[time-nuts] LTE-Lite module

Bob Camp kb8tq at n1k.org
Mon Oct 20 07:48:28 EDT 2014


Hi

We tend to focus on this or that enhanced feature in a piece of code. It’s fun to talk about. That’s not what keeps most designs from doing what they should. By focusing on this rather than the testing required, we set people up to fail. If you start off the project believing you mostly need fancy code when you mostly need long term testing instead, you hit a wall pretty fast. Setting up for one is not at all the same as setting up for the other. 

Bob

> On Oct 20, 2014, at 5:51 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> 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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