This is a frequency standard, suitable for use in metrology, communications, and many other applications in engineering or science.Ī cesium atomic clock needs a few other parts. After the small remaining biases are measured and eliminated, the output frequency is a very accurate 10000 000 Hz, accurate to about 5 parts in one hundred thousand billion when averaged over a day. The quartz oscillator is adjusted automatically by the servo control to maximize the number of cesium ions collected, keeping the microwaves in step with the spinning of the cesium atoms. The other atoms are absorbed by another carbon getter. The B magnet deflects the in-step atoms towards a detector, the hot wire cesium ionizer and ion collector. The B magnet collects the cesium atoms that stayed in step with the microwaves, and which now have their magnetization pointing the other way (the cesium-133 atoms in the f=4 level). The spinning is stopped by the microwaves at the other end of the Ramsey cavity. (Quantum mechanics describe these cesium-133 atoms as an oscillating combination of the two hyperfine levels, f=4 and f=3.) Magnetic shielding isolates the atoms from outside magnetic fields. Allowing for tiny corrections, their magnetization spins at 9 192 631 770 rotations per second in a very uniform magnetic field, the C field of less than 1/10 the Earth's magnetic field. Some atoms have their magnets set spinning by microwaves in the Ramsey cavity. The A magnet selects cesium atoms with their atomic magnets pointing one way (those in the f=3 level of the ground state of the cesium-133 atom), and sends other atoms to be absorbed by a carbon getter. How does it all work?Ĭesium is evaporated at the cesium source to form a beam of well-separated cesium atoms that travel without collisions at about 250 m/s, through a vacuum maintained by the vacuum pump. The clock is located in a copper room to isolate it from radio interference. Cesium atoms are emitted at one end of the tube and pass through two microwave cavities (the copper waveguide which feeds the microwaves to these cavities can be seen above the tube) before they are analyzed and detected at the other end. The large aluminium tube is a vacuum vessel which contains the heart of the clock. Officers in the Frequency and Time group, make adjustments to one of the NRC-built cesium atomic clocks. Jean-Simon Boulanger and Rob Douglas, Research Ultimately, we show how to use this corrections-ADEV as a diagnostic to help identify the source of disturbances and drift observed on the clock output.Drs. In this article: we 1) derive the baseline shot-noise-limited noise floor for this ADEV, 2) validate and adjust for the complexities of our control loop with a computer model, and 3) examine model results and laboratory data that lie on or diverge from the noise floor to understand what divergences reveal about LO and/or clock behavior. However, the ADEV of the corrections reveals somewhat different information, specifically more direct information about all disturbances that the measurement system detects and compensates for, from the LO or elsewhere. Some of this can be diagnosed using the output frequency's Allan deviation (ADEV), the traditional measure of clock performance. To optimize clock performance, it is important to determine whether disturbances on the output are due to variations of the LO that the control loop failed to remove or variations of the reference frequency itself. This article focuses on the shot-to-shot corrections themselves. Fortunately, this slow drift is mitigated by repeatedly comparing the atomic reference frequency to the LO and applying corrections each iteration through a control algorithm. Most microwave atomic clocks in operation today use quartz crystal LOs with excellent short-term noise variation but large unwanted long-term drift. As atomic clocks and frequency standards are increasingly operated in situations where they are exposed to environmental disturbances, it becomes more necessary to understand how variations of each clock component impact the clock output, in particular the local oscillator (LO).
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