[time-nuts] Thunderbolt Loop Damping

Tom Van Baak tvb at LeapSecond.com
Tue Jul 8 13:55:04 UTC 2008


Mark Sims writes:
> My ADEV plotter takes the PPS error in nanoseconds and
> adds 1.0 to it to get a per-second phase error.  To get

I'm not sure why you would add 1.0 here? The PPS error is
a per-second time-interval (phase) error already, no?

> comparable values from the OSC PPB value,  divide it by
> 1E9 and multiply by 100.  Add that to 1.0 seconds to get a
> per-second phase error of the OSC signal.  The resulting
> ADEV plots of PPS and OSC PPB are off by a decade
> below 20 tau,  and match very closely above 100 tau.  The
> OSC PPB is worse below 100 tau (4.5e-9 vs 4.5 e-10 at
> 1 tau,  both around 4e-11 at 100 tau)  by 10,000 tau the
> OSC PPB is slightly better than the PPS signal (both in the
> 5e-13 range).

It's not clear what you're doing here. Are you comparing your
TBolt to an external reference (Rb or Cs or other GPSDO)?

Or are you just processing the internal TBolt numbers and
assuming you can derive ADEV stability numbers of GPS
or the GPSDO or the OCXO itself from that?

> As far as messing with the discilining constants,  I figure
> Trimble has them fairly well optimized for their OXCO.  They
> probably only need to be changed if you graft a different
> OCXO onto the unit.  

Yes, it's quite flexible and you can have a lot of fun replacing
the OCXO. I'm assuming a number of you will be trying this
in the coming months.

But even with the onboard OCXO, it allows you to change the
time constant which can make a noticeable difference in the
mid-term stability of the GPSDO. I'll dig out plots I made last
year if you're interested.

/tvb





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