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

Robert Darlington rdarlington at gmail.com
Sat Sep 19 23:29:45 UTC 2009

Ah ha!  That explains the changing numbers I see on the 5370B (jitter), and
explains why I see no change on the scope other than the jitter.  Reading
the manual, I don't see any clear explanation like what you just gave me.  I
see my mistake in manual interpretation but not until after reading your
reply.  Thank you.


On Sat, Sep 19, 2009 at 5:18 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
> wrote:

> Bob
> 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
> receiver.
> 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
> >
> Bruce
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