Petr Panek

Accuracy of two-way time transfer via a single coaxial cable

In order to find limits of the accuracy of the two-way time transfer (TWTT) via a single coaxial cable, we have carried out a detailed analysis which is presented in this paper. We applied the TWTT concept when a transmission line is driven by pulse current drivers and the times of arrival of the pulses are measured at the ends of the line. In addition to the estimation of the accuracy, the analysis provides several rules for proper design of a TWTT system with optimal performance. Based on this concept, a TWTT system for highly accurate time distribution or comparison has been designed and realized. For distances up to 1?km the accuracy was better than 100?ps without any additional correction or adjustment. After the influence of the non-symmetry of input?output circuits was corrected, the errors were lower than 20?ps for distances up to 2?km. The TWTT system is designated for keeping unified time in a net of event timers distributed in one building or in a relatively small area. The timing units forming the system guarantee the time transfer parallel to the time tagging of external pulses.

Nonlinear effects in the time measurement device based on surface acoustic wave filter excitation

A transversal surface acoustic wave filter has been used as a time interpolator in a time interval measurement device. We are presenting the experiments and results of an analysis of the nonlinear effects in such a time interpolator. The analysis shows that the nonlinear distortion in the time interpolator circuits causes a deterministic measurement error which can be understood as the time interpolation nonlinearity. The dependence of this error on time of the measured events can be expressed as a sparse Fourier series thus it usually oscillates very quickly in comparison to the clock period. The theoretical model is in good agreement with experiments carried out on an experimental two-channel timing system. Using highly linear amplifiers in the time interpolator and adjusting the filter excitation level to the optimum, we have achieved the interpolation nonlinearity below 0.2 ps. The overall single-shot precision of the experimental timing device is 0.9 ps rms in each channel.

Two-way time transfer via optical fiber providing subpicosecond precision and high temperature stability

We are reporting on analysis, design, construction, and key parameters of the device for a two-way time transfer via an optical fiber. The dominant source of errors in the two-way optical time transfer (TWOTT) via relatively short optical fibers is the temperature dependence of internal delays within the terminal units. We have performed an analysis of the influence of the internal delays and their temperature dependence on the TWOTT process considering two different configurations of the terminals with the feedback coupling in optical and electrical domains. The achieved results have been used for the optimal design of a TWOTT system implementing standard small form-factor pluggable optical transceivers. The operational tests of this new TWOTT system confirmed the precision of the time transfer on the subpicosecond level, the time transfer stability characterized by TDEV better than 60 fs for averaging intervals from 100 s to 10 000 s, and the temperature stability better than 100 fs K−1.

Note: electronic circuit for two-way time transfer via a single coaxial cable with picosecond accuracy and precision.

We have designed, constructed, and tested the overall performance of the electronic circuit for the two-way time transfer between two timing devices over modest distances with sub-picosecond precision and a systematic error of a few picoseconds. The concept of the electronic circuit enables to carry out time tagging of pulses of interest in parallel to the comparison of the time scales of these timing devices. The key timing parameters of the circuit are: temperature change of the delay is below 100 fs/K, timing stability time deviation better than 8 fs for averaging time from minutes to hours, sub-picosecond time transfer precision, and a few picoseconds time transfer accuracy.

Optical two-way timing system for space geodesy applications

Until now, time itself is not an observable in space geodesy. The major reason for this fact is the considerable difficulty to keep track of the phase of the clock oscillation between the point of origin and the point of the measurement. However, if geodesy will attempt to provide a reference frame fully based on general relativity, a proper treatment of time is mandatory. The Geodetic Observatory Wettzell is currently in the process to modernize the timing system such that the phase of the master clock can be established at all times. The ultra-short pulses of an optical frequency comb are transporting both time and frequency from the master clock of the observatory to the individual space geodetic techniques, namely Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Global Navigation Satellite System (GNSS), using a two-way approach. In order to verify the functionality of this system not only in sense of delay stability but also accuracy, we have developed a new TWTT system based on the exchange of timing signal via standard optical telecommunications Small Form-factor Pluggable (SFP) transceivers to transfer timing information between two or more terminals with the accuracy below 1 ps via optical fibers of a length of up to several tens of kilometers. The heart of the measurement device is an event timing module using surface acoustic wave filters as a time interpolator, which allows the registration of the times-of-arrival of electrical pulses with sub-picosecond timing resolution, linearity and stability. These pulses are derived from the optical signal, which is used for the communication between the terminals. Great care was taken in order to minimize terminal internal delays instability, which can be the result of temperature changes inside terminals. The design, applications and the first experiments at GO Wettzell will be discussed.

Note: Precise phase and frequency comparator based on direct phase-time measurements

We are reporting on the design, performance, and application results of a phase and frequency comparator based on the direct phase-time measurement using a high performance event timer. The advantages of this approach are the simple implementation, a broad frequency range, and the clear interpretation of the measured results. Primarily we analyzed the background instability of the instrument in a common-clock test when a 200 MHz clock signal was connected to both inputs and the noise bandwidth was kept at 5 Hz by a preprocessing of the measured data. The results show that the Allan deviation of the background instability follows 4 × 10(-14)/τ for a wide range of averaging intervals from 0.1 s up to 10(4) s. These results are better than background instability of commercially available state-of-the-art instruments based on the phase difference multiplication. Finally the instrument was used for comparison of two H-masers. This experiment proofed that one of possible applications is a comparison of low-noise highly stable frequency sources and measurement of their frequency stability in the time-domain. The noise background of the instrument was negligible for averaging intervals longer than 100 ms.

Event timing device providing subpicosecond precision

We are reporting on the latest experimental results achieved with an event timing device using a surface acoustic wave filter as a time interpolator. During the tests of the first version of the device, the noise of the filter excitation was identified as the dominant source of the measurement error. Therefore a new concept of the excitation with very low level of the noise energy was designed. This new solution led to considerable improvement of the device performance. It results from the experimental measurements that the single shot precision is repeatedly lower than 500 fs RMS when time marks generated synchronously with the time base are measured. When asynchronous time marks are split into two event timers and the resulting time difference is measured, the single shot precision is below 700 fs RMS per channel. In this case the measurement is affected not only by random errors, but also by non-linearity of the time interpolation. The temperature dependence is below 0.1 ps/K. Operating the device in a common laboratory environment without temperature stabilization, the stability TDEV better than 3 fs has been routinely achieved for range of averaging intervals from 10 s to several hours.

VLBI receiver chain monitoring

The most demanding goal of the Global Geodetic Observing System initiative is the definition of station positions to an accuracy of 1mm and the corresponding velocities to 0.1 mm/year. The main remaining sources of error are caused by systematics, leading to intra- and inter- technique biases. In this work, we have focused on Very Long Base Interferometry (VLBI) and phase calibration generator currently in operation. This unit is injecting calibration tones into the detection chain through an input coupler located near the input of the antenna. The tones propagate further through entire detection chain and are recorded with the observed signal. Then they are extracted in post processing. These tones are generated out of an atomic frequency standard. The supplied frequency is significantly influenced by temperature and mechanical changes since usually a long cable is employed to bring the frequency to the calibration unit. To monitor the electrical length of the cable, calibration with a picosecond precision is essential. We have redesigned a phase calibration unit so that it enables the implementation of the Two Way Time Transfer (TWTT) method on single coaxial cable using two event timers to monitor the electrical length of the critical cable. Such a system has been installed in parallel to the unit currently in operation. The comparison of the TWTT method with previous measurement method is presented.

Local ties control in application of laser time transfer

In many fundamental physical experiments time plays an important role. The standard way for the comparison of time and frequency is the application of GNSS signals and the Two-Way Satellite Time and Frequency Transfer - TWSTFT. This technique is based on radiofrequency signal transmission. Recently, there was a rapid increase of optical time comparison development, which uses the Satellite Laser Ranging network (SLR). Currently the French project T2L2 is in operation on board of Jason 2 and the European Space Agency project ELT in support of the Atomic Clock Ensemble in Space (ACES) is under development. The goal of both projects is the time synchronization with a precision below 40 ps rms and an absolute error well below 100 ps. Comparing the results of the optical time transfer with the GNSS time comparison requires unprecedented control of the local ties between the different observation techniques. One of the possible methods is the application of the Two Way Time Transfer (TWTT) on a single coaxial cable. Such a system can be implemented using two or more event timers, which are interconnected by a standard coaxial cable. The event timers are exchanging pulses between each other and time tagging them. Out of the measured result one can evaluate the difference of time scale represented by the event timers. It was shown that such a technique can be used for time transfer with a precision of a few picoseconds of rms and an absolute error below 20 ps for distances reaching several hundreds of meters. We have implemented this technique for establishing and monitoring the absolute time delays between the SLR system and the time keeping laboratory on the Geodetic Observatory Wettzell. The event timers were developed at the Czech Technical University. The measurement principal is based on SAW filter excitation. The Event Timers were located in different buildings 50 meters apart and with the help of the TWTT technique, the delay between them was measured. The second input sockets of the event timers were used to monitor the respective local timescale.

Two way time transfer with picoseconds precision and accuracy

The first results of a two way time transfer between two timing devices over modest distances with sub-ps precision and a few ps accuracy were the major tasks of this work. The two way time transfer scheme employing a single coaxial cable was applied. Based on our previous results we have designed and constructed a timing device which allows registration of Times of Arrivals of pulses with sub-ps timing resolution, linearity, and stability. The concept of the timing devices enables to carry out time tagging of pulses of interest in parallel to the comparison of the time scales of these timing devices. The two timing devices were located in different laboratories and buildings within one institution. The common clock 5 MHz was referenced to a Cesium clock. The analysis of systematic errors contribution shows that the accuracy of the time transfer well below 10 ps may be achieved. The time transfer was accomplished over distances of the order of hundreds of meters. The main limitation for longer distances is the quality of the coaxial cable used for time transfer together with the amount of radiofrequency interference within the experiment.