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Comment by generuso | Hacker News Reader

There are different types of atomic clocks, but in most common types, the output comes from a crystal oscillator, or more generally frequency synthesizer, which is then slaved to some spectral feature in the "physics package". That is to say, the phase noise is as good as that of the crystal in the unit, but the longer term frequency stability is much improved by the slaving.

An exception is an active hydrogen maser, which directly outputs the frequency of atomic transition. It has very good phase noise, but is a rare beast, which is only used where it is absolutely necessary.

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How do you discipline a crystal without introducing significant phase noise?

You can pull the frequency of a crystal resonator circuit by changing the loading capacitance, for example. You may use a varactor or any semiconductor junction. Doing so doesn't really affect phase noise.

I'm probably missing the right terminology. I would have thought the feedback loop creates jitter of its own. Phase locked VCOs are generally noisier than a crystal alone. But maybe I'm overthinking it and the loop bandwidth can be made very narrow with trimmed crystal and long integration times or whatever.

Your own answer is right. For atomic clocks, the integration times of the feedback loop are typically much longer than a second.

The contribution of the crystal to the overall tank circuit Q is vastly higher, by several orders of magnitude, than the contribution of the varactor diode.

So while making an oscillator steerable inevitably increases its phase noise slightly, the increase is either ignorable in practice or cancelled by the control loop itself.

A very good 10 MHz ovenized crystal oscillator, Hewlett Packard 10811D, ages by up to a few Hertz per year, and has a mechanical capacitor for trimming the frequency by up to 10 Hz, and an electronic frequency adjustment by 1 Hz using a varactor. I am sure the varactor does not improve jitter, but considering that the full range of adjustment is 0.1 ppm, it also should not add that much jitter, assuming everything is well designed.

So we are talking about a reasonably high stability crystal to begin with, and a very narrow adjustment range. In an atomic clock, the feedback loop uses the electronic frequency adjustment to more or less completely remove the aging. This requires a very tiny and a rather slow acting feedback.

In the atomic clock, the output of the crystal is used as a reference for a microwave sweep generator, which then scans the spectrum of atomic transitions. The absorption peak in a cesium clock is something like a kiloherz wide, but with a good signal to noise ratio and with a lot of averaging, one can measure the position of the peak to a very tiny fraction of its width. Comparing the measured and the expected positions reveals the deviation of the reference frequency from its design value, and that is what generates the tuning feedback for the crystal. I do not know off the top of my head how slow the feedback is exactly -- it is something that one could look up in the Hewlett-Packard service manual, but I am sure it is slow enough to be irrelevant for the cycle-per-cycle jitter.

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