[time-nuts] question Alan deviation measured with Timelab and counters
magnus at rubidium.se
Sun Jan 18 17:10:36 EST 2015
On 01/18/2015 10:34 PM, Stéphane Rey wrote:
> Thanks a lot Bob and Magnus for your very helpful comments.
> The HP5370a was indeed in TI mode. By the way what is the difference with +/-TI, the button just aside...
> But I guess I understand where I've missed something : I've tried to put the Rb on channel A and the DUT on channel B but result was always the same but I do understand now that there is indeed a switch to change from COMmon to SEParate and it was always on COM meaning I believe that channel B wasn't used. This explains a lot of things I did not understand. I'm sorry for these so basic issues that might have been solved if I had read carefully the HP5370a manual first.
Good. This confirmation makes sense to be and Bob, now we can relax as
the mystery is solved.
> So possible conclusions until now are that I have actually measured the ADEV floor of the system rather than my DUT... which is already nice. The second conclusion from these oscillations seen with the GPSDO under test is that there is very likely in this GPSDO design a systemic noise added to the 10 MHz output (power supply, PCB coupling, ... I'll make further investigations on it later on).
It's a great opportunity to learn the tools, and once you have the
tools, you can see if you can't improve things.
> I will experiment all the suggestions you made and will come back. For information the 1PPS from the HP58503b has a positive pulse width that is only few us length.
This only makes it hard to view on a scope, but long enough to reliably
trigger your counter and scope.
> Now, when considering that the method is to compare the DUT to an other source, I assume then that the other source shall be at least 1 order of magnitude better than the DUT. Otherwise this will be impossible to distinguish who is the instability contributor between the source and DUT, right ?
For a simple setup, yes. But then we are the time-nuts, we have ways of
handling these things. :) Let's get you started with the basic
measurement, it will be a good start.
> Then the second question is what kind of very stable source can be used to measure DUT which could be Rb or GPSDO which are already in the range of 10E-10 to 10E-12 < 100s ?
Time-nuts tend to spend their time and money getting even more stable
clocks and tools. If you have the right tool, you can measure near and
*under* the noise-level of your reference, but not without running into
issues. One such trick is called cross-correlation, while another is to
use three-corner hat techniques.
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