[time-nuts] yet another GPSDO

Arthur Dent golgarfrincham at gmail.com
Fri Aug 7 14:53:35 EDT 2015

I picked up yet another version of a GPSDO on Ebay the other
day. I thought that I'd mention this because sometimes when
one unit shows up there are a lot more that will be listed
for sale later, like with the many Lucent RFTG units that
are currently being sold on Ebay.

This one is a Symmetricom CGBA with absolutely no info other
than the photos in the listing. The board is about 9"x13" and
has date codes from 2003. It has a Symmetricom 5Mhz OCXO that
feeds a doubler with L/C filtering and a 10Mhz crystal to
further filter the 10Mhz and give a nice clean sine wave
feeding a 27 ohm termination resistor. Then the signal goes
to 3 EL2257 dual gated opamps That seem to make the signal
look much worse so there may have been more than one design
engineer on this project. ;-)

I traced out 2 lines for power, each one goes thru its own
fuse to a diode bridge so it doesn't care about polarity.
>From the bridge the power goes to several discreet onboard
switchers that give +12, +12 OCXO, -12, +5, and +3.3VDC. I
found that the minimum voltage to make the board operate was
22-28VDC so I'm running it on 36VDC. The current draw is 0.5A
cold to 0.25A with the OXCO warmed up.

The GPS is made by Furumo and has what looks like an HP part
number 0960-1060(HP31) and has +5VDC for an active antenna on
the antenna connector. There are unused pads on the main board
for SMA size connectors that are marked 10M, PP2S, 4.096M, 100Hz,
30.72M, 40.96S, and 8K. Not all of these are active and the
3 dual opamps are not 'on' either so some of the pins in the
connectors on the back of the assembly may do some controlling
of signals or these may be options not implemented on this board.

When powered up it goes through a light flashing sequence then
the GPS LED goes flashing green when it sees the satellites, then
solid green when it locks, and the ACT LED turns from flashing red
to flashing green. Using my modified Tbolt to test the unit shows
it stays within +/-200PPT for about the first 2 hours then settles

There is an RJ-45 connector on the front plus there is a 15 pin
'D' connector behind the panel that may have been for some
testing purpose because it can't be accessed from the front.
At this time I'm just happy to see how well it works and later
I'll see if I can communicate with it somehow.



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