[time-nuts] GPSDO with all-digital phase/time measurement?

[Mark Haun]

Thanks to everyone who responded.  I had this bright idea that I would turn
off mail delivery and just read the list through the pipermail interface. 
It works great, until you want to reply to a message.  Sorry if this breaks
the thread in two.

Language is imprecise even when carefully thought out, and I can see I gave
some erroneous impressions.  What I am really trying to get at, is whether
there is any advantage to using an outboard phase detector like the
4046-based circuit recently posted in the "arduino GPSDO" thread.  I get
that this is going to provide a much faster measurement of phase error than
the digital counter method.  (The RC time constant on that circuit looked
pretty fast, ~ 1 second.)  But if my loop filter has a time constant
measured in the 10s or 100s of seconds, I'm not clear what difference this
makes.  That was the gist of my "100ns/T" thought experiment.

Ditto on the comment re: long averaging times != infrequent tuning updates. 

To clarify my "requirements" a bit more, I am mostly interested in the
learning exercise, but it would be nice to end up with a frequency standard
capable of supporting symbol periods (coherent) on the order of seconds for
VLF and shortwave digital comms experiments.  So, a pretty modest goal of,
say, 1E-9.  Of course, being an engineer I want to do the best job possible
with the parts at hand.

Toolkit: I have a handful of the 26-MHz Pletronics ebay OCXOs.  The spec
sheet says +/- 0.5 ppb over 30 seconds short-term stability.  I will try to
use a GPS module enabling SW sawtooth correction, an NV08C if I could get
it.  MCU of choice is STM32 (ARM Cortex-M4).  I am still reading the fine
print but I believe interrupt handling is strictly deterministic. 

So, based on the OCXO short-term spec and a 1E-9 performance "requirement,"
could I not then estimate the loop time constant at ~100 seconds, and
furthermore argue that my TIC has an effective measurement accuracy of 
1/(26 MHz)/(100 s) = 0.4 ns ?  Meaning, also, that SW sawtooth correction
would be worthwhile?

Mark
KJ6PC