Comparison Of Frequency Standards Research Articles

A Study on Autonomous Integrity Monitoring of Multiple Atomic Clocks

Abstract A stable and reliable time keeping system depends on the integrity monitoring of the atomic frequency standard. This paper reports a scheme for autonomous integrity monitoring of multiple atomic clocks, which combines the frequency standard comparison method and the frequency jump detection method. The frequency standard comparison method uses multi-channel synchronous acquisition technology and digital frequency measurement technology to realize the precise measurement of multiple atomic frequency standards. The frequency jump detection method uses adaptive filtering to predict the relative frequency difference and give an accurate and timely alarm for the abnormal of frequency jump. The results show that the noise floor frequency standard comparator is better than 6.5×10−15 s. For a relative frequency deviation of 2.0×10−6 Hz, the probability of anomaly detection is almost 100 %. The system has high frequency measurement resolution and fast alarm of frequency jump, which can meet the real-time requirements of a time keeping system for the integrity monitoring of multiple atomic clocks.

Continuous IPPP links for UTC

The technique of precise point positioning with integer ambiguity resolution (IPPP) has been developed for many years and has been shown to significantly improve the long-term performance of time and frequency transfer with respect to other GNSS-based techniques. In this paper, we present results of GPS IPPP links over a period of 22 months for a dozen time laboratories participating to UTC. We show that continuous links, in which the continuity of the GPS phase measurements is preserved, can be maintained for periods exceeding one year and further extended if data from two receivers per station are available. We quantify the frequency transfer uncertainty of IPPP by comparison to optical links and show how IPPP could improve UTC links to below 1 × 10−16 relative frequency uncertainty over averaging times of up to one month, i.e. the characteristic period of UTC publication. Comparisons of primary and secondary frequency standards reported for TAI indicate that IPPP could somewhat improve the accuracy of TAI/UTC. Comparisons of IPPP to two-way time transfer techniques reveal very long-term ns-size instabilities which must be further studied. Use of IPPP for UTC links is possible only if integer GNSS satellite products become available with a short delay and we report on such experimental products. Finally, we discuss the practical implications of using IPPP link in UTC and describe how the necessary steps could be implemented.

High-Resolution Multi-Channel Frequency Standard Comparator Using Digital Frequency Measurement.

The rapid improvement accuracy of the atomic frequency standard puts forward higher requirements for the measurement resolution of the atomic frequency standard comparison system. To overcome the defect that the single zero-crossing point detection is sensitive to noise in the traditional dual mixer time difference measurement method, a digital frequency measurement method is proposed. This method combines sinusoidal beat technology, multi-channel synchronous acquisition technology, and digital frequency measurement technology, and uses differential compensation of system error to realize the precision measurement of atomic frequency standard. The frequency measurement accuracy is less than 2.5 × 10−14 and the noise floor is better than 6.5 × 10−15/τ = 1 s. The system has a high frequency measurement accuracy and a low noise floor, which can realize the precise measurement of a highly stable frequency source.

Development the Measurement System of the Multi-Channel Frequency Standard Comparison

This paper introduces a measurement system and method of high-speed sampling and full digital for the comparison of multi-channel frequency standard. The hardware of the system consists of 4 parts: A: Impedance matching module (After matching with the impedance of the acquisition circuit, the two input signals are transformed into suitable amplitude, and then transformed into digital signals by the high-speed acquisition circuit). B: High speed data acquisition module. C: Data real-time processing module. D: Calculation module for phase difference and frequency stability. The system not only can achieve real-time processing under the minimum computing resources, but also provide the upper computer with complete information and equivalent frequency data with small amount of data. Moreover, the fast and high-precision calculation of phase difference and frequency stability can be achieved by improving the calculation process. The software of the system meets the requirements for the technical specifications of national metrology. Compared with domestic and foreign frequency standard comparators, the interface of software is more friendly, the operation is more convenient, and it has a strong ability to analyze and process data. The test results show that the uncertainty of the system is better than 1.5 ×10^-13 (sampling time is 1 second), which is consistent with Russian VCH-315 index.

Direct comparisons of European primary and secondary frequency standards via satellite techniques

We carried out a 26-day comparison of five simultaneously operated optical clocks and six atomic fountain clocks located at INRIM, LNE-SYRTE, NPL and PTB by using two satellite-based frequency comparison techniques: broadband Two-Way Satellite Time and Frequency Transfer (TWSTFT) and Global Positioning System Precise Point Positioning (GPS PPP). With an enhanced statistical analysis procedure taking into account correlations and gaps in the measurement data, combined overall uncertainties in the range of 1.8 × 10−16 to 3.5 × 10−16 for the optical clock comparisons were found. The comparison of the fountain clocks yields results with a maximum relative frequency difference of 6.9 × 10−16, and combined overall uncertainties in the range of 4.8 × 10−16 to 7.7 × 10−16.

New Frequency Standard Comparison System Based on the Group Quantization Phase Processing

A new frequency standard comparison system is proposed based on the group quantization phase processing. By shortening the width of phase coincidence fuzzy area and capturing the best group phase coincidences, we reduce the randomness of the counting gate and improve the measurement precision of the comparison system. The method is based on the Fieldprogrammable gate array (FPGA), which not only retains the advantage using phase synchronization detection technology to overcome the ±1-word counting error, but also accelerates the system response time, simplifies the measurement equipment, and reduces the development cost and power consumption. The experimental results show that the method is reasonable and scientific, and the comparison accuracy of the system can reach the E-12 s-1 level, obviously superior to the measurement accuracy of the traditional frequency standard comparison method, which has a wide application and popularization value.

Development of a Miniaturized Frequency Standard Comparator Based on FPGA

Frequency standard comparison measurement has important practical significance for the rational use of frequency standard in engineering. This paper was devoted to the study of frequency standard comparison measurement based on classical dual mixing time difference method. However, in the actual system design and implementation, the commonly used counter was discarded and the phase difference was measured by a digital signal processing method based on Field Programmable Gate Array (FPGA). A miniaturized 10 MHz frequency standard comparator with good noise floor was successfully developed. The size of the prototype circuit board is only about 292.1 cm2. The experimental results showed that the noise floor of the frequency standard comparator was typically better than 7.50 × 10-12/s, and its relative error of phase difference measurement was less than 1.70 × 10-5.

First international comparison of fountain primary frequency standards via a long distance optical fiber link

We report on the first comparison of distant caesium fountain primary frequency standards (PFSs) via an optical fiber link. The 1415 km long optical link connects two PFSs at LNE-SYRTE (Laboratoire National de métrologie et d’Essais—SYstème de Références Temps-Espace) in Paris (France) with two at PTB (Physikalisch-Technische Bundesanstalt) in Braunschweig (Germany). For a long time, these PFSs have been major contributors to accuracy of the International Atomic Time (TAI), with stated accuracies of around . They have also been the references for a number of absolute measurements of clock transition frequencies in various optical frequency standards in view of a future redefinition of the second. The phase coherent optical frequency transfer via a stabilized telecom fiber link enables far better resolution than any other means of frequency transfer based on satellite links. The agreement for each pair of distant fountains compared is well within the combined uncertainty of a few 10−16 for all the comparisons, which fully supports the stated PFSs’ uncertainties. The comparison also includes a rubidium fountain frequency standard participating in the steering of TAI and enables a new absolute determination of the 87Rb ground state hyperfine transition frequency with an uncertainty of .This paper is dedicated to the memory of André Clairon, who passed away on 24 December 2015, for his pioneering and long-lasting efforts in atomic fountains. He also pioneered optical links from as early as 1997.

Border effect-based precise measurement of any frequency signal**Project supported by the National Natural Science Foundation of China (Grant Nos. 10978017 and 61201288), the Natural Science Foundation of Research Plan Projects of Shaanxi Province, China (Grant No. 2014JM2–6128), and the Sino-Poland Science and Technology Cooperation Projects (Grant No. 36-33).

Limited detection resolution leads to fuzzy areas during the measurement, and the discrimination of the border of a fuzzy area helps to use the resolution stability. In this way, measurement precision is greatly improved, hence this phenomenon is named the border effect. The resolution fuzzy area and its application should be studied to realize high-resolution measurement. During the measurement of any frequency signal, the fuzzy areas of phase-coincidence detection are always discrete and irregular. In this paper the difficulty in capturing the border information of discrete fuzzy areas is overcome and extra-high resolution measurement is implemented. Measurement precision of any frequency-signal can easily reach better than 1 × 10−11/s in a wide range of frequencies, showing the great importance of the border effect. An in-depth study of this issue has great significance for frequency standard comparison, signal processing, telecommunication, and fundamental subjects.

High-resolution frequency measurement method with a wide-frequency range based on a quantized phase step law

A wide-frequency and high-resolution frequency measurement method based on the quantized phase step law is presented in this paper. Utilizing a variation law of the phase differences, the direct different frequency phase processing, and the phase group synchronization phenomenon, combining an A/D converter and the adaptive phase shifting principle, a counter gate is established in the phase coincidences at one-group intervals, which eliminates the ±1 counter error in the traditional frequency measurement method. More importantly, the direct phase comparison, the measurement, and the control between any periodic signals have been realized without frequency normalization in this method. Experimental results show that sub-picosecond resolution can be easily obtained in the frequency measurement, the frequency standard comparison, and the phase-locked control based on the phase quantization processing technique. The method may be widely used in navigation positioning, space techniques, communication, radar, astronomy, atomic frequency standards, and other high-tech fields.

Comparison of Frequency Standards Used for TAI

In this paper, we study the performance of the ensemble of high-accuracy primary frequency standards (PFSs) that ensure the accuracy of International Atomic Time (TAI). We analyze the statistics of the results of PFS evaluations compared with their stated uncertainties, and we discuss different components of the uncertainty. We also consider the use of reporting evaluations of secondary frequency standards and how they could contribute to TAI.

A DPTD frequency standard comparison system

A dual phase-coincidence-detection time difference (DPTD) system for frequency standard comparison is proposed in this paper. Based on the concept of the greatest common factor frequency and a high-stability common frequency source, a high-resolution phase comparison between frequency standards can be implemented by utilizing the phase coincidence detection approach. The resolution of this simple instrument is better than 10 ps and the resolution of theoretical analysis is much higher than 10 ps. Compared with dualmixer time difference systems, a comparison system based on this method is simple in structure and low in cost.

Ultra-resolution phase comparison method combining phase synchronous detection and common frequency source

With the improvement of frequency standard comparison and precise frequency and phase processing technique, the high-precision frequency measurement, phase comparison, phase locked loop and signal processing are necessary. Combining phase synchronous detection principle and common frequency source, an ultra-high resolution phase comparison method is presented in this paper. Using the concepts of the equivalent phase comparison frequency, group period phase processing, phase group synchronization and phase quantization step and so on, and the high stability of common frequency source, the resolution with picosecond or subpicosecond can be easily obtained and ±1 count error in traditional phase comparison approach can be eliminated. Experimental results show that the measuring precision better than femtosecond in one hour can be achieved in the long-term frequency standard comparison.

A Novel Super-High Resolution Phase Comparison Approach

With the improvement of the accuracy of atomic frequency standard and satellite navigation, the high-resolution phase comparison method is necessary. Using the phase synchronous detection principle, a super-high resolution phase comparison method between frequency standards is proposed based on the greatest common factor frequency, phase group processing and a common frequency source and so on. This method is mainly dependent on the stability of the common frequency standard and its frequency. The ±1 count error can be eliminated effectively. Therefore, higher than 1 ps resolution can be easily reached with a simple instrument. Experimental results show higher than 10−15/h precision can be obtained in the long-term frequency standard comparison and the measuring precision can reach 10−17 for several days of comparison.

Atomic clock ensemble in space

Atomic Clock Ensemble in Space (ACES) is a mission using high-performance clocks and links to test fundamental laws of physics in space.Operated in the microgravity environment of the International Space Station, the ACES clocks, PHARAO and SHM, will generate a frequency reference reaching instability and inaccuracy at the 1 · 10−16 level. A link in the microwave domain (MWL) and an optical link (ELT) will make the ACES clock signal available to ground laboratories equipped with atomic clocks. Space-to-ground and ground-to-ground comparisons of atomic frequency standards will be used to test Einstein's theory of general relativity including a precision measurement of the gravitational red-shift, a search for time variations of fundamental constants, and Lorentz Invariance tests. Applications in geodesy, optical time transfer, and ranging will also be supported.ACES has now reached an advanced technology maturity, with engineering models completed and successfully tested and flight hardware under development. This paper presents the ACES mission concept and the status of its main instruments.

Colloquium: Comparison of astrophysical and terrestrial frequency standards

We have re-analyzed the stability of pulse arrival times from pulsars and white dwarfs using several analysis tools for measuring the noise characteristics of sampled time and frequency data. We show that the best terrestrial artificial clocks substantially exceed the performance of astronomical sources as time-keepers in terms of accuracy (as defined by cesium primary frequency standards) and stability. This superiority in stability can be directly demonstrated over time periods up to two years, where there is high quality data for both. Beyond 2 years there is a deficiency of data for clock/clock comparisons and both terrestrial and astronomical clocks show equal performance being equally limited by the quality of the reference timescales used to make the comparisons. Nonetheless, we show that detailed accuracy evaluations of modern terrestrial clocks imply that these new clocks are likely to have a stability better than any astronomical source up to comparison times of at least hundreds of years. This article is intended to provide a correct appreciation of the relative merits of natural and artificial clocks. The use of natural clocks as tests of physics under the most extreme conditions is entirely appropriate; however, the contention that these natural clocks, particularly white dwarfs, can compete as timekeepers against devices constructed by mankind is shown to be doubtful.

A theoretical and experimental analysis of frequency transfer uncertainty, including frequency transfer into TAI

Results are presented from a theoretical and experimental investigation of the frequency transfer uncertainty (FTU) in long-distance comparisons of frequency standards. The FTU can be an important component of the total uncertainty in such comparisons. The use of the Allan deviation in characterizing the FTU is analysed theoretically and it is shown that for certain noise types the Allan deviation is biased high. A potentially more accurate first difference statistic that can be used in certain situations is also discussed. In addition, an experimental determination of the noise types and levels in common transfer techniques is presented. It is shown that FTUs approaching 1 part in 1016 at 30 days are possible with current transfer methods. Finally, a method is presented for estimating the FTU in calibrating International Atomic Time (TAI) with a primary frequency standard.

A super-high resolution frequency standard measuring approach based on phase coincidence characteristics between signals

A method for super high resolution comparison measurement is proposed in this paper with a comparison between the frequency standards of different nominal frequencies, which is based on phase coincidence detection of the two compared signals. It utilizes the regular phase shift characteristics between the signals. The resolution of the measurement approach can reach 10−13/s at 5 MHz, and the self-calibration resolution can achieve 10−14/s in the comparison between 10 MHz and 100 MHz, or even can reach 10−15/s in the comparison between 10 MHz and 190 MHz. This method implies significant progress in the development of the high precision frequency standard comparison technology.

Fiber-laser-based clockwork for long-term measurements and comparisons of visible optical frequency standards

A long-term clockwork capable of linking visible optical frequency standards to the radio frequency domain is demonstrated with a continuous measurement stretching eight days. The clockwork is based on a femtosecond fiber laser frequency comb. Using this system, an uncorrected measurement of the 88Sr+ frequency standard at 445 THz (674 nm) is completed and is in excellent agreement with previous measurements.

Long-term comparison of caesium fountain primary frequency standards

There are currently nine caesium fountain primary frequency standards regularly reporting calibrations of International Atomic Time to the Bureau International des Poids et Measures (BIPM). An investigation has been carried out using data from the BIPM publication Circular T to evaluate the frequency differences among these standards and to determine whether these offsets are consistent with the stated uncertainties. The fractional frequency uncertainties of some Cs fountains are now in the range of 4 × 10−16 to 5 × 10−16. The results of this investigation show that the standards agree well with each other. An overall estimate of the caesium frequency is made using the weighted mean of all the fountains.