【正文】 gets longer， since some noise types can be removed by averaging. At some point， however， more averaging no longer improves the results. This point is called the noise floor， or the point where the remaining noise consists of no stationary processes such as flicker noise or random walk. The device measured in Fig. has a noise floor of ~5 10 11 at τ = 100 s. Practically speaking， a frequency stability graph also tells us how long we need to average to get rid of the noise contributed by the reference and the measurement system. The noise floor provides some indication of the amount of averaging required to obtain a TUR high enough to show us the true frequency where xi is a set of phase measurements in time units containing x1， x2， x3， and so on is the number of values in the xi series， and the data are equally spaced in segments τ seconds long. Note that while standard deviation subtracts the mean from each measurement before squaring their summation， the Allan deviation subtracts the previous data point. This differencing of successive data points removes the time dependent noise contributed by the frequency offset. An Allan deviation graph is shown in Fig. . It shows the stability of the device improving as the averaging period (τ) gets longer， since some noise types can be removed by averaging. At some point， however， more averaging no longer improves the results. This point is called the noise floor or the point where the remaining noise consists of no stationary processes such as flicker noise or random walk. The device measured in Fig. has a noise floor of ~5 10 11 at τ = 100 s. Practically speaking， a frequency stability graph also tells us how long we need 中北大學 2021 屆畢業(yè)設計說明書 第 10 頁 共 16 頁 to average to get rid of the noise contributed by the reference and the measurement system. The noise floor provides some indication of the amount of averaging required to obtain a TUR high enough to show us the true frequency offset of the DUT. If the DUT is an atomic oscillator (section ) and the reference is a radio controlled transfer standard (section ) we might have to average for 24 h or longer to have confidence in the measurement result. Five noise types are monly discussed in the time and frequency literature： white phase， flicker phase， white frequency， flicker frequency， and random walk frequency. The slope of the Allan deviation line can help identify the amount of averaging needed to remove these noise types (Fig. ). The first type of noise to be removed by averaging is phase noise， or the rapid， random fluctuations in the phase of the signal. Ideally， only the device under test would contribute phase noise to the measurement， but in practice， some phase noise from the measurement system and reference needs to be removed through averaging. Note that the Allan deviation does not distinguish between white phase noise and flicker phase noise. Table shows several other statistics used to estimate stability and identify noise types for various applications. Identifying and eliminating sources of oscillator noise can be a plex subject， but plotting the first order differences of a set of time domain measurements can provide a basic understanding of how noise is removed by averaging. Figure was made using a segment of the data from the stability graph in Fig. . It shows phase plots dominated by white phase noise (1 s averaging) ， white frequency noise (64 s averages) ， flicker frequency noise (256 s averages)， and random walk frequency (1024 s averages). Note that the white phase noise plot has a 2 ns scale， and the other plots use a 100 ps scale. 中北大學 2021 屆畢業(yè)設計說明書 第 11 頁 共 16 頁 外文 文獻中文翻譯 時間 和頻率的基本原理 介紹 時間和頻率標準應用于三種基本信息類型 ： 時間 ， 時間間隔和頻率 .時間信息有小時 ， 分 ， 秒 .通常還包括日期 (年 ， 月 ， 日 ).用來顯示和記錄時間的器件叫做鐘表 ， 如果鐘表標記了一件事的發(fā)生 ， 那么這個標記叫做時間標簽或時間印記 .日期和時間能確保事情的同步或同時發(fā)生 . 時間間隔是兩個事件持續(xù)或斷續(xù)的時間 ， 時間間隔的標準單位是秒 ， 然而許多工程上應用要求更短的時間間隔 ， 像毫秒 ， 微秒 ， 納秒 ， 和皮秒 ， 時間是七個基本物理量之一 ， 并且秒是國際單位體制制定七個基本單位之一 .許多區(qū)其他物理量的定義是依靠秒而定義的 .秒曾經(jīng)定義根據(jù)地球回轉(zhuǎn)率 .原子時代正式開始在1967 年目前 SI 定義秒為 ： 秒是銫 133 原子 (Cs133)基態(tài)的兩個超精細能級之間躍遷所對應的輻射的 9， 192， 631， 770 個周期所持續(xù)的時間 。 UTC 與 TAI 執(zhí)行相同的頻率 ， 然而它有區(qū)別的 TAI 是整數(shù)秒 ， 這個不同處總在增長隨著皮 秒的跳變 .皮 秒增加到 UTC 的每年 6 月 30 日或 12 月 31 日 .增加跑秒的目的在于是原子時間的誤差在 +/ 老的時間標準叫 UT1， 它根據(jù)地球的回轉(zhuǎn)率