[time-nuts] GPS discipline oscillator vs phase lock

Bob kb8tq kb8tq at n1k.org
Fri Jun 16 16:32:59 EDT 2017


A *lot* depends on your definition of “phase locked”.  If indeed you are after 0.1 degree at 100 MHz, that gets into the “no can do” range. To put some numbers on it, 0.1 degree at 100 MHz is 2.7 ps. GPS time as received simply is not stable to that level … If you drop back to about 20 degrees, you start to get into the “maybe can do” range. 


> On Jun 16, 2017, at 2:40 PM, life speed via time-nuts <time-nuts at febo.com> wrote:
> I may already know the answer to this, but I figured I would ask the time nuts anyway.  I have an application where I would like to phase-lock two oscillators together, probably 10MHz OCXOs as they have particularly good Allen Deviation compared to what I would ultimately like to use, a 100MHz crystal oscillator locally PLL'd to the 10MHz oscillators.  These oscillators will be separated by a distance of a few yards up to a few miles.  The requirement is not that their phases align perfectly, as in the conventional locally-connected phase-locked loop sense, but rather that any phase difference between the two oscillators resulting from arrival times of GPS signals are held constant.  Perhaps this shows my lack of understanding of GPS time, I don't know if travel time is accounted for in commercial GPS 1Hz outputs, it may well be corrected.
> I suspect the result of a GPSDO is not the same as phase-locking two oscillators together.  Perhaps it is frequency locking?  Which, if the phase difference were held constant to within 0.1 degree, would be acceptable.  Not sure this is the result either.
> Thanks for the info, - Lifespeed
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