1、 毕业设计 说明书 英文文献及中文翻译 学 院： 信息与通信工程 专 业： 电子信息科学与技术 2011 年 6 月 第 1 页 共 16 页 外文文献原文 Fundamentals of Time and Frequency 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
2、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
3、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 intervals， such as milliseconds (1 ms = 10 -
4、3 s) ， microseconds (1 s = 10 -6 s) ， nanoseconds (1 ns = 10 -9 s) ， and picoseconds (1 ps = 10 -12 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 re
5、ly upon the definition of the second. The second was once defined based on the earths 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
6、 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
7、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 第 2 页 共 16 页 standard unit for frequency is the hertz (Hz) ， defined as events or cycles per second. The frequency of electrical signal
8、s is often measured in multiples of hertz， including kilohertz (kHz)， megahertz (MHz)， or gigahertz (GHz)， where 1 kHz equals one thousand (103) events per second， 1 MHz equals one million (106) events per second， and 1 GHz equals one billion (109) events per second. A device that produces frequency
9、 is called an oscillator. The process of setting multiple oscillators to the same frequency is called synchronization. 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 determi
10、ne 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 w
11、ith uncertainties of 1 10-15.Physical realizations of the other base SI units have much larger uncertainties. Coordinated Universal Time (UTC) The worlds major metrology laboratories routinely measure their time and frequency standards and send the measurement data to the Bureau International des Po
12、ids 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
