仪表与量测PPT课件-Chapter 5 Time and Frequency Measurement.ppt

Chapter 5Time and Frequency Measurement P. Pataki Introduction Frequency is by definition the number of occurences of a given periodic phenomenon in a given time. Time has two meanings in practice: the position of an event on a conventional time axis. astronomy, space research, navigation, etc, a redio signal emitted from a very accurate frequency standard (the Atomatic Clock) synchronized to a well defined starting point (1958.1.1 00:00:00) the time elapsed between two events (time-length or time-interval) Topics: time, frequency, and phase. Frequency Standards - Accuracy Frequency accuracy of 10-13 can be taken as a theoretical limit in the Earth-fixed coordinate system. The relativistic effects and the change of the gravitational potential cause a time shift in this range. In space, it is 10-10 (Reinhard 1974, Ramsey 1987, ...) Frequency Standards - Accuracy In early 60, different mathematical descriptions were usedto characterize the accuracy of high-stability. 1964, NASA and IEEE has a conference on the standardization - (1) fractional frequency offset (2) phase-time Frequency Sources - Atomic Frequency Standard The most precise way to produce a stable frequency generator is the atomic clock. It is based on the principle of quantum mechanics where frequenc is determined by an atomic resonator. The frequency of the microwave radiation is proportional to the energy difference between the discrete p and q energy level. h=6.63x10-34 J is Plancks constant. The atomic clock elements are used from the first column of Mendelejevs periodical system (Alkali metals) The material have only one valence electron on their outer energy level. The rediation frequency depends on the energy difference between two outer levels. (Hayatt 1973; Kartaschoff 1978, Ramsey 1991). Frequency Sources - Quartz Crystal Oscillator The most generally used frequency standard of practical measurement techniques. The piezoelectric effect of the quatz crystal makes it easy to excite mecha