Hi Ignacio
I have removed a "faulty" oscillator from one of these, not one of my more
relaxed moments and quite amazed when the board emerged totally undamaged,
but proof at least that it can be done:-)
With the oscillator removed there's access to two sets of connector pads
that will either accept SMA or SMB connectors and after conducting tests on
the original oscillator via flying leads just soldered to the board I
decided not to fit a replacement to the board after all but to fit a couple of
SMB connectors to make the setup more versatile, and to avoid the
possibility of having to remove another oscillator in the future:-)
Another advantage of these pads is that if the board does get damaged
removing the oscillator they can still be used as an alternative.
The connectors take the 10MHz from the oscillator into the board and the
EFC control voltage out from the board to the oscillator, now there's a
surprise:-), and oscillator power can be completely external if required.
>From this it would seem that the reference supply from the oscillator
itself is not used in practice, certainly not in the "offboard" case anyway.
As others have suggested it seems likely your problem may not be the
oscillator itself, but it still might be worth removing anyway to make testing
and fault finding more straightforward.
The maximum positive excursion of the NTGS50AA should be 6 volts, not 5 as
you're seeing, and another indication it might be worth removing the
oscillator to see how the board behaves stand alone.
I've not seen what seemed to be the repeated attempts at lock that you
mentioned previously, but then I wasn't even aware for a long time that the
control voltage could drive below 3 volts as well as above it:-)
This is my note from previous observation of my "faulty" unit----
-------------------------------------------------------------
When first powered it brings up all LEDs and then switches to a green LED
for a few seconds and then amber. It starts a self survey and acquisition
process with all appearing ok, and the DAC voltage reported as 3.000002
volts.
Sometime later, I've seen as short as 6 minutes but as long as 12 to 15
depending on how long the oscillator has been turned off and allowed to cool,
once enough satellites are being tracked, the DAC voltage starts to
increase, presumably seeking to drive the oscillator frequency to 10MHz, but
the
frequency doesn't reach 10MHz and the DAC voltage ramps up to 6.000004 volts
over a period of approx 30 seconds where it remains.
As the DAC voltage crosses approx 5.6 volts the Red "fault" LED is
switched on, as opposed to green that would normally be expected to indicate
all
was well, and Lady Heather's "OSC:" report switches from Good to Bad and
highlights red. Similarly "Normal OSC age" changes to "OSC age alarm" and also
highlights red.
---------------------------------------------------------------
I've attached a Lady H plot that shows this, hopefully it will get through
OK.
The above DAC voltages were as reported by Lady Heather but I've checked
these and, when the board is working as it should be anyway, they're very
close.
In my case the problem described above was an oscillator that had aged
beyond the upper 6 volt limit, needing approx 6.54 volts to reach 10MHz, and
once removed from the board I was able to add a simple op amp level shifter
to bring it back into range just to prove all else was ok, which it was,
but obviously Lady H now indicated the EFC into the level shifter rather than
at the oscillator itself.
Whilst your problem sounds like it might not be quite such an obvious fix,
removing the oscillator would open the loop and make testing both the
oscillator and the board much easier, so much as it's a pain I do feel that's
probably your best next step.
Regards
Nigel
GM8PZR
In a message dated 12/11/2014 02:06:54 GMT Standard Time,
eb4...@cembreros.jazztel.es writes:
Hi,
Removing the oscillator for testing and replacing it with other if it
was the culprit was my first option. I have a spare Trimble oscillator
that probably came from other NTGS50AA since it still have the foam band
attached, but this oscillator is really aged, it needs 7.91 V to bring
it on spot and the maximum control voltage of the NTGS50AA is 5 V.
I was trying to avoid removing the oscillator but probably it must be
done to clarify things.
Thank you,
Ignacio EB4APL
.
El 12/11/2014 a las 2:40, Mark Sims escribió:
I have seen this caused by the oscillator not responding to the EFC
signal. Fixed it by swapping in a MV-89 oscillator.
The oscillators used in these units don't output an oven temperature
monitor signal.
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