[time-nuts] ✘NEO-M8N vs. NEO-M8T

Bob kb8tq kb8tq at n1k.org
Mon May 21 14:48:16 EDT 2018


> On May 21, 2018, at 2:08 PM, Gary E. Miller <gem at rellim.com> wrote:
> Yo Bob!
> On Mon, 21 May 2018 14:00:41 -0400
> Bob kb8tq <kb8tq at n1k.org> wrote:
>>>> Ok, are you trying to hold close to UTC or simply have a second
>>>> that is as close to 1 second as possible?  
>>> Yes.  One follows the other.  
>> Not really, you can have a source of seconds that are all within 0.1
>> ns of the right length but are offset from UTC by 200 ns. ( stable
>> but not accurate)
>> You can have a series of seconds that are all within 10 ns of UTC,
>> but one may be 20 ns to short and the next is 20 ns to long.
>> ( accurate but not stable )
>> So, which of the two is more important?
> UTC is most important (to me), but if one has perfect UTC, then one also
> has perfect seconds.

Except that you are doing a design. That involves tradeoffs. Pre-processing a thing message
that comes in 800 ms before a pulse does not sound like a big deal to me. In your design it
apparently *is* a big deal. If you indeed want very tight UTC, that involves very similar
sorts of things. There are a *lot* of delays to be worked out. The offsets between GPS time
and UTC need to be downloaded and summed into the servo as well. 

A GPSDO ( or anything that works like one) will have accurate second to second timing. In a 
very general sense, it does not care about a time offset. A fixed delay of 100 ns is no different 
than a fixed delay of 200 ns as far as it’s output or it’s function. 

So, no, it’s not a drop dead simple choice.


> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
> 	gem at rellim.com  Tel:+1 541 382 8588
> 	    Veritas liberabit vos. -- Quid est veritas?
>    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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