[time-nuts] ✘NEO-M8N vs. NEO-M8T

Bob kb8tq kb8tq at n1k.org
Mon May 21 11:05:02 EDT 2018


Hi


> On May 20, 2018, at 11:49 PM, Mark Sims <holrum at hotmail.com> wrote:
> 
> I think what Gary really wants is a GPS receiver with the most stable PPS output available.


Unfortunately that’s not how any of these devices are designed to be used. They all ( including the
Furuno ) have a sawtooth issue. It’s just how the fundamental process inside them works.

>  That is probably the Furuno GT-8736... 1.7 nsec sawtooth error.  Typical PPS span is +/- 4 nsec.   Also, the Trimble Thunderbolt has 0 sawtooth error.

The TBolt is a GPSDO, which is a very different beast. It takes the “sawtooth" error it measures and shoves
it into the control loop for the OCXO. The net result is a zero average error vs GPS. That’s how all phase
lock based GPSDO’s  do things. 

The tradeoff is the magic word “average” that snuck in there. Depending on the control loop parameters 
( and a few other things ) the time out of the GPSDO may be off from GPS time by quite a bit. If “time 
right now” is what you are after ( this is TimeNuts after all ), a GPSDO may not be the ideal answer ….
If “time right now” is the goal, the real time clock corrections you can grab on the internet may well be
part of the total solution. 

Bob

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