[time-nuts] Continuous timestamping reciprocal counter question

[Tijd Dingen]

Magnus Danielson wrote:
> There are many things you can get away with, just how much trouble you 
> want to verify it versus doing the proper thing is another issue.

Define "proper thing". ;-) From what I understand taking the exact Nth edge,
and then do linear regression is equivalent to taking roughly every Nth
edge and then do linear regression.

Equivalent in the sense that the frequency estimates of the two will be
the same, to within the usual numerical uncertainties. Or to put that another way:
The first method of doing things is not inherently better or worse than the second
method. After all, that is the whole thing I am trying to be sure of right now.

Of course I can make sure that I take exactly every Nth edge. It is just that there
are some considerable implementation advantages if that constraint does not have
to be so strict.

One advantage being that if this constraint can be fairly loose, then using the
ISERDES2 in the spartan-6 as part of the coarse counter is fairly simple. I did
a couple of test with that, and all looks good. The main advantage there being
that if I use the serdes, this translates into a higher input frequency without the
need for a prescaler. Which translates into better precision.

Hence my current (over)focus to make absolutely sure that all the results are also
valid if one takes almost the Nth edge, but not quite right all the time... However, you
still know which edge is which. You just don't know it early enough in the pipeline
to use as basis for a triggering decision.


> I have not looked on detail performance comparison between these 
> algorithms lately. However, they should not be used naively together 
> with AVAR and friends since they attempt to do the same thing, so the 
> resulting filtering will become wrong and biased results will be produced.

Well, for the AVAR calculation I only use the raw time-stamps. So nothing
preprocessed. Then I should not have to worry about this sort of bias, right?

regards,
Fred