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

Robert Darlington rdarlington at gmail.com
Sat Sep 19 23:08:29 UTC 2009

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.
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.

Bob, N3XKB

More information about the time-nuts mailing list