[time-nuts] Question about the PLL of Trimble Thunderbold
Tom Van Baak
tvb at LeapSecond.com
Wed Oct 31 17:45:06 EDT 2018
> I have in mind a project which consists in synchronizing two or more stable
> clocks (OCXO) disciplined by GPS.
> However, would be great to have the option to disable the GPS on both sides
> at a given time and to synchronize them in a Master-Slave or directly by means
> of a protocol they could correct each other and synchronize themselves.
Given your desire to synchronize the clocks at picosecond levels consider using 10 MHz instead of 1PPS. What you are designing then is just a very tight PLL to keep the oscillators in sync. Leave GPS and 1PPS out of the equation; just focus on the RF signals. Once you have meet your 100 or 10 or couple of ps goal then adding the coarse timing is quite simple. There are several ways to do the UTC/1PPS part:
1) Out of 10 million cycles you pick the cycle that's closest to your best GPS/1PPS. And then steer the synchronized OCXO by +/- 50 ns to match GPS. I don't know how happy the PLL will be sliding 50 ns when it is designed to lock within ps, but I'm sure that's a solvable problem.
2) Out of 10 million cycles you pick the cycle that's closest to your best GPS/1PPS. And then just record the +/- 50 ns offset as data and send it over a serial port to the other oscillators in your ensemble.
The point is there are two ways to do timing. The hard way is to generate 10 MHz and 1PPS signal that is exactly UTC. The easy way is to generate 10 MHz and 1PPS along with a message telling you what the offset is (or was). It's similar to how saw tooth correction is done; you don't need an exact on-time pulse as long as you are given information to calculate the exact time of the pulse.
Question for the list -- who of you have done multi-oscillator PLL's? Can two 10 MHz OCXO be locked to within 10 or 1 ps? For now, ignore cable issues and assume they're right next to each other.
Years ago John Miles did a write-up on Warren Sarkinson's prototype TPLL.  If that achieves resolution of 1e-13 @ 1 s would that imply the PLL is locking to sub-ps levels?
Warren -- Are you still on the list? Syncing multiple 10811A oscillators to extreme levels sounds like something you would have tried. Either just for fun, or to create an N-way ensemble of OCXO for the purpose of reducing phase noise.
Rick -- Do you remember the 8-way (?) 10811A phase noise reference standard that Len used in the 5071A lab in Santa Clara?
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