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

Tom Van Baak tvb at leapsecond.com
Sun Mar 13 23:05:37 EST 2005

> Very interesting, Bill.  Thanks for that info.  I'm a bit surprised as I 
> would have thought that deriving the 1 PPS from the 10MHz would yield 
> the best performance.  Of course, this means that using the 1 PPS to 
> measure the stability of the 10MHz oscillator isn't correct, so I've 
> done a bunch of non-meaningful measurements recently. :-)

Not to worry. The positive way to look at your plot
is that you have confirmed the two outputs agree to
within +/- 1 ns peak-to-peak over more than a day.

That's good. And it means the difference between
using the 1 PPS and the 10 MHz output for any
measurement is on the order of a few parts in
10^14 at a day.

Nothing you measure is that low so my conclusion
is that you can safely use either the 1 PPS or the
10 MHz as a reference for any measurement you like.

Note that 1 ns is easiliy within thermal variations.

I'm running your experiment here just to be sure. It
would be good also to show the 1 ns excursions are
not something in your counter. Was this a 5334A?

> The CPU must be applying some smoothing to the GPS 1PPS as the jitter is 
> significantly less than the raw pulses, but it can't be doing too much 
> as there's still about ~50ns noise (over timescales of hours) in both my 
> units.  I wonder what the algorithm is?

The CPU, like all of us, does software averaging on
the raw Oncore 1 PPS (but I don't know the details).

> Another interesting experiment suggests itself... measuring both the CPU 
> generated 1 PPS, and the 10MHz derived 1 PPS, against a third source to 
> see the relative jitter of each.

I may have done this and will look for the old data. My
understanding was always that the 1 PPS output was
tied tightly to the OCXO. I guess I don't buy into the
idea that the CPU is generating the 1 PPS on its own.

To me, 1 ns agreement between two signals suggests
it's all hardware-based, not software. But I'd be happy
to learn I'm wrong.


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