National Measurement Standards

Time and Frequency

Time and Frequency Metrology is concerned with the realization of the second, which is the basic unit of the SI system. It is defined as the duration of 9,192,631,770 periods of the electromagnetic wave corresponding to the transition between the hyperfine energy levels of the caesium atom 133 in the ground state. This SI definition is most often realized by primary caesium atomic beam frequency standards, which are most common and traditional. More recent and more accurate are those based on caesium fountains.

In caesium fountain primary frequency standards, caesium atoms are first laser cooled and concentrated, then launched upwards to a height of about one meter. Under the influence of a gravitational field they fall back and slip past the resonator and the detector, where transitions are made between their quantum states. The spectral line from such a standard is much more precisely defined, since the collisions between atoms are low, as are the unwanted Brown motions due to very low temperatures of atoms.

 

Caesium fountain primary frequency standards are maintained in the laboratories of PTB in Germany, NPL in Great Britain, NIST in the United States of America, LNE-SYRTE in France and IEN in Italy. The accuracy of primary standards is currently somewhere around 2 × 10-15 Hz / Hz.

Smaller laboratories mainly use commercial high performance caesium atom beam frequency standards, most of which are of type HP 5071A. These standards or clocks are grouped and compared with each other to obtain an average clock or frequency of higher accuracy and to ensure the necessary redundancy and verification of the system operation. Typical accuracy of such standards is around 10-13 Hz/Hz.

In Slovenia, the unit of second is realized by commercial frequency standard Hewlett Packard, type 5071A.

The standard for time and frequency in SIQ

The reference standard for time and frequency used in SIQ is commercial frequency standard Hewlett Packard, type 5071A. This standard is compared with that of the PTB via a "common-view" comparison, where GPS satellites have the role of a transfer standard. Transfer standards are used solely for the transfer of values between laboratories and their absolute accuracy does not affect the final outcome of the comparison. However, they must be stable and repeatable, which GPS satellites are.

The data obtained from the "common-view" comparison are also sent to BIPM (Bureau International des Poids et Mesures), where, based on the data from all laboratories involved in this comparison, the calculation of international atomic time scale (TAI - International Atomic Time) takes place. This is done in such a way that the closest possible fit to the definition of the SI second is achieved. The Coordinated Universal Time (UTC) is also calculated there. The calculation is based on TAI, but in such a manner that it fits the uneven rotation of the Earth.

Local realizations of the UTC time are maintained in metrology laboratories around the world. The source of UTC (SIQ) is the standard that also serves as a frequency standard. A physical source of UTC (SIQ) is 1 PPS (pulse per second) output on the front panel of this standard.

SIQ has been included in the calculations of TAI and UTC since 1. 8. 2007. BIPM publishes the results of these calculations in their Circular-T monthly publication, which is also published on their website. Other data affecting these calculations are available on their web pages a well.

Based on the "common-view" comparison, we have been able to adjust the frequency of our standard to the frequencies of TAI and UTC. In this way we have improved the long-term accuracy of our standard by around 10 times. Before the comparison, the relative deviation of the frequency was -2.7 ×10-13 Hz/Hz. After the adjustment, it is within ±2×10-14 Hz/Hz. Even better accuracy currently cannot be obtained due to temperature variations in the laboratory where the standard is located.

Measurement capabilities of our Time and Frequency Laboratory

Our frequency and time interval calibration and measurement capabilities (CMCs) are recorded in the BIPM key comparison database (KCDB). We can generate and measure frequencies within the range from 0.1 MHz to 26.5 GHz. Routine calibrations of these frequencies are performed using frequency counters and signal generators. We have five counters in the laboratory to cover this frequency range.
For the purposes of the calibration of frequency standards the method of a phase comparison is used. This method is the most accurate, but also very time-consuming.

A time interval is measured within the range from -1s to 100000s. These measurements are carried out using time interval counters or oscilloscopes.

Currently, we are preparing procedures in the field of absolute time scale to be able to calibrate absolute difference with regard to the UTC time.

Dissemination of UTC (SIQ) time scale

SIQ has one NTP server that is synchronized to the national standard. The NTP server has a secondary backup source of time trough a GPS receiver. The NTP server is currently freely accessible for all users at:

  • ntp.siq.si
  • time.siq.si

Upon a prior agreement, it is also possible to authenticate the server using MD5 keys.

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