Page 348 - The Mechatronics Handbook

TABLE 17.1 Uncertainties of Physical Realizations
of the Base SI Units
SI Base Unit Physical Quantity Uncertainty
Candela Luminous intensity 1 × 10 -4
Kelvin Temperature 3 × 10 -7
Mole Amount of substance 8 × 10 -8
Ampere Electric current 4 × 10 -8
Kilogram Mass 1 × 10 -8
Meter Length 1 × 10 -12
Second Time interval 1 × 10 -15

6
9
equals one million (10 ) events per second, and 1 GHz equals one billion (10 ) events per second. A
device that produces frequency is called an oscillator. The process of setting multiple oscillators to the
same frequency is called syntonization.
Of course, the three types of time and frequency information are closely related. As mentioned, the
standard unit of time interval is the second. By counting seconds, we can determine the date and the
time-of-day. And by counting events or cycles per second, we can measure frequency.
Time interval and frequency can now be measured with less uncertainty and more resolution than
any other physical quantity. Today, the best time and frequency standards can realize the SI second with
uncertainties of ≅ 1 × 10 – 15 . Physical realizations of the other base SI units have much larger uncertainties,
as shown in Table 17.1 [1–5].

Coordinated Universal Time (UTC)
The world’s major metrology laboratories routinely measure their time and frequency standards and
send the measurement data to the Bureau International des Poids et Measures (BIPM) in Sevres, France.
The BIPM averages data collected from more than 200 atomic time and frequency standards located at
more than 40 laboratories, including the National Institute of Standards and Technology (NIST). As a
result of this averaging, the BIPM generates two time scales, International Atomic Time (TAI), and
Coordinated Universal Time (UTC). These time scales realize the SI second as closely as possible.
UTC runs at the same frequency as TAI. However, it differs from TAI by an integral number of seconds.
This difference increases when leap seconds occur. When necessary, leap seconds are added to UTC on
either June 30 or December 31. The purpose of adding leap seconds is to keep atomic time (UTC) within
±0.9 s of an older time scale called UT1, which is based on the rotational rate of the earth. Leap seconds
have been added to UTC at a rate of slightly less than once per year, beginning in 1972 [3,5].
Keep in mind that the BIPM maintains TAI and UTC as ‘‘paper’’ time scales. The major metrology
laboratories use the published data from the BIPM to steer their clocks and oscillators and generate real-
time versions of UTC. Many of these laboratories distribute their versions of UTC via radio signals, which
are discussed in section 17.4.
You can think of UTC as the ultimate standard for time-of-day, time interval, and frequency. Clocks
synchronized to UTC display the same hour, minute, and second all over the world (and remain within
one second of UT1). Oscillators syntonized to UTC generate signals that serve as reference standards for
time interval and frequency.

17.2 Time and Frequency Measurement

Time and frequency measurements follow the conventions used in other areas of metrology. The fre-
quency standard or clock being measured is called the device under test (DUT ). A measurement compares
the DUT to a standard or reference. The standard should outperform the DUT by a specified ratio, called
the test uncertainty ratio (TUR). Ideally, the TUR should be 10:1 or higher. The higher the ratio, the less
averaging is required to get valid measurement results.

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