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

Tom Van Baak tvb at LeapSecond.com
Thu Oct 25 11:14:18 EDT 2018

> I have in mind a project which consists in synchronizing two
> or more stable clocks (OCXO) disciplined by GPS.

If the OCXO are disciplined by GPS then you have a GPSDO. There is no need, then, to synchronize the OCXO directly; they are all synchronized indirectly through GPS. Or perhaps I don't understand what you're actually trying to do.

It would also help, particularly on this list, to tell us if your synchronization goals are at the ms or us or ns level.

> 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.

It's easy to disable GPS. Choose one of:

1) Remove the gps antenna connector, or
2) Cover the antenna with a RF shield, or
3) Use a RF relay inline with the GPS antenna feed, or
4) Kill the power to the antenna, or
5) Hack the PCB and disable the 1PPS from the receiver, or
6) Use s/w commands to disable the discipline loop. The Trimble TBolt has such a feature. It's very handy.

> After some research, the Trimble Thunderbold board got my attention, as has everything I need to get the project started.

Yes, the Thunderbolt, aka TBolt, is a favorite. I assume you're talking about getting used ones on eBay rather than buying new from Trimble?

> Before getting a pair of the boards, on the datasheet is explained that one can
> get the unit on a disconnected state and adjust the ADC which drives the OCXO
> directly (that’s one of the desired capabilities !).

Yes, there are several states you can configure. And yes, you can jam sync or set the DAC manually if you wish. There is no ADC; is that a typo or are you thinking there's something to read from the OCXO?

> The question is: does anybody know if the phase difference (input of the PLL)
> can be read, in order to know how to steer the ADC ??

Yes, the phase difference, often called TI (as in Time Interval), is reported by the TBolt. This is the recent delta between the GPS/1PPS and the OCXO/1Hz. Most GPSDO give you this information. You could then do your own steering using the DAC commands to close the loop. You won't match native performance but it's fine for testing & playing.

For the TBolt you can play with all of this using:

1) Trimble's TBoltmon.exe program, a simple GUI that let's you read/write any modes and settings.
2) Mark Sim's Heather program, a "feature rich" tool that many on this list use.
3) Roll your own command using TSIP, the serial protocol that Trimble uses. Sample code on the web.

> Alternatively, could you please suggest another board that would fulfil the following?
> - GPS can be disabled.
> - has a serial com port, so commands can be sent to the unit and information can be retrieved.
> - provides 1PPS and 10MHz signals.

Most GPSDO will do this. If you want something cheap, simple, and direct control over everything inside consider one of the Arduino-based GPSDO kits/products. That way you get source code and can do anything you want to the s/w.

And again, let us know if your synchronization goals are at the ms or us or ns level. Or perhaps describe in technical detail what problem you're working on, and what your constraints are.


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