毕业设计精品]脉冲宽度测量仪设计 外文及翻译.doc

毕业设计说明书
英文文献及中文翻译
学生姓名: 学号:
学院: 信息与通信工程
专业: 电子信息科学与技术
指导教师:
2011年 6月
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 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 intervals, such as milliseconds (1 ms = 10 -3 s) , microseconds (1 μs = 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 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 freque