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

John Ackermann N8UR jra at febo.com
Sun Mar 13 12:38:45 EST 2005

Bill Jones, K8CU wrote:

>On the Z3801A the 1 PPS from the Motorola GPS receiver is sent to pin 82 of
>the CPU (U32). From there, the Z3801A 1 PPS output is sourced from the CPU
>on pin 78 and eventually drives the output line driver at U6.
>The 1 PPS is definitely not simply divided from the 10 MHz oscillator
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. :-)

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?

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.


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