[time-nuts] Allan deviation -> finally some data!!!

Didier Juges didier at cox.net
Thu Oct 26 01:43:24 UTC 2006


Tom Van Baak wrote:
>> Since I have only one readily accessible spare OCXO at the moment and I 
>> want to get the procedure right before I take anything else apart, I 
>> will feed the output of the 10 MHz reference from the 5370 into the 
>> START channel, and my DUT into the STOP channel and select SEP instead 
>> of START COM.
>>     
>
> If you have only one oscillator, yes, this is what you
> have to do. But as soon as you have two, you can
> leave the 5370's own oscillator out of the picture and
> compare the two external oscillators using the 5370
> as a simple TIC.
>
>   
That will be in phase 2 for sure :-)
>> I believe that once I have the setup correct, I will rapidly find that I 
>> need dividers on both OCXOs... 10 MHz will be too fast and I will be 
>> skipping periods rapidly.
>>     
>
> Dividers are nice, but often not really necessary.
>
> Even if the two 10 MHz OCXO differ in frequency by
> as much as 1e-9 that's only 1 ns per second of drift.
> That gives you about 100 clean samples before a
> cycle wrap.
>
> Two solutions. 1) simply adjust them closer than 1e-9;
> this should be easy, or 2) take care of cycle wrap in
> software. I have code or tools to do this if you need.
> It is the most reliable solution -- because you don't
> have to touch, adjust, or lock either of the OCXO.
>
>   
Solution 2 would be good because it would be pretty universal, and even 
work with oscillators that are not easily adjusted. However, attempts to 
adjust the OCXO on my 3586 did not give me a warm fuzzy feeling that 
getting to 1e-9 would be easy...

I assume the code looks for a sudden phase jump and adds the modulo 
(period of the START signal)
>> The easiest from there will probably be to hook up the GPS into the 
>> START channel, and a divided down 10 kHz or 1 kHz on the STOP channel.
>>     
>
> Yes. But don't wait for a divider. The other advantage
> of processing cycle wrap in software is that you can
> collect clean, long-term, OCXO data with just this:
>     5370 function TI, N=1
>     START: GPS 1PPS (0.5 VDC, 50R)
>     STOP: OCXO 10 MHz (0.0 VAC, 50R)
>
>   
I would like to try that tonight, if I can find the GPS antenna...
>> Maybe I can close the loop using the PC, driving the EFC input of the 
>> OCXO from the serial port. That would allow me to conveniently adjust 
>> the loop parameters.
>>     
>
> Don't close the loop. It will surely introduce humps
> in the ADEV plots. Closing a loop means you are
> measuring the performance of a disciplined oscillator
> instead of a stand-alone oscillator.
>
> /tvb
>
>   
Well, the idea was precisely to adjust the loop parameters in software 
on the PC, which would probably be easier than in hardware, with the 
time constants involved, then once those are established, implement them 
in a uC or in analog hardware.


Didier



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