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

Stewart Cobb stewart.cobb at gmail.com
Mon Oct 20 00:53:07 EDT 2014


Fellow time-nuts,

This (long) post is a review of the HP/Symmetricom Z3810A (or Z3810AS)
GPSDO system built for Lucent circa 2000.  I wrote it because I looked
for more information before I bought one, and couldn't find much.
It's relevant because (as of this writing), you can buy a full system
on the usual auction site for about $150 plus shipping.  For those of
you lamenting the dearth of cheap Thunderbolts, this looks like one of
the best deals going.  The description of these objects does not
include "GPSDO", so time-nuts may have missed it.  Search for one of
the part numbers in the subject line and you should find it.

So what is it?  It's a dual GPSDO built by HP as a reference
(Redundant Frequency and Time Generator, or RFTG) for a Lucent
cell-phone base station, built to Lucent's spec KS-24361. Internally,
it's a close cousin of a later-model Z3805A.  Externally, it looks to
be almost a drop-in replacement for the earlier RFTG system built to
Lucent's spec KS-24019.  That was a redundant system containing one
rubidium (LPRO, in the one I have) and one OCXO in two
almost-identical boxes.  That spec went through several revisions with
slightly different nameplates and presumably slightly different
internals.  You can generally find one or two examples on the auction
site (search for RFTG or KS-24019).

This system is similar, but the two boxes each contain a Milliren
(MTI) 260-0624-C 5.000MHz DOCXO, and neither contains a rubidium.  The
Milliren DOXCO is the same one used in the later models of the HP
Z3805A / 58503A.  It's a very high-performance DOCXO, in the same
class as the legendary HP 10811, and better than the one in most
surplus Thunderbolts.  The 5 MHz output is multiplied up to 10 MHz in
at least one unit, and 15 MHz in both units.  I don't have the ability
to measure phase noise on these outputs, but I'd be interested to see
the results if someone could.

Nomenclature:  The Z3810AS (there always seems to be an "S" at the
end) is a system consisting of the Z3811A (the unit containing a GPS
receiver), the Z3812A (the unit with no GPS receiver), and the Z3809A
(a stupid little interconnect cable).  The GPS receiver inside the
Z3811A is a Motorola device, presumably some version of an OnCore.
Where the Z3811A has a TNC GPS antenna input, the Z3812A has an SMA
connector labeled "10MHz TP".  That is indeed a 10 MHz output.  It
comes active as soon as power is applied to the unit, and its
frequency follows the warmup curve of the OCXO.  The two units have
identical PCBs (stuffed slightly differently), and I have no doubt
that someone can figure out how to add a 10 MHz output to the Z3811A
as well.

Operation:  From the outside, these units are broadly similar to
earlier units in the Lucent RFTG series. The (extremely valuable)
website run by Didier, KO4BB, has a lot of information on those
earlier units, much of which still applies here.  The purpose of these
units was to provide a reliable source of frequency and timing
information to the cell-site electronics.  The 15 MHz outputs from
both units were connected to a power combiner/splitter and directed to
various parts of the transmitter.  The units negotiate with each other
so that only one 15 MHz output is active at a time.  The outputs
labeled "RS422/1PPS" contained a 4800 baud (?) serial time code as
well as the PPS signal, which were sent to the control computer.

Power is applied to the connector labeled "+24VDC" and "P1", in
exactly the same way as the earlier RFTG units. Apply +24V to pin 1
and the other side of the power supply (GND or RTN) to pin 2.  In
these units, that power supply goes directly to an isolated Lucent
DC/DC converter brick labeled "IN: DC 18-36, 1.9A".  Presumably you
can run both units with a 4-amp supply.

Once you have applied power, connect the Z3809A cable between the
jacks labeled "INTERFACE J5" on each unit.  The earlier RFTG units
used a special cable between two DE-9 connectors, and it mattered
which end of the cable connected to which unit.  The interconnect for
these units is a high-density DE-15 connector (like a VGA plug).  The
Z3809A cable is so short that the two units need to be stacked one
above the other, or the cable won't reach.  It doesn't seem to matter
which end of the cable goes to which unit.  I don't know whether it's
a straight-through cable, or whether you could use a VGA cable as a
substitute.

When you apply power, all the LEDs on the front panel will flash.  The
"NO GPS" light will continue flashing until you connect a GPS antenna.
Once it sees a satellite, the light will stop flashing and remain on.
The unit will conduct a self-survey for several hours.  Eventually, if
all is well, the Z3812A ("REF 0" on its front panel) will show one
green "ON" light and the Z3811A ("REF 1") will show one yellow "STBY"
light.  This means that the Z3812A is actually transmitting its 15MHz
output, and the other one is silently waiting to take over if it
fails.

Most time-nuts want to see more than a pretty green light.  The old
RFTG series allowed you to hook up a PC to the "RS422/PPS" port and
peek under the hood with a diagnostic program.  The program is
available on the KO4BB website.  It is written for an old version of
Windows, and I had no luck getting it to run under Windows 7.  It does
run under WINE (the Windows emulator for Linux) on Ubuntu 12.04 LTS.
To use it, you need to make an adapter cable to connect the oddball
RS-422 pinout to a conventional PC RS-232 pinout.  The adapter cable
looks like this:

RFTG          PC

DE-9P         DE-9S

7 <----------> 5

8 <----------> 3

9 <----------> 2

(According to the official specs, this is cheating, because you're
connecting the negative side of the differential RS-422 signals to the
RS-232, and ignoring the positive side of the differential signals.
However, it's a standard hack, and it's worked every time I've tried
it.)

With that adapter, you can see the periodic timetag reports from the
unit.  The RFTG program will interpret these timetags when it starts
up in "normal mode".  However, when I try to use any of the diagnostic
features built into the program, it crashes WINE.  The timetag output
was required for compatibility, but I suspect that HP didn't bother to
implement the Lucent diagnostics.

Instead, they added a connector which is not on the previous RFTG
series.  That connector is labeled, logically enough, "J8-DIAGNOSTIC".
It too is wired with RS-422, so you need to use the same adapter cable
as before.  Once you do, you'll find that this connector speaks the
usual HP SCPI command set (Hooray!).  I used the official SATSTAT
program (again under WINE on 12.04 LTS), but I'm sure that other
programs written for this command set will work as well.  The default
SATSTAT serial port settings of 9600-8-N-1 worked for me.

After about 24 hours, with a poorly-sited indoor GPS antenna, my
system has converged to TFOM=3, FFOM=0 (the best possible numbers),
and a "predicted 24-hour holdover uncertainty" of 5.2 microseconds,
which is not too shabby.  It found the correct day and year without
any assistance, so if it has a "GPS week number rollover" problem,
it's still in the future.  I don't currently have the ability to
compare the 10 MHz output to anything else.  Again, if someone else
can, I'd be interested to see the results.

Additional Notes: The parts on the boards all have date codes of 1998
or 1999.  The Motorola GPS receiver has a firmware label that reads
"02/04/00".  The SCPI error logs inside the HP units were virgin when
I first got them.  They had 84 and 94 power cycles, respectively.
Before the GPS receiver acquired time, the error log timestamps read
"2000-05-09 00:00:00", which I interpret as a firmware release date.

The PCB has an interesting feature.  Next to each soldered-in pin of
the Milliren OCXO is a single-pin socket soldered into the board.  I'm
guessing this was used in manufacturing, to temporarily install a
Milliren and confirm that the system worked before permanently
soldering it in.  (At production prices, the Milliren would have cost
far more than the rest of the PCB.)  You might be able to use this in
reverse, if you have a set of Millirens to test from another source.

The Z3809A interconnect cable has three of the 15 pins on each end
clipped a bit shorter than the rest.  Not so short that they won't
eventually make contact, but short enough to make contact later than
the rest.  Don't know why, but it's clearly deliberate.  A lot of
hot-plug connectors are built that way, including USB connectors.  I
have no idea what the pinout of the interconnect is.

The redundant system slaves both DOCXOs to the same GPS reference.
Inside the GPS loop bandwidth, the two oscillators will have almost
the same frequency and will differ only by phase noise and short-term
stability.  This is almost a perfect setup for experimenting with
certain kinds of time-nut measurements, assuming someone can figure
out how to get 10MHz out of the Z3811A unit.  If you then command both
units into holdover, you could measure longer-term stability as well.

The units are described as "new in factory sealed box".  After an
archeological investigation of the various strata of labels and tape
on the boxes, I would say that's probably accurate.  My set seems to
have been shipped from the Agilent factory in Korea to Symmetricom in
Sunnyvale, CA sometime in August, 2000, shortly after it was built,
and remained untouched until I opened it.  I'm guessing it was built
and saved as part of a spares program for Lucent, and kept until
Lucent decided they didn't need spares any more.

I have no connection with the current seller of these units (or any
other sellers, for that matter) except as a satisfied customer.  I
think I'll order another set as a spare, before the feeding frenzy
hits.

Request for help:  Both the SatStat and RFTG programs run under WINE
on stock Ubuntu 12.04 LTS (32-bit) without any tricks or special
configuration.  Neither seems to run under WINE on Ubuntu 14.04 LTS
(64-bit). I am a WINE novice.  Any hints from WINE experts would be
appreciated.  Also, I've been able to run TimeLab under WINE, but I
can't connect it to my USB-to-488 interface, so I can't take data.  If
anyone can tell me how to set that up, I'd be extremely grateful.

Cheers!
--Stu



More information about the time-nuts mailing list