. It is demonstrated that, in the presence of the sawtooth noise induced by the GPS receiver, the unbiased FIR estimates with thinning out fit the clock states better than the Kalman filter, in terms of the Allan deviation and precision time protocol deviation.

In a modern power system, photovoltaic as distributed generated source is growing larger and it can cause a variety of problems. The most important problem is that of the islanding phenomenon. In order to prevent islanding phenomenon, three kinds of active islanding detection methods have been studied. These are, respectively, to change magnitude, frequency, and the start phase of inverter output current. Among those, both a frequency variation method and a start-phase variation anti-islanding method make the islanding frequency drift away from the trip window of the frequency relay if islanding occurs. This paper presents a novel combined active anti-islanding method, which consists of a frequency variation method active frequency drift (AFD) and a start-phase variation method slip-mode frequency shift (SMS). Clearly, the proposed anti-islanding method shows the islanding detection ability to IEEE 1547 Standard. To validate the performance of the proposed method, simulations and experiments were performed. Possible islanding conditions are implemented with a unity quality factor by the IEEE Standard 1547. The methodology presented can be extended to the other active anti-islanding methods.

Passive radio spectroscopy is employed to examine plasma process instabilities generated by the interaction between the power source oscillator and the plasma load. A fixed frequency of 13.56 MHz and a 170–180 kHz Flyback transformer are considered. The carrier frequencies are interrogated using a resolution bandwidth that constitutes ∼1/7000–1/580 of the target oscillator frequencies with a sweep time of less than 0.06 s across the phase noise disturbance. Within these spectrum analyzer measurement parameters, oscillator phase noise (1/ f n= 1–3 , discrete spurs and raised noise floor) is shown to be linked to plasma load mismatch and periodic instabilities. In the case of the Flyback circuit, it is found that the oscillator frequency pulling and modulation are linked to the plasma reactance. These results indicate that oscillator phase noise can be used as a non-invasive plasma process metrology tool.