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RE: Rubidium clock not locking

Posted by AbeCollins (A) on January 7, 2015 at 12:18:39

In Reply to: Rubidium clock not locking posted by eduardoo on January 5, 2015 at 18:40:12:

First of all you need to determine if the Rubidium oscillator itself is in lock, independent of the device it is attached to.

There is a Rubidium lamp that is normally heated and excited by RF. The gas mixture in the lamp will determine it's color when excited. That lamp shines through a glass Rubidium cell which is also heated. Rubidium is a soft metal and it needs to be heated to create a vapor. A photo detector detects the atomic hyperfine transition state of the Rubidium atom (~6.8GHz) as the light shines through the glass Rubidium cell, and through a PLL circuit it 'steers' the output of an ovenized voltage controlled crystal oscillator (using a varactor or similar diode).

If the crystal oscillator is not in 'lock' with the Rubidium circuit, the PLL will cause the crystal oscillator to 'hunt'. This literally means that the crystal oscillator will deliberately swing it's frequency trying gain 'lock' with the Rubidium electronics.

So the first thing to do is to determine if the Rubidium oscillator box can achieve lock. It doesn't matter what is attached to the output of the Rubidium oscillator. If it cannot achieve internal lock, it will not work properly as a frequency reference for your DAC.

At least this is how it was done back in the 1980's when I worked for Efratom, one of the pioneers of Rubidium oscillators. You'll find Rubidium oscillators used in GPS satellites and other communications gear where timing accuracy is critical.

I can't say that a Rubidium oscillator is of any use for an audio DAC. Besides, the crystal oscillator itself has more influence on phase noise and short term accuracy. But it sure sounds cool to have a DAC that is sync'd to an atomic clock! ;-)

Edits: 01/07/15

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