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

John Ackermann N8UR jra at febo.com
Mon Mar 14 08:17:32 EST 2005

Tom Van Baak wrote:

[ snip ]

> Note that 1 ns is easiliy within thermal variations.

Absolutely true.  All the gear was in the same thermal environment, but 
the external 10MHz divider was sitting on top of the rack, rather than 
in it, so was probably exposed to more temperature variation due to air 

> 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?

This was the 5370B, with averaging to 100 seconds, so the resolution 
plus noise ought to be in the 10ps range.

[ snip ]

> 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.

That's sure what I would have thought -- unless the CPU is being clocked 
from the OCXO and thus is effectively transferring its stability to the 
1pps (sort of like your PIC divider).  But I suspect it would be hard 
for a CPU doing several tasks to maintain the cycle timing of a 
dedicated divider.

> 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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