[time-nuts] FW: Question about the PLL of Trimble Thunderbold

Tom Van Baak tvb at LeapSecond.com
Mon Oct 29 06:13:43 EDT 2018


> The synchronization goal is in the order of ps level.

I'm glad you mentioned your requirements. Note that time synchronization at a "ps level" is 3 to 4 orders of magnitude beyond what the typical commercial or eBay or DIY GPSDO will do.

But as you imply, you're only using GPS to get close (ns levels) and then using your own two-way communication to narrow it down to ps levels, right?

> Within the Report Packet 0x8F-AC, the bytes 16-19 indicate “Estimate of UTC/GPS offset”, is this the time difference ?

Yes. Section "A.10.31 Report Packet 0x8F-AC Secondary Timing Packet" says:

   "PPS Offset: This field carries the offset of the PPS output relative to UTC or GPS as
    reported by the GPS receiver in nanoseconds. Positive values indicate that the
    ThunderBolt’s PPS is coming out late relative to GPS or UTC."

The range and precision of that time interval value would look something like this:


I can send you the raw data if you want to play with it. Attached is a histogram. Note the standard deviation is about 2 ns.

> My idea is to develop the control loop which will be able to synchronize one oscillator to another.

Remember that all the ingredients in your system will then need to be stable to ps levels: the oscillator, the DAC, the 1PPS pulse, the cables, the input signal conditioning, and time or phase comparator, etc. That's a pretty tall order but not impossible.

You may want to synchronize using 10 MHz instead of 1PPS to reduce your noise and improve the tight lock. How far apart are your two oscillators?

In case it helps your effort, see https://en.wikipedia.org/wiki/The_White_Rabbit_Project and read the PDF's. It's an open h/w project.

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