[time-nuts] LTE-Lite module

Charles Steinmetz csteinmetz at yandex.com
Mon Oct 20 05:51:32 EDT 2014


Poul-Henning wrote:

>PLLs are really not that hard  [context: we have been discussing 
>all-digital PLLs ("ADPLLs")]

Yes, I know -- I have designed more than a few.  I have also reviewed 
more than a dozen hobbyist designs and modeled some of them, and 
found that few hobbyists seem to have mastered the art.  Judging also 
by on-list responses over the years, it does not appear that many 
time nuts are interested in designing and building their own 
ADPLLs.  So, I conclude that disciplining a good OCXO with GPS and 
getting the best stability the OCXO can deliver is not practicable 
for most hobbyists.

The OP in this sub-thread indicated that he was considering using an 
LTE-Lite to discipline a "really good" 10811, and it appeared that 
his expectation was to end up with a GPSDO more or less as good as 
his 10811.  My point was simply to put realistic bounds on the expectation.

Said posted that a quick lash-up with an OCXO produced stability 
about 10x better than with the on-board TCXO.  That is a useful 
improvement, but a good OCXO (certainly, a "really good" 10811) will 
have stability about 3 orders of magnitude better than a TCXO 
(1000x), so two decades of possible improvement were not realized.

Said's experiment was a proof-of-concept exercise and not a careful 
optimization, so it is almost certain one could do better than 10x 
with some further work.  But I very much doubt that optimization can 
gain the entire two decades of potential improvement (short of 
designing a full ADPLL, in which case you don't need the LTE-Lite at 
all -- all you need is a source of PPS), and I doubt it is possible 
to gain even one whole decade.

So, I am inclined to think that there are better (and easier) ways to 
discipline a 10811 to reach its ful potential, that's all.

Best regards,

Charles






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