[time-nuts] Lucent KS-24361/Z3812A GPSDO initial setup

Bob Stewart bob at evoria.net
Sat Oct 25 14:44:38 EDT 2014

Hi Bob,
The thing is: I don't really need a frequency standard other than as a reference for my GPSDO project.  I'll have to look into pulling out that message every second to see if the correction makes it a suitable phase reference.  No, the nanosecond level probably isn't suitable for your needs, but I think it fits mine.  My target audience remains the hobbyist, not the professional.

It will be interesting, as it ages in, to see how it likes the antenna at the south window that it's sharing with my project.  If I don't see any glitches, it may be time to pull the wire through the attic, rather than through the window.  I guess I could also get some good information running it with the antenna in the attic for awhile.  But you do make a good point about power supplies.  Santa may bring a small UPS for Christmas to power this, my project, and the splitter.
At the very least, this gives me a lot of information about GPSDOs that I didn't have in the past.  And there's that EFC pin-out back near the OCXO that I could watch with my 3456A, to see what the dynamics are on a "real" GPSDO, once it ages in.

      From: Bob Camp <kb8tq at n1k.org>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Saturday, October 25, 2014 1:22 PM
 Subject: Re: [time-nuts] Lucent KS-24361/Z3812A GPSDO initial setup

Remember - these gizmos are designed as a CDMA base station reference, not as a Time Nut frequency (or time) standard. They (likely) had a +/- 100 ns spec on the gizmo for static time error when locked to GPS. The little trained squirrel inside makes an executive decision to move the PPS when it gets to close to that (or some other) limit. 

The filter algorithm in these adapts to the rate of change of the OCXO. On a unit that has been on the shelf since 2000 or 2001, it probably will take a while for the OCXO to settle down and hit a low aging rate. Until it does, the filter will not “stretch out” to it’s longest tau / lowest bandwidth. You can watch the thing switch, it’s pretty obvious on a phase plot when it does. The switch points are where the back and forth phase change slows way down compared to what it was doing. 

> On Oct 25, 2014, at 12:05 PM, Bob Stewart <bob at evoria.net> wrote:
> Regarding my comment earlier that my GPSDO and this Z3812A don't agree on phase.  I see just now a fairly quick phase movement of the phase between the two, and I see that there is a line on the Satstat program that may explain this:  1PPS TI +50.0 ns relative to GPS.  Just a few minutes ago, it said -90.0 ns.  Watching a bit more closely, the phase difference seems to track this figure +/- the phase error on my unit.
> Can anyone shed any light on this?

Bottom line: Hook it up on an independent power supply. Give it it’s own antenna. Put it in a corner away from drafts and crazy temperature changes. Just forget about it. Let it run forever and ever. It will (eventually) settle down and do a pretty good job. How far it settles depends on a lot of things, including just how good the particular OCXO you have is. 


> Bob - AE6RV
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