[time-nuts] Absolute phase

Bob kb8tq kb8tq at n1k.org
Fri Nov 17 14:23:20 EST 2017


Most GPSDO’s use a divider to get straight from the 10 MHz to the PPS. There are some that 
derive both the PPS and the 10 MHz off a higher frequency, they are not common in the eBay
world (yet). The divider is going to have a fixed delay based on it’s design. That might be 35 ns,
it might be 3.5 ns. It all depends on how they did this or that. There easily could be an inversion 
that moves everything 50 ns. 

If the GPSDO wanders around over a 30 ns range, that’s a pretty big phase shift at 10 MHz. If
they both are doing this you are up to (maybe) half a cycle or more. 

Now toss in devices that “skip” a 10 MHz cycle to keep the PPS aligned. This is pretty uncommon 
in normal operation. Uncommon does not mean that it never happens. 

So, if you have two of the same model / same firmware GPSDO’s, the 10 MHz phase is probably
“in the ballpark”. It’s certainly not close in terms of < 1 degree or something like that. If you have
GPSDO’s from multiple outfits …. who knows what you will see.


> On Nov 17, 2017, at 2:05 PM, Jerry Hancock <jerry at hanler.com> wrote:
> Granted I expect someone on this list to reply with something that makes me feel stupid, if you have two GPSDO units running side by side, should the phase delta on the 10Mhz output be zero (ideally)?  Is there an absolute phase standard kept between GPSDO units as all it would take is one extra inverter in the chain to flip the phase, no?
> Thanks
> Jerry
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