[time-nuts] Thunderbolt question regarding PPS and 10MHz phase relationship

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Sep 19 23:18:43 UTC 2009


Robert Darlington wrote:
> Hello all,
> I'm trying to adjust the time between the leading edge of the PPS square
> wave and the 10MHz zero crossing outputs on a Trimble Thunderbolt.  The
> manual seems to indicate the cable delay compensation setting will do this.
No, it doesnt.
This only adjusts the timing of the output PPS signal with respect to
the GPS time as seen by the receiver.
It is only intended to compensate for delay in the cable (and
potentially the active antenna group delay) between the antenna and the
The PPS output is synchronous with the 10MHz signal and the delay from
the preceding zero crossing of the 10Mhz signal to the leading PPS edge
cannot be altered.
The PPS signal has considerable jitter (~300ps rms ) with respect to the
10MHz signal zero crossing.
> As I understand things, this is also called the "PPS Offset" in the manual.
> I've changed this from 58ns to -58ns with no visible change over the course
> of a 12 hour period on a scope.  My test setup is using an old Tek TDS 210
> scope triggering on the PPS on channel 1 through a 50 ohm terminator,
> trigger set to 400mV and horizontal position adjusted so that 400mV crosses
> the center vertical graticule marking.   Channel 2 is attached to the 10MHz
> signal through another pass-through 50 ohm terminator.  What I see on the
> screen looks like 15ns delay between the triggered PPS signal and the zero
> crossing 10MHz signal.  My 5370B shows 18ns delay between them when the
> start input level is set for 400mV.
> The manual indicates that it could take a while to see a change, but how
> long is a while?  My tbolt has default settings for everything except the
> PPS Offset value (again, now sitting at -58ns).  I expected to at least see
> some change over the course of 12 hours or so, but I get nothing.  Am I
> completely misunderstanding something?  I accidentally cycled the power
> after making the settings change after the tbolt was up for a couple of
> months.  I learned to not use a normal serial cable with an APC ups, it cut
> the power on the UPS immediately upon plugging it into the back of my PC!
> Settings were verified after power was brought back up.  Total downtime was
> perhaps 10 seconds, but that was enough for it to lose almanac data.
> Thanks,
> Bob, N3XKB


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