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Due to the collision-shift effect, helium permeation into the vapor cell causes a time-dependent frequency drift in rubidium atomic clock. To quantify this shift, a cylindrical vapor cell with dimensions of 1.8 cm in diameter, 1.6 cm in height, and 1 mm in thickness, operating at a temperature of 65 ℃, is selected for analysis. Numerical methods are employed to simulate the temporal variation of helium gas pressure within atomic vapor cells composed of Pyrex (Corning 7740) and low helium-permeable aluminosilicate (Corning 1720). The results of the calculations indicate that for Pyrex vapor cells, after 12 years of operation, the drift rate attributable to helium permeation decreases to less than 1.0×10-14/day. In contrast, the frequency drift rate due to helium permeation in aluminosilicate vapor cell rubidium atomical clocks remains below 3.0×10-17/day throughout their operational lifetime, rendering its contribution to the drift rate negligible. This computational approach is also applicable to the investigation of the permeation processes of other gaseous species in various glass materials.

With their high reliability, small size and low power consumption, rubidium atomic clocks are widely used in the fields of navigation, communication, etc. In particular, rubidium atomic clocks for navigation satellites have developed excellent stability. However, their inherent frequency drift characteristics (about E-12 to E-13/day) will deteriorate their long-term performance and affect the autonomous punctual timing of satellites. In this paper, we fully analyzed the frequency data of high-performance rubidium atomic clocks, aiming to figure out the physical mechanism behind the frequency drift. A high-precision frequency drift compensation scheme is proposed and experimentally verified. The results show that the drift rate of high-performance rubidium clock can be maintained in the order of E-15/day within 60 days without external taming, and the day stability can reach the order of E-15 (Allan variance), which greatly improves the autonomous punctual timing ability of rubidium atomic clock．

Rubidium fountain clocks are operated by a control system. In this study, first, the control requirements of a rubidium fountain clock are analyzed; then, a control system is designed and divided into a timing sequence control system, a data acquisition system, and a servo control system. Multiple boards based on PCI extensions for the instrumentation (PXI) bus and Labwindows/CVI software 2019 are adopted. The timing sequence control system outputs 16 transistor–transistor logic (TTL) signals and three arbitrary waveforms. The results show that these signals are synchronized within 380 ns. Moreover, the digital locking frequency of the master laser is triggered by one of the timing signal, sweeping the laser frequency in a wide range during polarization gradient cooling. Two time-of-flight signals are acquired, and Ramsey fringes are scanned using the data acquisition system. An error signal is obtained every two cycles, and it is feed back to the frequency synthesizer and a high resolution offset generator using the servo control system. Then the frequency synthesizer is locked to the Ramsey center resonance. The rubidium atomic fountain clock exhibited a frequency stability of 7.4 × 10−16 at 86,400 s. Moreover, the phase of the Rb fountain clock is compared with that of UTC (NTSC) in real time, demonstrating a frequency drift of 0.00044 ns/day/day. The whole control system is full-featured, robust and flexible, thus satisfying the requirements of rubidium fountain clocks. [ABSTRACT FROM AUTHOR]

The effect of atomic motion on the optical properties of atomic ensembles cooled in special laser traps was analyzed. The possibility was studied to model the continuous displacement of atoms under the assumption of stationary coupled oscillators, in which slow motion is modeled, firstly, by averaging over their random spatial distribution, and, secondly, by introducing a random shift of their frequencies, modeling Doppler effects. A direct comparison of the results obtained taking into account the continuous displacement of atoms with the model ones revealed a very limited range of applicability of the latter. [ABSTRACT FROM AUTHOR]

In technical systems, in addition to mechanical oscillations, it is also worth noting electrical sinusoidal processes in alternating current circuits, in electronic devices, in particular, in the industrial power supply network, oscillatory circuits, signal generators, communication systems, etc. In radar systems, sinusoidal signals with frequency drift can also be observed, for example, linear frequency-modulated or Doppler signal reflected from a maneuvering target, the speed of which has at least one derivative. Among the possible ones, in the future we limit ourselves to comparative studies of two types of SL-oscillations - harmonic (SG) with all constant parameters and sinusoid with linear frequency drift (SD). At the same time, we will consider two types of their mathematical models - trigonometric (T-models) and autoregressive (AR-models). The paper analyzes sine-like trigonometric models and features of their application. The peculiarities of the task of measuring the parameters of the movement of an object with uniform acceleration based on the estimation of the Doppler frequency of the radar probing signal reflected from it are considered. A synthesis of simplified and approximate ARD models was carried out. An important feature of the synthesized ARG model is the fact that only one parameter of the trigonometric model appears in it - the normalized frequency. The main merit of the synthesized simplified and approximate ARD models is their invariance to the amplitude and initial phase of the sinusoidal signal. This reduces the parametric dimension of the estimation problem from four to two parameters, which contributes to a significant improvement in the speed of calculations. Based on mathematical modeling, statistical errors were obtained for different sampling intervals in the triplet and sample size. Based on mathematical modeling, statistical errors were obtained for different sampling intervals in the triplet and sample size. The selection of pairs of triplets was probabilistic. Calculations show that the error of the obtained model does not exceed 8%. [ABSTRACT FROM AUTHOR]

PurposeHeating of gradient coils and passive shim components is a common cause of instability in the B0 field, especially when gradient intensive sequences are used. The aim of the study was to set a benchmark for typical drift encountered during MR spectroscopy (MRS) to assess the need for real-time field-frequency locking on MRI scanners by comparing field drift data from a large number of sites.MethodA standardized protocol was developed for 80 participating sites using 99 3T MR scanners from 3 major vendors. Phantom water signals were acquired before and after an EPI sequence. The protocol consisted of: minimal preparatory imaging; a short pre-fMRI PRESS; a ten-minute fMRI acquisition; and a long post-fMRI PRESS acquisition. Both pre- and post-fMRI PRESS were non-water suppressed. Real-time frequency stabilization/adjustment was switched off when appropriate. Sixty scanners repeated the protocol for a second dataset. In addition, a three-hour post-fMRI MRS acquisition was performed at one site to observe change of gradient temperature and drift rate. Spectral analysis was performed using MATLAB. Frequency drift in pre-fMRI PRESS data were compared with the first 5:20 minutes and the full 30:00 minutes of data after fMRI. Median (interquartile range) drifts were measured and showed in violin plot. Paired t-tests were performed to compare frequency drift pre- and post-fMRI. A simulated in vivo spectrum was generated using FID-A to visualize the effect of the observed frequency drifts. The simulated spectrum was convolved with the frequency trace for the most extreme cases. Impacts of frequency drifts on NAA and GABA were also simulated as a function of linear drift. Data from the repeated protocol were compared with the corresponding first dataset using Pearson's and intraclass correlation coefficients (ICC).ResultsOf the data collected from 99 scanners, 4 were excluded due to various reasons. Thus, data from 95 scanners were ultimately analyzed. For the first 5:20 min (64 transients), median (interquartile range) drift was 0.44 (1.29) Hz before fMRI and 0.83 (1.29) Hz after. This increased to 3.15 (4.02) Hz for the full 30 min (360 transients) run. Average drift rates were 0.29 Hz/min before fMRI and 0.43 Hz/min after. Paired t-tests indicated that drift increased after fMRI, as expected (p < 0.05). Simulated spectra convolved with the frequency drift showed that the intensity of the NAA singlet was reduced by up to 26%, 44 % and 18% for GE, Philips and Siemens scanners after fMRI, respectively. ICCs indicated good agreement between datasets acquired on separate days. The single site long acquisition showed drift rate was reduced to 0.03 Hz/min approximately three hours after fMRI.DiscussionThis study analyzed frequency drift data from 95 3T MRI scanners. Median levels of drift were relatively low (5-min average under 1 Hz), but the most extreme cases suffered from higher levels of drift. The extent of drift varied across scanners which both linear and nonlinear drifts were observed.

Due to the working principle of MEMS resonant accelerometers, their thermally induced frequency drift is an inevitable practical issue for their extensive application. This paper is focused on reducing the thermally induced packaging effects on the frequency drift. A leadless ceramic chip carrier package with a stress-buffering layer was proposed for a MEMS resonant accelerometer, and the influences of packaging structure parameters on the frequency drift were investigated through finite element simulations and verified experimentally. Because of the thermal mismatch between dissimilar materials, the thermo-mechanical stress within the resonant beam leads to a change in the effective stiffness and causes the frequency drift to decrease linearly with increasing temperature. Furthermore, our investigations reveal that increasing the stress-buffering layer thickness and reducing the solder layer thickness can significantly minimize the thermo-mechanical stress within the resonant beam. As the neutral plane approaches the horizontal symmetry plane of the resonant beam when optimizing the packaging structure, the effects of the compressive and tensile stresses on the effective stiffness of the resonant beam will cancel each other out, which can dramatically reduce the frequency drift. These findings provide guidelines for packaging design through which to improve the temperature stability of MEMS resonant accelerometers. [ABSTRACT FROM AUTHOR]

It is now widely recognized that energy storage enables increased integration of renewable resources. One of the uses of storage is to provide synthetic inertia, making up for some of the inertia lost from displaced conventional generation, thereby maintaining frequency stability. However, energy storage systems continue to be very expensive, and this motivates the development of innovative approaches to mitigate some of the challenges posed by renewable resources. This article presents a novel method called “grid-scale virtual energy storage” that harvests free energy storage from properties inherent to control of multiarea power systems, thereby increasing the amount of renewable generation that a system can tolerate before its frequency stability is compromised. The concept of virtual energy storage proposed here is based on the surplus of necessary energy that is required to restore the system frequency to within a safe range of the nominal frequency. In a dynamic sense, virtual energy storage is very responsive and is not limited by the operation time and capacity. Moreover, it can be exchanged easily among control areas and does not require installation and implementation expenses as it takes advantage of the existing diversity of area control errors in interconnected systems and employs an appropriate frequency control method. It also helps in reducing the reserve requirement, regulation capacity, transmission loading, and wear and tear on the power system by functioning as normal energy storage. In this article, the proposed new approach is explained mathematically, and its operation is analyzed using a mathematical model and simulation in order to validate its effectiveness. [ABSTRACT FROM AUTHOR]

Accuracy of range measurements using navigation satellites primarily depends on on-board clock correction parameters which are broadcasted to users periodically. The on-board clock offset, a critical correction parameter which accumulates due to fractional frequency offset as well as frequency drift, has to be maintained within the allocated broadcast limit throughout the satellite's service. In IRNSS satellites, the clock offset is maintained within the broadcast limit by applying frequency offset corrections to the on-board atomic clock monitoring unit. In this paper, we discuss the methodology implemented for computing the clock offset using one-way range measurement data. Further, we report the time keeping methodology employed in IRNSS satellites that enables maintaining the satellite clock offset within the broadcast limit with minimum number of intervening frequency offset corrections throughout their operational life time. [ABSTRACT FROM AUTHOR]

Due to the working principle of MEMS resonant accelerometers, their thermally induced frequency drift is an inevitable practical issue for their extensive application. This paper is focused on reducing the thermally induced packaging effects on the frequency drift. A leadless ceramic chip carrier package with a stress-buffering layer was proposed for a MEMS resonant accelerometer, and the influences of packaging structure parameters on the frequency drift were investigated through finite element simulations and verified experimentally. Because of the thermal mismatch between dissimilar materials, the thermo-mechanical stress within the resonant beam leads to a change in the effective stiffness and causes the frequency drift to decrease linearly with increasing temperature. Furthermore, our investigations reveal that increasing the stress-buffering layer thickness and reducing the solder layer thickness can significantly minimize the thermo-mechanical stress within the resonant beam. As the neutral plane approaches the horizontal symmetry plane of the resonant beam when optimizing the packaging structure, the effects of the compressive and tensile stresses on the effective stiffness of the resonant beam will cancel each other out, which can dramatically reduce the frequency drift. These findings provide guidelines for packaging design through which to improve the temperature stability of MEMS resonant accelerometers.

The protection against the unintentional islanding of Grid-Tied inverters is an important electrical security issue addressed by the main Standards. This concern is justified in face of the fact that unintentional islanding can lead to abrupt variations of voltage and frequency, electrical damages, professional accidents, power quality degradation, and out-of-phase reclosure. In response to the islanding concern, the literature has proposed several Anti-Islanding Protection (AIP) schemes that can be divided in passive and active methods. Many of the active AIP is based on the insertion of some disturbance in the inverter current in order to deviate the frequency out of the allowed thresholds, tripping the inverter internal disconnection system. Thus, the main objective of this paper is to analyze the performance of the Active Phase Jump with Positive Feedback (APJPF) algorithm compared to other well-known frequency drift-based solutions. More than that, this work covers the Non-Detection Zone (NDZ) problem, analyzing its main mapping methodologies and the normative requirements, exposing the minimum normative recommendations a given AIP must reach to be considered functional. The last contributions of this paper are the proposal of a parametrization criterion for the Active Frequency Drift with Pulsating Chopping Factor (AFDPCF) and for the APJPF. [ABSTRACT FROM AUTHOR]

Purpose: Heating of gradient coils and passive shim components is a common cause of instability in the B0 field, especially when gradient intensive sequences are used. The aim of the study was to set a benchmark for typical drift encountered during MR spectroscopy (MRS) to assess the need for real-time field-frequency locking on MRI scanners by comparing field drift data from a large number of sites. Method: A standardized protocol was developed for 80 participating sites using 99 3T MR scanners from 3 major vendors. Phantom water signals were acquired before and after an EPI sequence. The protocol consisted of: minimal preparatory imaging; a short pre-fMRI PRESS; a ten-minute fMRI acquisition; and a long post-fMRI PRESS acquisition. Both pre- and post-fMRI PRESS were non-water suppressed. Real-time frequency stabilization/adjustment was switched off when appropriate. Sixty scanners repeated the protocol for a second dataset. In addition, a three-hour post-fMRI MRS acquisition was performed at one site to observe change of gradient temperature and drift rate. Spectral analysis was performed using MATLAB. Frequency drift in pre-fMRI PRESS data were compared with the first 5:20 minutes and the full 30:00 minutes of data after fMRI. Median (interquartile range) drifts were measured and showed in violin plot. Paired t-tests were performed to compare frequency drift pre- and post-fMRI. A simulated in vivo spectrum was generated using FID-A to visualize the effect of the observed frequency drifts. The simulated spectrum was convolved with the frequency trace for the most extreme cases. Impacts of frequency drifts on NAA and GABA were also simulated as a function of linear drift. Data from the repeated protocol were compared with the corresponding first dataset using Pearson's and intraclass correlation coefficients (ICC). Results: Of the data collected from 99 scanners, 4 were excluded due to various reasons. Thus, data from 95 scanners were ultimately analyzed. For the first 5:20 min (64 transients), median (interquartile range) drift was 0.44 (1.29) Hz before fMRI and 0.83 (1.29) Hz after. This increased to 3.15 (4.02) Hz for the full 30 min (360 transients) run. Average drift rates were 0.29 Hz/min before fMRI and 0.43 Hz/min after. Paired t-tests indicated that drift increased after fMRI, as expected (p < 0.05). Simulated spectra convolved with the frequency drift showed that the intensity of the NAA singlet was reduced by up to 26%, 44 % and 18% for GE, Philips and Siemens scanners after fMRI, respectively. ICCs indicated good agreement between datasets acquired on separate days. The single site long acquisition showed drift rate was reduced to 0.03 Hz/min approximately three hours after fMRI. Discussion: This study analyzed frequency drift data from 95 3T MRI scanners. Median levels of drift were relatively low (5-min average under 1 Hz), but the most extreme cases suffered from higher levels of drift. The extent of drift varied across scanners which both linear and nonlinear drifts were observed.

Objectives: In October 2011 the Russian GLONASS （global navigation satellite system） orbital constellation of 24 satellites is restored, enabling global coverage for positioning, navigation and timing for civil users once again. It plays key roles in navigation system performance evaluation, integrity monitoring, determination and prediction of satellite clock correction. To analyze the time‐frequency characteristics of GLONASS in ‐ flight satellite clocks, GLONASS satellites are equipped with cesium clocks, which are different from GPS, Galileo and BDS （BeiDou navigation satellite system） satellites, so GLONASS in‐flight Cesium clock may have quite different time‐frequency characteristics. However, analysis and evaluation are rarely reported about GLONASS satellite clocks.Methods: In order to estimate the behavior and state of GLONASS in‐flight satellite clocks, the long‐term characterization of the GLONASS in‐orbit Cesium clocks is carried out in terms of the long‐term variations of the five indices, namely clock phase, frequency, frequency drift, frequency stability and clock model noise. In view of the availability and accuracy of GLONASS satellite clock data, the precise GLONASS satellite clock products from Jan. 1, 2016 to May 11, 2019 with 5‐minute interval released by the Russian GLONASS Information and Analysis Center for Positioning, Navigation and Timing are used as data base to derive the clock phase, frequency, frequency drift and clock model noise from the quadratic polynomial model, as well as calculate the clock frequency stability at the short interval according to the classical Hadamard deviation. Results: It is clearly shown that the phase and frequency of the GLONASS on‐board Cesium clocks are very stable. The average values of the model noise and frequency drift of the GLONASS in ‐ orbit clocks are 0.7 ns and 5.94×10-15/d, respectively. The results demonstrate that GLONASS Cesium clocks have good physical characteristics.The frequency stability of the GLONASS satellite clocks in orbit is at the 10-13 level at short interval, and there is noticeable relationship between the frequency stability and clock model noise, which will be needed to clarify further. Conclusions: The results show that the physical characteristics of new satellite clocks are better than that of older clocks, and the clock model noise are also substantially lower. [ABSTRACT FROM AUTHOR]

• We improve the ranging accuracy of FMCW systems using the spectral refinement method CZT. • The experimental system constructed improves the ranging stability of the FMCW lidar when performing high-precision ranging. • Accurately measures the speed of target motion using binomial fitting. • The constructed experimental system can determine the direction of motion when measuring the motion state of a moving target. The conventional coherent FMCW lidar system has the problem of frequency drift to meet the high accuracy range. When measuring the moving target, it cannot judge the target motion status in real-time. This paper proposes a high-stability coherent heterodyne FMCW lidar detection method to solve the above-proposed problems. The combined single/double sideband modulation method significantly suppresses the frequency drift and, combined with the spectrum refinement processing method, achieves high stability of laser ranging and speed measurement. The result shows that the target is located at an 8 m distance from the system, the range accuracy is 5 cm, the range error is less than 1.5 cm, and the root mean square(RMS) error is 6.6 × 10-3 m, which is an order of magnitude higher than that of the conventional FMCW laser detection system. We use the binomial fitting method to obtain a measurement speed error of fewer than 1.15 × 10-2 m/s. [ABSTRACT FROM AUTHOR]

The protection against the unintentional islanding of Grid-Tied inverters is an important electrical security issue addressed by the main Standards. This concern is justified in face of the fact that unintentional islanding can lead to abrupt variations of voltage and frequency, electrical damages, professional accidents, power quality degradation, and out-of-phase reclosure. In response to the islanding concern, the literature has proposed several Anti-Islanding Protection (AIP) schemes that can be divided in passive and active methods. Many of the active AIP is based on the insertion of some disturbance in the inverter current in order to deviate the frequency out of the allowed thresholds, tripping the inverter internal disconnection system. Thus, the main objective of this paper is to analyze the performance of the Active Phase Jump with Positive Feedback (APJPF) algorithm compared to other well-known frequency drift-based solutions. More than that, this work covers the Non-Detection Zone (NDZ) problem, analyzing its main mapping methodologies and the normative requirements, exposing the minimum normative recommendations a given AIP must reach to be considered functional. The last contributions of this paper are the proposal of a parametrization criterion for the Active Frequency Drift with Pulsating Chopping Factor (AFDPCF) and for the APJPF.

In order to solve the problem of synchronization addressing and the inadequacy of power interaction among nodes in the current mobile wireless sensor network,a new data transmission algorithm for mobile wireless sensor network was proposed.Firstly,the broadcast mechanism was adopted to realize synchronous control packet transmission and reduce the pressure caused by the synchronization flow to the addressing process.Then regional node flow valve control mechanism was used,and data traffic between sink nodes and regional node links were recorded and got by interception,flow link equalization was further adopted to promote traffic equalization.in the further step.Finally,the limited bandwidth based on wheel number-sink link jitter screening mode could be identified,and the transmission failure due to limited bandwidth was reduced.Simulation results show that compared with the bandwidth of the root zone node and widely used in limiting transmission algorithm,node link stability encoding algorithm and hierarchical encoding algorithm,the data transmission bandwidth can be improved,the packet loss rate can be reduced,and the practical needs can be better met.

Optical-pulse-coding (OPC) technique can break the trade-offs between key parameters of distributed fiber-optic sensing systems, therefore it has received great attention in recent years. OPC Φ-OTDR has achieved significant performance improvements over conventional Φ-OTDR; however, scan-rate, interference fading and frequency drift remain critical issues for this type of sensing systems. In this article, for the first time bipolar Golay coding Φ-OTDR with heterodyne detection is proposed and realized to address the scan-rate issue, the laser frequency drift is compensated by a novel real-time compensation method, and the interference fading is eliminated by incorporating the spectrum extraction and remix method. Theoretical analysis is described in detail, and the benefits provided by bipolar code are experimentally demonstrated in 10 km sensing range with 0.92 m spatial resolution, achieving 7.1 dB SNR improvement and halving measurement time compared with unipolar code case. By addressing frequency drift and fading issues, the distributed sensing capability of OPC Φ-OTDR becomes available, at a reasonable expense of enlarged spatial resolution (6 m). It should be noted that, such a bipolar scheme can also be applied in other coding schemes, opening up more possibilities for OPC in Φ-OTDR. [ABSTRACT FROM AUTHOR]

A novel temperature-compensated LC-VCO is presented for RFID applications. By employing a low pass passive filter and an auxiliary varactor loop, the frequency drift is reduced by 70% without using any other active device. The effectiveness of the technique is demonstrated on a frequency synthesizer of a RFID transceiver. The frequency synthesizer is fabricated on a 0.18 μm RF process with a single supply of 1.8 V. The empirical measurements show that the VCO achieves a current consumption of 6 mA, while the passive temperature-compensated circuit does not increase the power consumption. The VCO has a tuning range of 1.6–2.1 GHz and phase a noise of − 124 dBc/Hz at 1 MHz offset from the 1.824 GHz carrier. [ABSTRACT FROM AUTHOR]

An improved harmonic detection method was proposed to realize the synchronization and control of active power filter in three-phase four wire system． The method can extract the frequency，phase， positive and negative sequence components of grid voltage and the harmonic components of load current when there are three-phase unbalance or other disturbances in the system． The voltage model and discrete complex phasor state space equation were established of three-phase four wire system，and proposes a master-slave Kalman filter was proposed to estimate the frequency and noise covariance of system voltage， so as to quickly track the frequency and amplitude of the grid voltage in the system with measurement noise and frequency offset; meanwhile，a load harmonic detection method based on adaptive neural network was proposed，which can accurately obtain the amplitude and phase of each harmonic component in the load current in a power frequency cycle． The results show that the method is superior to the traditional Kalman filter algorithm in frequency tracking time and accuracy． Simulation and experiment prove effectiveness of the algorithm． [ABSTRACT FROM AUTHOR]

• A low noise capacitive MEMS accelerometer is designed and characterized. • The sensitivity and non-linearity of fabricated MEMS accelerometer are respectively 53 fF/g, 0.03% by the sigma-delta modulator, while the CVC method is 3.78 mV/g with the non-linearity 0.04%. • The frequency dynamic range is 0–158 Hz with a sensitivity non-distortion of ±0.1 dB. This work reports a capacitive MEMS accelerometer designed for seismic applications. The asymmetrical anti-spring structure is used and the device is microfabricated using SOI technology. Two capacitive readout methods, namely, capacitance-to-voltage-converter (CVC) and a commercial sigma-delta modulator, are introduced for comparison in the device characterization. For both methods, the accelerometer has a frequency dynamic range of 0-158 Hz. The sensitivity and non-linearity of the sensor is 53 fF/g (21.3 mV/g), 0.03% by the sigma-delta modulator, while the CVC method is 3.78 mV/g with the non-linearity 0.04%. As for the resolution, both methods have detected the lab's background noise of 11.5 μ g / Hz at 0.03 Hz and have exhibited the resolution of the sensor is better than 10 μ g / Hz. Regardless of the difference in parameter selection for the circuits, the CVC method has a much lower noise floor (51.8 ng/ Hz at 1 Hz) than sigma-delta readout configuration (10 μ g / Hz at 1 Hz). The proposed MEMS accelerometer based on anti-spring structures shows its high sensitivity and low noise performance, demonstrating its potential in seismic applications. [ABSTRACT FROM AUTHOR]

Purpose: To assess the performance, in the presence of scanner instabilities, of three dynamic correction methods which integrate ∆B0 mapping into the chemical exchange saturation transfer (CEST) measurement and three established static ∆B0‐correction approaches. Methods: A homogeneous phantom and five healthy volunteers were scanned with a CEST sequence at 7 T. The in vivo measurements were performed twice: first with unaltered system frequency and again applying frequency shifts during the CEST acquisition. In all cases, retrospective voxel‐wise ∆B0‐correction was performed using one intrinsic and two extrinsic [prescans with dual‐echo gradient‐echo and water saturation shift referencing (WASSR)] static approaches. These were compared with two intrinsic [using phase data directly generated by single‐echo or double‐echo GRE (gradient‐echo) CEST readout (CEST‐GRE‐2TE)] and one extrinsic [phase from interleaved dual‐echo EPI (echo planar imaging) navigator (NAV‐EPI‐2TE)] dynamic ∆B0‐correction approaches [allowing correction of each Z‐spectral point before magnetization transfer ratio asymmetry (MTRasym) analysis]. Results: All three dynamic methods successfully mapped the induced drift. The intrinsic approaches were affected by the CEST labeling near water (∆ω < |0.3| ppm). The MTRasym contrast was distorted by the frequency drift in the brain by up to 0.21%/Hz when static ∆B0‐corrections were applied, whereas the dynamic ∆B0 corrections reduced this to <0.01%/Hz without the need of external scans. The CEST‐GRE‐2TE and NAV‐EPI‐2TE resulted in highly consistent MTRasym values with/without drift for all subjects. Conclusion: Reliable correction of scanner instabilities is essential to establish clinical CEST MRI. The three dynamic approaches presented improved the ∆B0‐correction performance significantly in the presence of frequency drift compared to established static methods. Among them, the self‐corrected CEST‐GRE‐2TE was the most accurate and straightforward to implement. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the effects of frequency drift on chemical exchange saturation transfer (CEST) imaging at 3T, and to propose a new sequence for correcting artifacts attributed to B0 drift in real time. Theory and Methods: A frequency‐stabilized CEST (FS‐CEST) imaging sequence was proposed by adding a frequency stabilization module to the conventional non‐frequency‐stabilized CEST (NFS‐CEST) sequence, which consisted of a small tip angle radiofrequency excitation pulse and readout of three non‐phase‐encoded k‐space lines. Experiments were performed on an egg white phantom and 26 human subjects on a heavy‐duty clinical scanner, in order to compare the difference of FS‐CEST and NFS‐CEST sequences for generating the z‐spectrum, magnetization transfer ratio asymmetry (MTRasym) spectrum, and amide proton transfer weighted (APTw) image. Results: The B0 drift in CEST imaging, if not corrected, would cause APTw images and MTRasym spectra from both the phantom and volunteers to be either significantly higher or lower than the true values, depending on the status of the scanner. The FS‐CEST sequence generated substantially more stable MTRasym spectra and APTw images than the conventional NFS‐CEST sequence. Quantitatively, the compartmental‐average APTw signals (mean ± standard deviation) from frontal white matter regions of all 26 human subjects were –0.32% ± 2.32% for the NFS‐CEST sequence and –0.14% ± 0.37% for the FS‐CEST sequence. Conclusions: The proposed FS‐CEST sequence provides an effective approach for B0 drift correction without additional scan time and should be adopted on heavy‐duty MRI scanners. [ABSTRACT FROM AUTHOR]

Featured Application: Absolute distance measurement. In frequency-sweep polarization-modulation ranging, distance is determined by the frequency of modulated waves and the corresponding wavelength multiple when emitted and returned waves are in phase. However, measurement of the frequency and the wavelength multiple is affected by thermally induced phase delay of the polarized wave. In this article we systematically discuss the principle of the ranging method and analyze the influences of thermally induced phase delay. New approaches to measurement are proposed to eliminate the impact on frequency and the wavelength multiple. Theoretical analysis and experimental results proved the efficiency and applicability of the methods. [ABSTRACT FROM AUTHOR]

A frequency-drift-compensated phase-locked loop (PLL) with an LC voltage-controlled oscillator (VCO) is fabricated in TSMC 40-nm CMOS process. The proposed frequency drift compensator employs an analog-to-digital converter to monitor the control voltage of the PLL in background. The capacitor banks are adjusted to compensate for the frequency drift of the LC-VCO. The measured reference spur is −65.15 dBc. The measured best phase noise of this PLL is −108.32 and −130.26 dBc/Hz at the frequency offset of 1 and 10 MHz, respectively, among five chips. This chip occupies 0.223-mm2 active area. The power dissipation of this PLL is 6.32 mW from a 0.9-V supply voltage. The average temperature coefficient is 2.43 ppm/°C from 20 °C to 100 °C. [ABSTRACT FROM AUTHOR]

Purpose: New frequency correction methods are required to achieve the accurate measurement of frequency drifts in MRS and MRSI. We present a prospective frequency correction method with outer volume suppression (OVS)‐based localization and selective water excitation for effective frequency correction with better SNR improvement compared to other techniques. Methods: An OVS‐localized navigator was developed to prospectively correct frequency drifts during MRS and MRSI measurements. The performance of the navigator was tested on the human brain and a solution phantom for frequency drifts induced by head motion or gradient heating by a preceding DWI experiment at 3T. Results: The OVS‐localized navigator could accurately track motion‐induced frequency drifts with an RMS error of 0.5 Hz. The SNR of MRS signals was not affected by use of the OVS‐localized navigator when compared with and without the navigator (P > 0.05). The frequency drifts induced by DWI experiments were 5.1 ± 0.3 Hz/min during MRSI measurements on humans, resulting in increased spectral linewidth, significant bias in metabolite concentrations, and significantly increased Cramér‐Rao lower bounds (P < 0.05). After prospective frequency corrections, the quality of MRSI was recovered to the level of those without any DWI‐induced frequency drifts, judged by the spectral linewidth, metabolite concentrations, and Cramér‐Rao lower bounds. Conclusion: The OVS‐localized navigator demonstrated effective prospective frequency corrections for large frequency drifts (5 Hz/min) without introducing any saturation‐induced SNR loss. These benefits can be particularly beneficial for the acquisition of MRS signals with long T1 and/or short TR, and spectral editing. [ABSTRACT FROM AUTHOR]

The properties of IIIb–III pairs observed by the URAN-2 radio telescope at frequencies 16-32 MHz are analyzed. Observations of these bursts were hold in April, June and September 2011. Durations, frequency drift rates, simultaneous frequency ratio of pairs components and their polarizations are analyzed. Pro and contra of IIIb–III harmonic connection are discussed.

Solar bursts with high frequency cut off were observed by the URAN-2 radio telescope (Poltava, Ukraine) on 18 August, 2012 in the frequency range 8-32 MHz. Durations of these bursts changed from 30 to 70 s. It is much longer than that for standard type III bursts. Drift rates are much smaller than those of type III bursts are, though much larger than those for decameter type II bursts. In some cases, the drift rate sign changes from the negative to positive one. Some of these bursts have fine structures. Stripes of the fine structures have small drift rates of 20-40 kHz/s. Polarizations of these bursts made about 10 % that apparently indicates that they are generated at the second harmonic of the local plasma frequency. The connection of bursts with the high frequency cut off with compact ejections from the behind-limb active regions is confirmed.

The control design of inductive power transfer (IPT) systems has attracted a lot of attention in the field of wireless power transmission. Due to the high-order resonant networks and multiple loads in IPT systems, a simplified model of an IPT system is preferred for analysis and control design, and a controller with strong robustness is required. Hence, an active disturbance rejection control (ADRC) for IPT systems is proposed in this paper. To realize the employment of ADRC, firstly a small-signal model of an LC series-compensative IPT system is derived based on generalized state-space averaging (GSSA), then the ADRC is implemented in the designed IPT system. The ADRC not only provides superior robustness to unknown internal and external disturbances, but also requires few knowledge of the IPT system. Due to the convenient realization of ADRC, the designed IPT system retains its simple structure without any additional circuits. Finally, a frequency domain analysis and experimental results have validated the effectiveness of the employed ADRC, especially its robustness in the presence of frequency drifts and other common disturbances. [ABSTRACT FROM AUTHOR]

A survey of the results of investigations of the frequency instability of output signals of the hydrogen frequency standard is presented. We analyze the causes of development of instability in the measurement range greater than 1000 sec. The influence of the ambient temperature and geomagnetic field is investigated. The methods used to compensate the frequency drift with the help of periodic corrections are described. [ABSTRACT FROM AUTHOR]

Burst-mode operation of laser arrays suitable for next-generation passive optical networks (PON2) employing time and wavelength division multiplexing (TWDM) is impaired by frequency drifts under variable burst duration. In this work, the issue of frequency drifts under burst and wavelength switching operation is addressed in a four-laser array integrated with a multimode interference coupler using a regrowth-free technology. Frequency drifts are shown to be significantly reduced in a TWDM-PON configuration thanks to the introduction of a thermal management technique based on the use of inactive lasers in the array. Thanks to this method, up to 22% drift reduction is obtained under burst and wavelength switching operation. [ABSTRACT FROM AUTHOR]

We obtain an analytic expression for the dependence of the relative error of frequency measure on the precision characteristics of this measure, time intervals of measurements, and periods between the measurements. This expression is characterized by the variance and the systematic component. [ABSTRACT FROM AUTHOR]

Specific emitter identification (SEI) is a technique for recognizing different emitters of the same type which have the same modulation parameters. Using only the classic modulation parameters for recognition, one cannot distinguish different emitters of a same type. To solve the problem, new features urgently need to be developed for recognition. This paper focuses on the common phenomenon of frequency drift, defines geometric features of frequency drift curve and, finally, proposes a practical algorithm of specific emitter identification using the geometric features. The proposed algorithm consists of three processes: instantaneous frequency estimation based on the adaptive fractional spectrogram, feature extraction of frequency drift curve based on geometric methods for describing a curve and recognition process based on support vector machine. Simulation results show that the identification rate is generally more than 98% above -5 dB of signal to noise ratio (SNR), and real data experiment verifies the practical performance of the proposed algorithm. [ABSTRACT FROM AUTHOR]

Accurate estimation of power signal frequency is an important requirement for many application areas that include system protection, energy quality monitoring and instrumentation. Though significant efforts have been made since long to develop potent algorithms for accurate estimation of power signal frequency, still their accuracy and convergence speed are a challenge under sudden frequency drift and variations. Therefore, this study focuses on a low complexity adaptive linear element filter using quadratic signal model, whose parameters are adjusted using a fast variable step size fuzzy logic‐based learning algorithm to provide better convergence and noise rejection properties for the estimation of frequency from noisy and distorted signals. In addition, the new filter has also been implemented on a field programmable gate array hardware and Xilinx 14.2 with Sysgen software for the tracking of dynamic signal parameters in single and three phase power networks. Various numerical and experimental results are addressed for estimation of frequency of time varying sinusoids. [ABSTRACT FROM AUTHOR]