[time-nuts] Synchronisizing a 100MHz TCXO with Tbold, pse help

Christian Vogel vogelchr at vogel.cx
Mon Sep 19 18:52:41 UTC 2011

Hi Tom,

Christian Vogel <vogelchr at vogel.cx>:

>> but of course: Whatever algorithms you put into your after-the-fact
>> software correction will have an influence similar to a hardware PLL.

Tom Van Baak <tvb at leapsecond.com>:

> I'm curious about this. What are the inherent limits of each
> approach? One difference comes to mind -- with a software
> post-processing solution you get the benefit of being able to
> see samples in both the past and future. A hardware PLL
> works in real-time with data from the past.

I have to admit that my comment was backed only by my "gut feeling"
about how I would implement such a post-processing, which
resembles a numerical PLL.

But your question about a non-causal correction/filter/... is a very
interesting one. I think it would be worthwhile to try and find it out
experimentally: Setup a thunderbolt that receives the GPS signal
but is undisciplined. Log both the internally measured PPS offset and
the measured PPS versus a known good reference. Try to find the algorithm
that gives the best estimate of the externally measured PPS offset
 from the internally measured (serial port logged) thunderbolt data.



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