【正文】 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 中北大學(xué) 2021 屆畢業(yè)設(shè)計(jì)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。 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(jì)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 interpolation schemes to detect parts of a time base cycle and have much higher resolution—1 ns resolution is monplace， and 20 ps resolution is available. Frequency offset can be measured in either the frequency domain or time domain. A simple frequency domain measurement involves