【正文】 integral number of seconds. This difference increases when leap seconds occur. When 中北大學(xué) 2021 屆畢業(yè)設(shè)計說明書 第 3 頁 共 16 頁 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 177。 協(xié)調(diào)全世界時間 (UTC) 世界的主要度量學(xué)實(shí)驗(yàn)室測量時間和頻率標(biāo)準(zhǔn) ， 并發(fā)送的 BIPM， 法國 BIPM中北大學(xué) 2021 屆畢業(yè)設(shè)計說明書 第 12 頁 共 16 頁 收集至少 40個實(shí)驗(yàn)室的 200多個原子時間和頻率標(biāo)準(zhǔn)包括來自國際標(biāo)準(zhǔn)和技術(shù)協(xié)會 (NIST).通過這些平均結(jié)果由 BIPM 產(chǎn)生兩個時間標(biāo)準(zhǔn) ， 國際原子時間 (TAI)和協(xié)調(diào)全世界時間 (UTC)， 這些時間標(biāo)準(zhǔn)盡可能地與 SI 標(biāo)準(zhǔn) 接近 。 大家認(rèn)為 UTC 將作為最終時間 ， 時間間隔 ， 頻率標(biāo)準(zhǔn) ， 時間同步使 UTC 在全世界范圍顯示相同的時 ， 分 ， 秒 .振蕩器同步使 UTC 產(chǎn)生應(yīng)用于時間間隔和頻率參考標(biāo)準(zhǔn)的信號 . 時間和頻率測量 時間和頻率測量可以 應(yīng)用于度量學(xué)的其他領(lǐng)域 .頻率標(biāo)準(zhǔn)或時鐘測量叫做終端。皮 秒作為一個小量增加到 UTC 每年一次從 1972 年開始的 。 頻率是一個事件的重復(fù)次數(shù) ， 如果 T 一個重復(fù)事件的周期 ， 那么頻率 f 是它的倒數(shù) ， 1/頻率的倒數(shù)是周期 ， T=1/ .很容易看出頻率和時間間隔關(guān)系很密切 .頻率的標(biāo)準(zhǔn)單位是赫茲 ， 定義為每秒發(fā)生的事件次數(shù)或循環(huán)次數(shù) ， 電信號的頻率通常用不同的赫茲測量 ， 包括千赫茲 ， 兆赫茲 ， 千兆赫茲 .1KHz 相當(dāng)于 每秒發(fā)生一千次事件 ， 1MHz 相當(dāng)于每秒發(fā)生一百萬次事件 .1GHz 相當(dāng)于每秒發(fā)生十億次事件 .產(chǎn)生頻率的裝置叫做振蕩器 ， 設(shè)置不同振蕩器具有相同的頻率叫做同步 。 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. 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 realtime versions of UTC. Many of these laboratories distribute their versions of UTC via radio signals which section are discussed in. You can think of UTC as the ultimate standard for timeofday， 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 simonized to UTC generate signals that serve as reference standards for time interval and frequency. Time and Frequency Measurement Time and frequency measurements follow the conventions used in other areas of metrology. The frequency standard or clock being measured is called the device under test (DUT). A measurement pares 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. The test signal for time measurements is usually a pulse that occurs once per second (1 ps). The pulse width and polarity varies from device to device， but TTL levels are monly used. The test signal for frequency measurements is usually at a frequency of 1 MHz or higher， with 5 or 10 MHz being mon. Frequency signals 中北大學(xué) 2021 屆畢業(yè)設(shè)計說明書 第 4 頁 共 16 頁 are usually sine waves， but can also be pulses or square waves if the frequency signal is an oscillating sine wave. This signal produces one cycle (360∞ or 2π radians of phase) in one period. The signal amplitude is expressed in volts， and must be patible with the measuring instrument. If the amplitude is too small， it might not be able to drive the measuring instrument. If the amplitude is too large， the signal must be attenuated to prevent overdriving the measuring instrument. This section examines the two main specifications of time and frequency measurements—accuracy and stability. It also discusses some instruments used to measure time and frequency. Accuracy Accuracy is the degree of conformity of a measured or calculated value to its definition. Accuracy is related to the offset from an ideal value. For example， time offset is the difference between a measured ontime pulse and an ideal ontime pulse that coincides exactly with UTC. Frequency offset is the difference between a measured frequency and an ideal frequency with zero uncertainty. This ideal frequency is called the nominal frequency. Time offset is usually measured with a time interval counter (TIC). A TIC has inputs for two signals. One signal starts the counter and the other signal stops it. The time interval between the start and stop signals is measured by counting cycles from the time base oscillator. The resolution of a low cost TIC is limited to the period of its time base. For example， a TIC with a 10MHz time base oscillator would have a resolution of 100 ns. More elaborate Tics use