help reviving Trimble UCCM-LPS GPSDO
hi,
I am currently trying to revive a Trimble UCCM-LPS ex-telco GPSDO.
The model is a 57963-D according to the PCB silkscreen.
Got the board without its OCXO but luckily my stash of surplus I have
discovered a Trimble OCXO labeled 72345-86. Given that that OCXO also has a
label UCCM-LPS it "should work".
Joining the two resulted in:
UCCM >syst:status?
57964-86 serial number S35DB68332 firmware ver 1.0.0.1-01 Ext
mode
Reference Status __________________________ Reference Outputs
XX Ext Ref : [LOS]
TFOM 2 FFOM
3
UCCM A Status[OCXO WARMUP]
GPS: [phase:-2.8E-09, settling]
ACQUISITION ...................................................[GPS 1PPS
Valid]
Tracking: 8 ____ Not Tracking: 3 ________ Time
PRN El Az C/N PRN El Az GPS 13:28:51 23 Dec
2021
15 71 212 37 5 6 195
24 45 272 43 30 7 85 ANT DLY 77 ns
17 28 97 37 19 19 124 Position
13 55 140 38 MODE Hold
14 39 58 39
23 33 291 44 LAT N 51:xxx
10 17 324 40 LON E 5:xxx
12 7 212 34 HGT +27.54 m
(MSL)
ELEV MASK 5 deg
Command complete
Unfortunately it never went GPS locked. Longer story short: the pin of the
OCXO that (I think) should produce a Vref for the EFC circuitry produced
some random ~ 266mV.
A quick experiment learned that the OCXO freq responds to the EFC voltage.
So, looks like the Vref circuit in the OCXO has died.
Now the question: has anyone a working unit that they can measure the
correct Vref voltage from?
For now I have fitted a 2.5V LM336 vref and I
see the EFC voltage go up and down a bit, with the output freq hovering
around 10MHz (+/- 4mHz roughly).
tia,
Wilko
Hi
It’s a pretty good bet that the Vref is 5V
Bob
On Dec 23, 2021, at 8:42 AM, Wilko Bulte wilko.bulte@xs4all.nl wrote:
hi,
I am currently trying to revive a Trimble UCCM-LPS ex-telco GPSDO.
The model is a 57963-D according to the PCB silkscreen.
Got the board without its OCXO but luckily my stash of surplus I have
discovered a Trimble OCXO labeled 72345-86. Given that that OCXO also has a
label UCCM-LPS it "should work".
Joining the two resulted in:
UCCM >syst:status?
57964-86 serial number S35DB68332 firmware ver 1.0.0.1-01 Ext
mode
Reference Status __________________________ Reference Outputs
XX Ext Ref : [LOS]
TFOM 2 FFOM
3
UCCM A Status[OCXO WARMUP]
GPS: [phase:-2.8E-09, settling]
ACQUISITION ...................................................[GPS 1PPS
Valid]
Tracking: 8 ____ Not Tracking: 3 ________ Time
PRN El Az C/N PRN El Az GPS 13:28:51 23 Dec
2021
15 71 212 37 5 6 195
24 45 272 43 30 7 85 ANT DLY 77 ns
17 28 97 37 19 19 124 Position
13 55 140 38 MODE Hold
14 39 58 39
23 33 291 44 LAT N 51:xxx
10 17 324 40 LON E 5:xxx
12 7 212 34 HGT +27.54 m
(MSL)
ELEV MASK 5 deg
Command complete
Unfortunately it never went GPS locked. Longer story short: the pin of the
OCXO that (I think) should produce a Vref for the EFC circuitry produced
some random ~ 266mV.
A quick experiment learned that the OCXO freq responds to the EFC voltage.
So, looks like the Vref circuit in the OCXO has died.
Now the question: has anyone a working unit that they can measure the
correct Vref voltage from?
For now I have fitted a 2.5V LM336 vref and I
see the EFC voltage go up and down a bit, with the output freq hovering
around 10MHz (+/- 4mHz roughly).
tia,
Wilko
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On Thu, 23 Dec 2021 14:42:27 +0100
Wilko Bulte wilko.bulte@xs4all.nl wrote:
A quick experiment learned that the OCXO freq responds to the EFC voltage.
So, looks like the Vref circuit in the OCXO has died.
A stupid side question: Why do have OCXO a Vref output in the
first place?
I can see that some form of reference might make stabilizing
the power in the crystal easier, but that still wouldn't make
it necessary to have an actual reference output.
And related to that: Would supplying the voltage reference
externally, in case of a broken Vref output, work for whatever
is inside that needs this reference voltage?
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
I've always assumed this is because they need to know the reference is
clean and under the OCXO manufacturer's control if it's to meet specs. If
the user had to supply the reference there's no knowing how clean it is.
Andy
www.g4jnt.com
On Thu, 23 Dec 2021 at 17:47, Attila Kinali attila@kinali.ch wrote:
On Thu, 23 Dec 2021 14:42:27 +0100
Wilko Bulte wilko.bulte@xs4all.nl wrote:
A quick experiment learned that the OCXO freq responds to the EFC
voltage.
So, looks like the Vref circuit in the OCXO has died.
A stupid side question: Why do have OCXO a Vref output in the
first place?
I can see that some form of reference might make stabilizing
the power in the crystal easier, but that still wouldn't make
it necessary to have an actual reference output.
And related to that: Would supplying the voltage reference
externally, in case of a broken Vref output, work for whatever
is inside that needs this reference voltage?
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send
an email to time-nuts-leave@lists.febo.com
To unsubscribe, go to and follow the instructions there.
On Thu, 23 Dec 2021 12:14:48 -0500
Bob kb8tq kb8tq@n1k.org wrote:
It’s a pretty good bet that the Vref is 5V
A quick google lead me to this forum post:
https://www.eevblog.com/forum/projects/a-look-at-my-symmetricom-gpsdo-(ocxo-furuno-receiver)/1325/
Looks like the Vref is supposed to be 2.5V
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
On 23 Dec 2021, at 18:47, Attila Kinali attila@kinali.ch wrote:
On Thu, 23 Dec 2021 14:42:27 +0100
Wilko Bulte wilko.bulte@xs4all.nl wrote:
A quick experiment learned that the OCXO freq responds to the EFC voltage.
So, looks like the Vref circuit in the OCXO has died.
A stupid side question: Why do have OCXO a Vref output in the
first place?
I can see that some form of reference might make stabilizing
the power in the crystal easier, but that still wouldn't make
it necessary to have an actual reference output.
Well, an external disciplining circuit might make good use of a oven stabilised reference, at least that is what I assume?
Disciplining circuit uses Vref from the OCXO for a DAC that in turn drives the EFC of the OCXO.
No doubt someone will correct me if that is a wrong assumption :-)
And related to that: Would supplying the voltage reference
externally, in case of a broken Vref output, work for whatever
is inside that needs this reference voltage?
Well, currently I have a 2.5V Vref externally connected. No smoke as of yet
Wilko
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-leave@lists.febo.com
To unsubscribe, go to and follow the instructions there.
On Thu, 23 Dec 2021 18:57:10 +0100
Attila Kinali attila@kinali.ch wrote:
On Thu, 23 Dec 2021 12:14:48 -0500
Bob kb8tq kb8tq@n1k.org wrote:
It’s a pretty good bet that the Vref is 5V
A quick google lead me to this forum post:
https://www.eevblog.com/forum/projects/a-look-at-my-symmetricom-gpsdo-(ocxo-furuno-receiver)/1325/
Looks like the Vref is supposed to be 2.5V
Ooops.. stupid me linked to the wrong page (too many tabs open)
https://www.eevblog.com/forum/projects/a-look-at-my-symmetricom-gpsdo-(ocxo-furuno-receiver)/1300/
Sorry about that.
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
On 23 Dec 2021, at 18:58, Attila Kinali attila@kinali.ch wrote:
On Thu, 23 Dec 2021 12:14:48 -0500
Bob kb8tq kb8tq@n1k.org wrote:
It’s a pretty good bet that the Vref is 5V
A quick google lead me to this forum post:
https://www.eevblog.com/forum/projects/a-look-at-my-symmetricom-gpsdo-(ocxo-furuno-receiver)/1325/
Looks like the Vref is supposed to be 2.5V
For the Symmetricom UCCM or for the Trimble?
These UCCM are form-fit-function compatible but that does not mean the circuits are the same. Or the OcXO.
Samsung also produced UCCM, of which I have 2, both working fine. Again different circuits, different GPS etc.
Wilko
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
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Hi
The main reason OCXO’s have a Vref is that an ovenized
voltage source is likely to be more stable over temperature
than what you can easily buy and mount on a PCB.
The gotcha is the common ground found on most OCXO’s.
Putting the oven current through that ground makes keeping
the Vref (and the EFC) well behaved a bit of a chore. For various
reasons OEM’s have a hard time dealing with multiple grounds
that have multiple purposes ….
Bob
On Dec 23, 2021, at 12:47 PM, Attila Kinali attila@kinali.ch wrote:
On Thu, 23 Dec 2021 14:42:27 +0100
Wilko Bulte wilko.bulte@xs4all.nl wrote:
A quick experiment learned that the OCXO freq responds to the EFC voltage.
So, looks like the Vref circuit in the OCXO has died.
A stupid side question: Why do have OCXO a Vref output in the
first place?
I can see that some form of reference might make stabilizing
the power in the crystal easier, but that still wouldn't make
it necessary to have an actual reference output.
And related to that: Would supplying the voltage reference
externally, in case of a broken Vref output, work for whatever
is inside that needs this reference voltage?
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-leave@lists.febo.com
To unsubscribe, go to and follow the instructions there.
Many of the older VTOCXOs have the internal reference voltage accessible
simply to use them stand-alone. A remote adjustment pot could be fed
from the reference, and set the fine tuning voltage, without needing any
other support from the system. Nowadays we typically tune from PLLs, and
such, and providing a stable reference is nearly trivial, but way back,
having a good reference right from the unit itself was (and still is) a
handy feature.
Ed