[time-nuts] Re: Using aliasing of reference clock to PPS to determine phase offset.

[Hal Murray]

> counter running at 10Mhz
> would have 10e6 counts on each capture
> around 1uS based

Looks like you dropped/added a 0 in there.  My guess would be you started 
at 1 MHz and missed a few edits with you changed to 10 MHz.


> i.e if I discipline the reference clock to be 10.0000005Mhz (10Mhz plus
> 0.5Hz) then the count of each clock should alternate 
> 10,000,000
> 10,000,001  

I think that approach is a wild goose chase.
Basically, what you are doing is counting over a longer period of time.
Looking for a pattern of 0 0 0 0 1 in the bottom bit rather than 0 0 0 0 0 
just moves the target frequency up a bit.

What you want is the Allan Intercept.  You want to average as long as you can but not so long that you can't follow things like temperature changes.

Note that the PPS signal is not synchronous with your clock.  If the PPS happens to be in phase with your clock there is a race condition.  You might get 9,999,999 followed by 10,000,001 or the reverse.


If I had the hardware for that sort of setup, I would write some test code that drove the DAC by hand and printed the counts between PPS pulses.  The idea is to figure out how big a step the bottom bit of the DAC makes and how stable things are in your lab.


-- 
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