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

Brooke Clarke brooke at pacific.net
Tue Mar 15 16:13:50 EST 2005

Hi Tom:

Does the PRS10 work in a similar way?
I have the Phase Lock Time Constant (PT)  value set to 14 which gives an 
18 hour natural time constant which Stanford Research said would be best 
for GPS steering.

Have Fun,


Tom Van Baak wrote:

>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.
>time-nuts mailing list
>time-nuts at febo.com

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