[time-nuts] Lucent KS-24361, HP/Symmetricom Z3809A, Z3810A, Z3811A, Z3812A GPSDO system

Bob Camp kb8tq at n1k.org
Sun Nov 2 12:16:20 EST 2014


> On Nov 2, 2014, at 4:42 AM, Stewart Cobb <stewart.cobb at gmail.com> wrote:
> I have now tested 3 pairs of these units (full Z3810AS system), always
> in pairs.  One Z3811 box was DOA.  It drew power at the appropriate
> level, but the lights did not light up and there was no response on
> the SatStat port.  AECI quickly swapped it for a working unit, and
> even paid return shipping for the dud.  Can't complain about the
> service.  One pair was built in the US in approximately March 1999; it
> has serial numbers in the old HP style: 3844A41xxx.  The others were
> built later, in Korea, and have new-style serial numbers like
> KR92840xxx.  They all seem to have the same software.
> My lab uses a dual USB-to-serial converter to talk to both boxes at
> once.  This does not use FTDI chips.  One side seems to talk perfectly
> both ways through the RS-422 hack wiring; the other side seems to
> receive perfectly but the GPSDO complains about erroneous commands
> about once or twice an hour.  If anyone wants to duplicate my setup,
> the converter comes from "Microconnectors" and can be found here:
> <http://www.amazon.com/Micro-Connectors-Serial-Adapter-E07-162/dp/B0032325LE>
> (Link provided for information only; I have no connection with any of
> these people.)
> It is NOT TRUE that only the box with the green light on will talk
> over its diagnostic port.  Both boxes can be interrogated
> simultaneously with SatStat, starting immediately after power-up.  The
> J8 diagnostic ports on both units "speak only when spoken to".  If
> you're using a scope to look for activity on the diagnostic port, you
> won't see any unless a PC is connected.  If you're switching one
> serial cable back and forth between two boxes, be sure to close the
> port (menu CommPort/PortOpen unchecked) before removing the cable from
> the first box, and open the port again (menu CommPort/PortOpen
> checked) after connecting the cable to the second box.  The PortOpen
> command seems to do more than merely opening the serial port; it seems
> to be necessary to get communications going with the box.  If you
> watch the window title, you can see that it's sending a *IDN? command
> and interpreting the results, but it may be doing other things as
> well.
> On the Z3812A, the front panel "10 MHz test point" J1 connects to a
> group of three 100-ohm SMT (0805?) resistors and one SMT (0805 again?)
> ceramic capacitor.  These are next to U206, a 74ACT14 in a SO-14
> package, on the bottom of the board behind J6.  On the Z3811A, these
> three resistors and one capacitor are missing.  It is quite probable
> that adding them would send a 10 MHz signal to the SMA jack footprint
> buried under the GPS antenna input TNC jack.  One could presumably
> solder a small coax cable to that footprint and route it around to a
> convenient location on the front panel.  (The capacitor is not marked
> with a value, but it's probably small.  It connects from the 10 MHz
> output to ground, and its purpose is probably just to slow down the
> edge rates of the 10 MHz output to reduce EMI.)
> The 10 MHz output seems to have a duty cycle of about 55% high / 45%
> low.  This may be related to the synthesis technique, or it may simply
> be due to asymmetrical thresholds in the 74ACT14 schmitt-trigger
> driver.
> The firmware in the Z3811 and Z3812 boxes appears to be exactly the
> same.  The PCBs are stuffed slightly differently, and of course the
> Z3812 has no GPS receiver.  It is possible that one could add a GPS
> receiver speaking the correct protocol to a Z3812, and turn it into a
> GPSDO.  It would be necessary to find out how the firmware detects
> what sort of box it is.  Perhaps it simply looks for a GPS receiver,
> or perhaps there is a pullup resistor somewhere on the board that is
> only stuffed on one version of the box.
> The GPS receivers in these boxes use fairly old technology.  In
> particular, they can only track 8 channels at a time, and they cannot
> make use of the WAAS signals.  WAAS transmits corrections for
> satellite ephemeris errors and for the changing ionosphere.  These are
> two of the most prominent error sources for GPS timing.  (The others
> are antenna position error and multipath, both of which are
> potentially under the control of the user, and troposphere delay,
> which is small but can't easily be measured or removed.)  It is
> theoretically possible to build a plug-compatible board carrying a
> modern GPS timing receiver and a small microcontroller to translate
> its communications into Motorola Oncore language.  With a well-sited
> antenna, such a system might display noticeably improved performance.
> The current GPS receivers in these boxes take a remarkably long time
> to settle down after power-up.  The GPS satellites transmit their full
> almanac every 12.5 minutes, which is normally all the time required
> for a GPS receiver to become completely happy.  These receivers seem
> to require over an hour to become happy enough to START their
> self-survey, and several more hours to complete it.

The Lucent and HP boys may have done something to slow things up. It’s quite possible that they decided to let the OCXO warm up for some number of hours before doing anything to it. If the warmup period is something like two or three hours and the survey is an hour, waiting on the survey isn’t all that silly. Let the super capacitor charge up, wait to be sure the power really is (back) on, get multiple passes at the GPS data frames to be sure they are correct. 

The point being - even with a new GPS, it may still just sit there for hours …. Either way, it will take a couple of days to settle down. An hour this way or that - no big deal.


>  The old rubidium
> RFTG units could take 24 hours or more to become fully operational;
> these units seem to take 4 to 12 hours, depending on GPS antenna
> quality and siting.
> Cheers!
> --Stu
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