[time-nuts] Where does the Z3801A 1 PPS come from?

Tom Van Baak tvb at leapsecond.com
Tue Mar 15 14:57:24 EST 2005

Hi John,

I have 40 hours of data now on the phase difference
between the 1 PPS and 10 MHz outputs. Attached
is a plot using a similar scale as yours. With 5 min
averaging the standard deviation is under 10 ps. So
I'm convinced it's a simple divider.

To further confirm this I pulled the antenna cable for
a few hours to see if both outputs keep the same
relative phase even as the 1 PPS drifts in phase and
the 10 MHz drifts in frequency. They do! Looking at
the plot you can't even tell when I did it.

The same is true coming out of holdover, during fine
frequency adjust and all. This would suggest HP does,
in fact, use a temporary frequency offset as a means
to fine adjust the phase within the 100 ns period.

As a final test I power cycled (!) the receiver during
this run and, sure enough, the 1 PPS and 10 MHz
keep their tight phase difference. The divider is
probably jam reset to the right 100 ns once after
the GPS 1 PPS and OCXO are stable and then
the frequency tweaked to the approximate ns.

So my conclusion is the 1 PPS output is exactly
as stable as the 10 MHz output and both are
derived from the OCXO.

Bill pointed out PCB traces going to/from U32 so
that PGA must contain, among other things, the
10 MHz to 1 PPS divider and output buffer.

In the old days one would use a string of 7490's
to implement a divider. I imagine the modern way
to do it in the 90's was a PGA (7 synchronous
resettable decade counters can't be that hard).

As you know I use a PIC-based divider, but that
just shows I'm more of a software person than
hardware. ;-)

Anyway, thanks again for bring this query up. It
turned out to be interesting. I don't think many
people would notice it because even with your
tempco (?) the difference was just a ns or two.

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