Electronics > Metrology

Two GPSDO to get more accuracy?

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FriedMule:
I had one GPSDO and I imagine I am not the first one to get one more? :-)

To no surprise for me do they not agree upon what 10MHz exactly is, but still does this give the question: "which one is most accurate?"
I remember having read an article translated from Russian, about combining two GPSDO in some way to let them help each other to gain 2-3 orders of magnitude better accuracy, but I have not been able to find it again.

Is there any truth to this? What is this configuration called?

Thanks in advantage. :-)

trobbins:
I'd recommend wading through the many GPSDO threads on this forum and extracting out pertinent posts that discuss GPSDO accuracy to get a better awareness of how the accuracy of one GPSDO can be assessed, especially those GPSDO that are aiming for much better than 1ppb, and how the various parts of a GPSDO can influence accuracy.

I'd suggest that there have been quite a few methods used in GPSDO's, and each has its benefits/limitations, and beyond that then the GPS receiver can also be a significant influence - especially for attainment of high accuracy.  So your query may well be too open-ended, as you haven't identified the GPSDO you use.

tkamiya:
I have more than half a dozen GPSDO in my lab.  If I average each for 24 hours, they are accurate and precise, but instantaneous reading is all over the place.  With two, you can potentially average.  With three or more, you can do N-cornered hat.  But verifying each method is actually working in home lab is entirely different matter.  You'll need a reference guaranteed to be accurate and precise than DUT.  Still, I think about putting dozen of them and do some kind of scheme, but I doubt there is any practical value in such attempt.

FriedMule:
Long time is fairly precise, down to a few nanoseconds, but my biggest problem is when I am trying to measure a crystal and do not have days or weeks to measure.
How can I let say be more sure a crystal is accurate in 1-2 hours of measurement time?

Would a combination with another type of oscillator crystal, I mean to remember that this fellow had combined 3-4 GPSDO's to a cesium but said 2 would be fine in most cases. I think he measured down to 13 digits.

Is it right that he used two long term precise crystals and connected them to a short term accurate crystal?

edpalmer42:

--- Quote from: FriedMule on June 15, 2022, 05:17:49 pm ---Long time is fairly precise, down to a few nanoseconds, but my biggest problem is when I am trying to measure a crystal and do not have days or weeks to measure.
How can I let say be more sure a crystal is accurate in 1-2 hours of measurement time?
--- End quote ---

Put some numbers on this.  What level of accuracy do you require?  Is 'accuracy' the correct word to define your requirements?


--- Quote ---Would a combination with another type of oscillator crystal, I mean to remember that this fellow had combined 3-4 GPSDO's to a cesium but said 2 would be fine in most cases. I think he measured down to 13 digits.

Is it right that he used two long term precise crystals and connected them to a short term accurate crystal?

--- End quote ---

Do you remember where you saw this?  We need more info to comment intelligently.

The only reason to use GPS is if you need to confirm absolute frequency accuracy over a long period.  As you've found, GPS isn't particularly good for short-term stability.  A really good quartz oscillator is better for short-term stability, but finding one is not a trivial task.  In fact, a good Rb is probably better than most GPSDOs for short-term stability.

But expecting to make a high resolution measurement of a crystal (?) in an hour or two is almost doomed to failure.  If you meant oscillator instead of crystal, you'll need to start with an oscillator that's worthy of consideration and then age it for a month or three to get the aging down to something approaching its long-term potential.  If you actually meant a bare crystal, it's even harder.  The performance of a bare crystal is heavily influenced by the oscillator and heater circuits that it's connected to.  You can't really measure the crystal in isolation.  And then you have to age it for a month or three before you can measure it.

Ed

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