Calibration

Time and frequency

People have been able to distinguish between days and nights, seasons, and other time units from the beginning of mankind. First civilizations have already invented basic clocks through the observation of the Sun and other celestial bodies. Since then, time has become practically the most important and best-known unit. But what is time and what does this parameter represent? At SIQ, we not only know how to answer this question, but we also know how to measure time very accurately.

Time is most often associated with clocks which are devices that measure, store or generate a specific interval. The evolution of clocks had begun with the clocks based on the water flow already known by Babylonian and ancient Egyptian and later also by ancient Greek and Roman. The first major step further occurred in the Middle Ages with the invention of the pendulum clock. During the First World War, the piezoelectric properties of quartz were investigated, which served as reference oscillator of the most accurate clocks until 1960, when the first atomic clocks appeared. Nowadays the most accurate measurements of time are still based that technology. Moreover, time became a physical quantity that can be nowadays the most accurately measured thanks to the accuracy of atomic clocks

At SIQ we also maintain a caesium atomic clock, which is included in the International Bureau of Weights and Measures (BIPM) in France, which is an international standardization organization that maintain international system of units. Our presence in the BIPM system implies that the SIQ’s atomic clock also contributes to the calculation of the reference time (i.e., UTC, Coordinated Universal Time) that is in use worldwide. SIQ as the holders of the national standard for time&frequency also provides distribution of the most accurate time in Slovenia. Our atomic clock has a relative frequency deviation less than 6×10^-15 which in the other words means that the clock would be off for less than 1 s in 5 million years.

Our daily lives significantly depend on the accuracy of measuring time without even realizing it:

we always want to know what time it is, when traveling we want to know precise time in different time zones, precise time is needed for daily schedules and meetings
in sports we need even more precise measuring of time
synchronization of measurements at different locations (country/continent), e.g., synchronous measurement of grid parameters at different locations
major global trading banks are increasingly relying on automated stock exchange trading that is based on powerful computers and complex trading algorithms (e.g., high‑frequency trading), and the legislation of some countries demands extremely accurate time-stamp records when transactions were performed, even with a resolution down to 1 μs
many instruments have their own internal time-base which in turn affects the accuracy of the instrument (counters, function generators, signal generators, oscilloscopes, spectrum analyzers, vector analyzers, etc.)
Nowadays, we have all used GPS navigation, but only a few people realize that the location accuracy on Earth depends on the accuracy of atomic clocks on satellites orbiting the Earth. In the other words, more advanced atomic clocks on satellites permit determination of location on Earth with a lower resolution, i.e., with a better accuracy.

At SIQ, we measure time at all levels of accuracy, from calibration of conventional analogue and digital stopwatches, calibration of time measuring function included in other measuring instruments (e.g., high voltage testers, multifunction instruments, acoustic dosimeters, etc.). We also perform the most accurate calibrations where we rely on our atomic clock (e.g., frequency counters, GNSS receivers, time interval/pulse/period meters and generators, frequency dividers). We also calibrate time-bases of different instruments (function generators, signal generators, oscilloscopes, spectrum analyzers, vector analyzers, etc.). The frequency can be measured directly from 1 mHz to 15 GHz, and from 15 GHz to 26.5 GHz using beat modulation technique.

Calibration of test equipment at SIQ

Frequency counters

We measure the following parameters:

Input sensitivity: The parameter defines the minimal required voltage when the counter starts measuring correctly. The sensitivity is checked at 50 Ω and /or 1 MΩ input impedance.
Time-base stability: We check the 24-hour time-base stability of the counter. The measurement requires at least two days; after initial 24-hour preheating a 24-hour time‑base stability is measured. Typical time-base frequencies are 10 MHz, 5 MHz, and 2 MHz, but other frequencies could be also measured if additional option is present. In addition to time-base stability we also check the counter’s ability to lock the frequency to the time-base.
Functional test: various tests are performed according to manufacturer recommendations (e.g., measurement of period/pulse length/time interval between the two inputs, frequency accuracy, etc.). A functional test is usually performed unaccredited.

GNSS receivers

We typically measure the relative deviation, a drift of the frequency and the stability of GNSS receivers:

Relative deviation and frequency drift are calibrated by phase comparison with a reference caesium frequency standard (e., atomic clock). The calibration requires at lasts three days; the instrument is stabilized for at least 48 hours to obtain operating temperature, which is followed by typical 24-hours calibration (per receiver output) while the device is locked to the GNSS signal. The calibration time could be prolonged if requested by the customer. The uncertainty calculation includes frequency drift during the calibration and according to this value the relative frequency drift could be estimated per day or per any other time interval (month, year).
Frequency stability is determined analytically using the measurements described above. Herein the Allan and modified Allan deviation are used, which are the key parameters that describe frequency stability of clocks.

Time-base calibration can be performed also at customer’s premises with uncertainty in the range of 2×10^-10 due to the use of a rubidium reference frequency standard instead of a caesium atomic clock.

Time interval/pulse/period/frequency meters and generators

In addition to time interval/pulse/period/frequency meters and generators, we also calibrate time stamp generators, short time meters, microprocessor system time-bases, time switches, time interval and tachometers, tariff pulse generators, time-base of signal generators, etc.

We calibrate the following parameters:

Input sensitivity: We define the input voltage when the instrument starts measuring correctly.
Time base: Like frequency counters, time interval meters and generators have their own internal time base which is checked with a reference counter connected to a reference frequency standard. The relative calibration uncertainties in the range 2×10^-12could be obtained
Functional test: The test is usually performed according to the procedures recommended by the instrument manufacturer. Additional functional tests can be also performed if requested. A functional test is usually performed unaccredited.
Pulse parameters: We calibrate different pulse parameters that are generated or measured by the instrument (pulse length, rise/fall time, positive/negative pulse length, pulse frequency, amplitude). Pulses can be generated and measured. If requested, we can perform other measurements.

Period/frequency: We check the accuracy of frequency/period generation and measurement.

Frequency divider

The key parameter of the frequency divider is the frequency division ratio which is the ratio between the input (i.e., generated) frequency and the output frequency. The frequency divider ratio is calibrated using a reference generator and frequency counter which are connected to the same time-base.

Stopwatches

We calibrate the following parameters for digital as well as analogue stopwatches:

Relative time: Relative time is measured directly using the standard clock calibrator. The calibrator measures the time deviation in a certain time interval (e.g., how many seconds the stopwatch deviates per day).
Functional test: Functional test is performed nonaccredited as a performance test.

Other instruments capable of measuring time, frequency, or other time related quantities

Several other types of instruments exist that could also measure time related quantities in addition to their primary measuring parameter (e.g., multifunctional instruments with adjustable measuring time, acoustic dosimeters, different test devices, digital/analogue multimeters and calibrators, AC power meters, frequency meters based on optical or rotational inductive meters, tachometers, etc.).

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