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

Stewart Cobb stewart.cobb at gmail.com
Sun Nov 2 04:42:27 EST 2014

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:


(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

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

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 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.


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