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In Reply to: RE: Rubidium clock not locking posted by eduardoo on January 05, 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: 01/07/15Follow Ups:
Hi, Abe!
I also have a feeling that the frequency reference itself is not locking, as I remembered that its lock indicator light would either dim or turn off when it's stable, but now it just stays lit. Someone on another forum say that this thing requires a slightly higher current starting up. I checked the output of the power supply, and it is outputting 14.8v and 0.62A, which seems to be about the constant power requirement of the module, and nowhere near its max requirement, which is about 2A, I think. Anyway,I'll likely take it to a repair shop to check it out.
Thanks.
I also have a feeling that the frequency reference itself is not locking,...Yes, that was my first point. You have to determine if the Rb clock is locking otherwise the 10MHz output signal is likely to wander (deliberately) as it tries to lock.
The OCXO oven should draw significantly more current at initial power ON while it's cool and then the current tapers off to a lower steady state level. If this is not happening, there's a chance that the oven heater is out. Let the Rb clock rest for a period of time so it is completely cool, then power it up and measure the current draw over time.
You really need a frequency counter and oscilloscope to trouble shoot. We had Tracor frequency difference meters to compare the output of a Rb clock against known reference.
You could poke around with a DMM to see if you have heater voltage. There are many other things to check including the Rb lamp. If you open the unit up, you should be able to see the light from Rb lamp but it might be buried within the physics module. If the lamp doesn't light a couple things could be wrong. There's no heat to create vapor from the Rubidium metal within the lamp. RF is used to excite an inert gas within the bulb creating a plasma to help heat the Rubidium. The RF might be out.
Most 'repair shops' have never seen an Rb oscillator so hopefully you'll find a very sharp tech.
Edits: 01/07/15
I sure hope that it is fixable on the cheap. Although I understand the skepticism on this type of device, I do hear a better sound using it. Who knows, maybe it's still placebo after all, but, like in medicine, if placebo makes the sick feels better, why not? :)
Found a guy who DIY's clock devices and audio equipment for sale. Hope he can deal with it.
Happy New Year -Abe,
do you use one of these device(s)?
Happy New Year!
No, I do not own an Rb oscillator but I used to work in an engineering lab at Efratom in Irvine CA with very sharp physicists developing Rubidium frequency standards and Hydrogen Maser frequency standards.
Rubidium oscillator modules are often an optional item when buying high-end test equipment. For example, GHz range frequency counters and signal generators are only as accurate as their timebase reference so some manufacturers offer Rb clocks as internal options and many also include an input BNC for running off an external reference.
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