Page 347 - The Mechatronics Handbook

17

Fundamentals of Time

and Frequency

17.1 Introduction
Coordinated Universal Time (UTC)
17.2 Time and Frequency Measurement
Accuracy • Stability
17.3 Time and Frequency Standards
Quartz Oscillators • Rubidium Oscillators
• Cesium Oscillators
17.4 Time and Frequency Transfer
Michael A. Lombardi Fundamentals of Time and Frequency Transfer
• Radio Time and Frequency Transfer Signals
National Institute of Standards
and Technology 17.5 Closing
17.1 Introduction

Time and frequency standards supply three basic types of information: time-of-day, time interval, and
frequency. Time-of-day information is provided in hours, minutes, and seconds, but often also includes
the date (month, day, and year). A device that displays or records time-of-day information is called a
clock. If a clock is used to label when an event happened, this label is sometimes called a time tag or time
stamp. Date and time-of-day can also be used to ensure that events are synchronized, or happen at the
same time.
Time interval is the duration or elapsed time between two events. The standard unit of time interval
is the second(s). However, many engineering applications require the measurement of shorter time
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intervals, such as milliseconds (1 ms = 10 s), microseconds (1 µs = 10 s), nanoseconds (1 ns = 10 s),
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and picoseconds (1 ps = 10 s). Time is one of the seven base physical quantities, and the second is one
of seven base units defined in the International System of Units (SI). The definitions of many other
physical quantities rely upon the definition of the second. The second was once defined based on the
earth’s rotational rate or as a fraction of the tropical year. That changed in 1967 when the era of atomic
time keeping formally began. The current definition of the SI second is:
The duration of 9,192,631,770 periods of the radiation corresponding to the transition between two
hyperfine levels of the ground state of the cesium-133 atom.
Frequency is the rate of a repetitive event. If T is the period of a repetitive event, then the frequency
f is its reciprocal, 1/T. Conversely, the period is the reciprocal of the frequency, T = 1/f. Since the period
is a time interval expressed in seconds (s), it is easy to see the close relationship between time interval
and frequency. The standard unit for frequency is the hertz (Hz), defined as events or cycles per second.
The frequency of electrical signals is often measured in multiples of hertz, including kilohertz (kHz),
3
megahertz (MHz), or gigahertz (GHz), where 1 kHz equals one thousand (10 ) events per second, 1 MHz

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