Standard Frequency Signal Research Articles

Microwave Frequency Dissemination over a 212 km Cascaded Urban Fiber Link with Stability at the 10−18 Level

To synchronize standard frequency signals between long-distance laboratories, we carried out a frequency dissemination experiment over a 212 km cascaded urban fiber link. This cascaded link was composed of two 106 km fiber links, in which the fiber noise was compensated by two microwave frequency dissemination systems. The two adjacent frequency dissemination systems used different frequency transmitted signals, preventing the influence of signal crosstalk between the received signal of the previous stage and the transmitted signal of the second stage caused by microwave signal leakage. The frequency dissemination over the cascaded link showed a dissemination fractional frequency instability of 6.2 × 10−15 at 1 s and 6.4 × 10−18 at 40,000 s, which is better than the transfer stability over the same 212 km single-stage link.

Research on a material for hydrogen purifying and flux controlling with application to space active hydrogen-masers

The Space Active Hydrogen-Maser (SAHM) can be used for the Space Very Long Baseline Interferometry Project to improve the resolution of the space astronomical telescope. The standard frequency signals provided by the SAHM have their stability dependent on the flux of effective H atoms. Therefore, a nickel (Ni) tube, one end closed, has been designed in this paper for the SAHM as a hydrogen (H2) purification and flux control device. Mechanisms of the H2 purification and flux control by a Ni tube were studied. H2 flux and ΔU, as a function of an input current, are given by experimental analyses. The diffusion activation energy of atomic H is much less than that of other atoms under the same conditions in Ni. The H atoms reversibly diffuse in Ni, and their diffusion speed has a high response to temperature. Experimental research shows that when the temperature of the tube was changed (from 27 to 700 °C), by an input heating electrical current (from 0 to 3.5 A), the H2 flux varied from 0 to 9.18 × 10−10 mol/s and no change in the phase structure of Ni. The data of flux should be useful for the frequency stability performance research of the SAHM. In addition, no phase change of the material indicates a stable performance of the tube for H-purifying and flux controlling. Finally, the above advantages make it a promising candidate for the reliability of the SAHM.

Real-time frequency transfer system over ground-to-satellite link based on carrier-phase compensation at 10−16 level* *Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0302101) and the Initiative Program of State Key Laboratory of Precision Measurement Technology and Instruments.

We demonstrate a novel and stable frequency transfer scheme over ground-to-satellite link based on real-time carrier-phase detection and compensation. We performed a zero-baseline measurement with the designed system, an uninterrupted frequency standard signal is recovered in the reception station without additional post-correction of delay error caused in the route, which is because the phase error of the entire route is tracked and compensated continuously in real-time. To achieve this goal, we employed two carriers in the system and the differential signal is transferred in order to eliminate the instability results from the local oscillator at the satellite transponder as well as the common-mode noise induced in the transfer route and microwave components. The stability of 3 × 10−16 with an integration time of 1 day was achieved and the time fluctuation during one day was measured to be about ±20 ps. Error sources and possible solutions are discussed. Our zero-baseline method shows a promising result for real-time satellite-based time and frequency transfer and deserves further research to find whether it works between long-baseline stations.

Метод трансляционного переноса для оценки стабильности нестационарного квантового стандарта частоты

Continuous wavelet transform is used to analyze the operation of a non-stationary signal of a quantum frequency standard. The method of translational transfer is proposed, with the help of which the boundary phenomena in this transformation are eliminated. The spectral integrals of the quantum frequency standard signal in various frequency ranges are calculated. A wavelet dispersion is introduced, which makes it possible to determine the moments of time when the signal fluctuations are the strongest. The comparison of the wavelet variance with the usual variance and with the Allen variance is carried out.

Increasing the Accuracy of Coaxial Cable Transmission of Standard Frequency and Time Signals

The accuracy of transmission of standard signals over a long coaxial cable is shown to depend on the temperature differentials in separate segments of the cable. The effects of temperature on the frequency instability of transmitted standard frequency and time signals are estimated in terms of the integral temperature of the cable. A method for increasing the accuracy in transmitting standard signals over a coaxial cable is proposed based on a method for determining the integral temperature of a long cable, a method for compensating the integral temperature of a long coaxial cable, and a method for thermal stabilization using a proportional-integral-differentiating algorithm and a distributed heater.

Optimization of Clock Synchronization System Architecture in Nuclear Power Plant

Clock Synchronization System (CSS) can generate standard time, time and frequency signals, and provide various high-precision time reference signals for systems and equipment requiring standard time scale in nuclear power plants. With the rapid development of power system, the requirement of time synchronization is becoming more and more urgent, which requires accurate, safe and reliable clock sources. The structure of nuclear power plant clock synchronization system is constantly optimized to meet the requirements of time synchronization of various systems and equipment, to ensure the time consistency of real-time data acquisition, to improve reliability and to adapt to the interconnection of large power grids in China. At the same time, the interconnection between time synchronization system and time-service equipment or system, and the interconnection of first-level time synchronization system network and operation mode of time synchronization devices from different manufacturers are standard

Precise Doppler shift measurement method based on motion radiation sources

Motion radiation sources, especially space motion radiation sources such as satellite, missile and aircraft and so on, have the Doppler shift relative to the ground detection equipment. A real-time and accurate localization of motion targets can be achieved using the Doppler shift. To reduce the noise and improve the positioning accuracy of motion targets under complex backgrounds, a precise Doppler shift measurement method is proposed based on motion radiation sources. Using frequency tracking and synthesis techniques, the captured signals of the motion radiation sources are converted into radio frequency signals as the measured signal, and by coarse measurement method, the traditional pulse counting method, the frequency nominal value of the measured signal can be obtained, and will be used as the frequency standard signal comparing pulse phase with the measured signal. The precise Doppler shift measurement of the measured signal is implemented using a high-resolution different-frequency group quantization phase processing. The experimental results indicate that the frequency stability of the system reaches the 4.3E−12/s level. Compared to the traditional Doppler shift measurement method, it has the advantages of a low cost, fast response, good noise suppression, high measurement precision and strong system reliability. This method has an important application in radio detection and ranging, location, tracking, early warning and surveillance of space motion targets, weapon control and navigation positioning and other high-tech fields.

High-precision frequency measurement system based on different frequency quantization phase comparison

Aiming at the requirements of phase comparison without frequency normalization, a high-precision frequency measurement system is proposed based on different frequency quantization phase comparison. Using group phase difference pulse in the least common multiple period, the period of frequency standard signal, the period of the measured signal and the pulse width of group phase difference can be respectively counted with the aid of FPGA technology. The obtained counter value is saved and sent to host computer. The measured frequency can be obtained by the data processing of the host computer. Experimental results show that the short-term frequency stability of the system better than E−12/s is achieved. Compared to the traditional frequency measurement system, the system have the advantages of wide measurement range, high frequency stability, low phase noise and low development cost. The system not only plays an important role in precise measurement physics fields such as high-precision transmission and comparison of time and frequency, measurement and suppression of phase noise, but also has a wider application in localization of spatial motion radiation source, satellite navigation equipment, precise time frequency measurement, communication, radar and other high-tech fields.

Measurement and control from frequency to phase based on virtual signal reconstruction.

A virtual reconstruction method of directly capturing phase information between different nominal frequency signals, without frequency transformation, is proposed in this paper, building a virtual standard frequency signal whose frequency equals the measured nominal frequency and then making continuous comparison in the measuring gate which is synchronous with multiple periods between the measured signal and the reference frequency signal. Phase variations of the measured signal in every continuous gate are determined, and continuous phase-measuring is implemented. The experimental result verifies this special method for directly processing the phase difference between different nominal frequency signals and realizes a comparison precision of 10-17/day in a wide range.

Direct precision frequency measurement and correction technology with double ADC.

A new method of direct precision frequency measurement is proposed in this paper, which uses the clock cursor effect between the sampling clock signal and input signal and ADC (analog to digital converter) quantization error suppression technique in the background of digital measurement with double ADC. On this basis, a precision frequency corrector is designed. Compared with the traditional frequency synthesizer, this device takes the non-standard frequency signal of the crystal oscillator as the reference to realize the standard frequency signal output through the frequency correction function. Meanwhile, the output signal is obviously narrower in range, from 0.0001 Hz to 0.1 Hz. The frequency corrector can also realize the second stability of less than 3 × 10-12 and a small frequency correction of 10-11 orders of magnitude.

Residual frequency drift in an atomic fountain clock

We report a locking mode in which the local oscillator (LO) is locked to an atomic fountain and calibration of the residual frequency drift (RFD). In this running mode, the locked LO outputs a standard frequency signal, and a short-term fractional frequency stability of 2.7 \times 10-13τ-1/2 is achieved. Due to the frequency drift of the LO in free running mode, a systematic frequency bias, or RFD, exists after being locked by the atomic fountain. We analyze and measure the RFD with a value of -3(2) \times 10-16. A sectionalized post-process method is adopted to calibrate the RFD.

Hanle detection for optical clocks.

Considering the strong inhomogeneous spatial polarization and intensity distribution of spontaneous decay fluorescence due to the Hanle effect, we propose and demonstrate a universe Hanle detection configuration of electron-shelving method for optical clocks. Experimental results from Ca atomic beam optical frequency standard with electron-shelving method show that a designed Hanle detection geometry with optimized magnetic field direction, detection laser beam propagation and polarization direction, and detector position can improve the fluorescence collection rate by more than one order of magnitude comparing with that of inefficient geometry. With the fixed 423 nm fluorescence, the improved 657 nm optical frequency standard signal intensity is presented. The potential application of the Hanle detection geometry designed for facilitating the fluorescence collection for optical lattice clock with a limited solid angle of the fluorescence collection has been discussed. The Hanle detection geometry is also effective for ion detection in ion optical clock and quantum information experiments. Besides, a cylinder fluorescence collection structure is designed to increase the solid angle of the fluorescence collection in Ca atomic beam optical frequency standard.

High-precision time and frequency measurement method combining time-space conversion and different frequency phase detection

According to the time & space conversion relations and different frequency phase detection principle, an ultra-high precision time & frequency measurement method is proposed in this paper. The higher accuracy and stability of the speed of light and electromagnetic signals during the transmission in space or a specific medium enable the measurement of short time interval which uses the coincidence detection of signal’s transmission delay in length. The measurement precision better than 10 picoseconds can be easily obtained. The method develops the length vernier utilizing the stability of signal’s transmission delay, minimizes the fuzzy region of phase coincidence between the standard frequency signal and the measured signal, approaches the best phase coincidences and therefore improves the measurement precision which is higher than the precision provided by the traditional methods based on frequency processing. Besides, the method costs less than the traditional methods and can also solve the problem of the measurement of super-high frequency. Experimental results show the method can improve the measurement precision to 10−12/s in the time & frequency domain.

Optical clocks based on stimulated emission radiation

Atomic clocks operating at microwave region, are divided into two categories according to the mode of operation, namely the passive mode, in which the standard frequency of a clock signal is locked on to a very narrow atomic spectral line such as in cesium beam and rubidium gas cell clocks, and the active mode, where the atomic ensemble itself can produce a high stable and accurate standard frequency signal such as hydrogen maser. The hydrogen maser has been the most stable atomic clock for nearly half a century, and has been applied in a variety of scientific research fields. The optical clocks, operating at frequency domain by about five orders of magnitudes higher than that of the microwave clocks, have been expected to surpass the performance of microwave clocks both in stability and uncertainty. With the development and maturity of femtosecond optical frequency comb, very recently, the ion optical clock and neutral Sr optical clock have demonstrated better stability and uncertainty than any microwave clocks. However, all the optical clocks realized up to date are based on the “passive mode” of operation, this means that the laser frequency is locked to the very narrow atomic spectral line, but not the lasing light (the stimulated emission radiation) frequency itself which is very stable and can be used as a frequency standard——the active mode. Recently, Chen has proposed the concept of active optical clock (see ref. [6] in this issue), which is an optical clock based on the stimulated emission radiation. This is the first extension of the principle of hydrogen maser from microwave domain to optical domain. There are twofold advantages of this active optical clock. On one hand, the linewidth of the output laser from active optical clock determined by the modified SchawlowTownes formula, is much narrower than the linewidth due to transit time, which is the ultimate limit of absorption laser spectroscopy. On the other hand, utilizing bad cavity in active optical clock, the shift and noise of the centre frequency of active optical clock caused by cavity length change is reduced to a cavity pulling effect. Currently, the cavity length noise is the ultimate limit of existing lasers with coherence time longer than one second, and thus the laser linewidth is at mHz level. Active optical clocks provide several new possibilities of applications: (i) more stable optical clock than any current atomic clocks; (ii) sub-natural linewidth laser spectroscopy; (iii) long coherence time laser with linewidth at mHz level; (iv) Ramsey laser combining stimulated emission process and Ramsey separated oscillatory fields method. Therefore, the prospect of active optical clock is attractive for the research at the field of optical clock, laser spectroscopy and metrology.

Design for the Sub-Core of an Antenna in a Radio-Controlled Watch

Radio-controlled watches that automatically adjust the time by using standard frequency and time signals becoming popular with consumers. To evaluate the receiving sensitivity of the antenna, we calculated the antenna output voltage by combining a magnetic field simulation and a circuit simulation. The output voltage of an antenna in a metal case is much smaller than that of an antenna in a non-conductive material case, because the leakage field induces an eddy current in the metal case. We present an example in which the sub-core increased the output voltage suppressing the leakage field. This effect of the sub-core depends on the size and arrangement of the sub-core in an antenna system, and thus optimization is essential. It is important to evaluate the quality factor (Q factor) of antennas, because the Q factor has a close relation to the receiving sensitivity. However, we show that an antenna with a higher Q factor might not always have a higher output voltage.

Investigation of the receivability of VLF standard time and frequency signals in Zaria, Nigeria

This study is on the receivability of some standard time and frequency signals in the VLF range in Zaria, Nigeria. Signal field strengths of various stations were estimated, and a suitable radio receiving system was set up to receive them. Using the assembled receiver, some of the transmissions, including the Omega W/L at Monrovia, Liberia, and GBR at Rugby, United Kingdom, were received and identified. Keywords: signal, receiver, standard time, standard frequency Nigerian Journal of Physics Vol. 17, 2005: 72-77

Wintertime survey of LF field strengths in Japan

A nationwide survey for measuring field strengths at 40 and 60 kHz was carried out in the winter months of 2004 to estimate the service area of the standard frequency and time signals. The data obtained were analyzed in comparison with the resultant field strengths of sky waves and the ground wave which were predicted using a method developed by one of the authors. Good agreement was seen between the measured and predicted field strengths.

The Effect of Radio Noise on Standard Frequency and Time Signal Emissions in LF Band

The standard frequency and time signal emissions in LF band began in 1999 in Japan. The bands of standard frequencies are rightly guarded from other intentional emission, but some reports show radio noise interferes with the receipt of the time signals. We observed LF band at several points, and found high level noises overlap at the some percents of time sequence. And additional search shows radio noise has two type origins. One is natural source, thunder noise, and it emits impulsive noise. But the other one, artificial origin, maintains emission during the origin works, and sometime becomes a strong disturbance.

TAD digital quadrature detection and its application to the receiver circuit for the LF standard time and frequency signal

TAD digital quadrature detection and its application to the receiver circuit for the LF standard time and frequency signal

Geostationary orbit determination for time synchronization using analytical dynamic models

A real time analytical orbit determination method has been developed for precision national time synchronization. The one-way time transfer technique via a geostationary TV satellite standard time and frequency signal (STFS) dissemination system was considered. The differential method was also applied for mitigating errors in geostationary satellite STFS dissemination system. Analytical dynamic orbit determination with extended Kalman filter (EKF) was implemented to improve differential mode STFS (DSTFS) service accuracy by acquiring better accuracy of a geostationary satellite position. The perturbation force models applied for satellite dynamics include the geopotential perturbation up to fifth degree and order harmonics, luni-solar perturbations, and solar radiation pressure. All of the perturbation effects were analyzed by secular, short, and long period variations for equinoctial orbit elements such as semimajor axis, eccentricity vector, inclination vector, and mean right ascension of the geostationary satellite. The reference stations for orbit determination were composed of four calibrated stations. Simulations were performed to evaluate the performance of real time analytical orbit determination in Korea. The simulation results demonstrated that it is possible to determine real time position of geostationary satellite with the accuracy of 300 m rms. This performance implies that the time accuracy is better than 25 ns all over the Korean peninsula. The real time analytical orbit determination method developed in this research can provide a reliable, extremely high accurate time synchronization service through setting up domestic-only benchmarks.