[time-nuts] Nortel GPSDO osc age alarm
GandalfG8 at aol.com
GandalfG8 at aol.com
Fri Jan 24 19:27:20 UTC 2014
In a message dated 24/01/2014 17:06:43 GMT Standard Time,
ailer2 at t-online.de writes:
Thanks so much, Nigel, for this very interesting mail.
Yes, the voltages are exactly as my ones. And yes, I've removed the
oscillator, already. I then removed the thermal isolation from the osc,
as well as the label, in slight hope for a hidden tuning screw. Of
course, there isn't any.
Nope, what you see is what you get, planned obsolescence wins again:-)
I did find the thermal insulation to be quite useful in reducing the
effects of drafts, as carefully observed by blowing on it:-), so well worth
keeping in place once all is up and running again.
I, too, had this idea to level shift the EFC voltage. So I took seat at
my computer to find out, which maximum EFC voltage would be ok. And saw
your mail. Do you have any further information about the osc? The number
on the sign (Trimble *0110-2450-T* *34310-T*) isn't really informative.
By the way: When removing the big Trimble label I found a smaller one
beneath, reading "DOC2127 0101/1940"; I think it's a Tekelc osc?
This is a comment from page 26 of the Nortel GPSTM General Spec available
from Didier's site......
-----------------------
XXX will identify which OCXO was used in the particular device.
XXX is to be either "T", "Oak", or "T2".
The T identifies the Tekelec Doc-1903 device and the "Oak" indicates the
OFC-4895 device from Oak Frequency Control Group.
A "T2" will indicate the Tekelec Doc-2127 device.
------------------------
-----which is interesting as it suggests your oscillator to be a T2 even
though it's marked "T".
I'll have to take a closer look at mine:-)
Is there a pull-up resistor behind the EFC input pin so that I could use
two diodes for level shifting? Or how did you?
I've got no information on the internal circuitry so I just assumed it
would be a reasonably high impedance and used a single IC as a non-inverting
unity gain summing amp powered from the onboard 12V supply for the
oscillator to add, in my case, approximately 2V to the DAC output from the circuit
board.
I used a TL071, but only because that's what I had to hand, with 12K
between the output and neg input and another 12k from the neg input to ground,
then two 12k input resistors feeding into the pos input, one of which was
connected to the EFC output from the board and the other biased very
approximately to 2 Volts using a 4k7 and a 1K resistor as a divder across the 12V
supply. Again no particular significance to the 12K, I just had some within
easy reach. The output from the op amp was then used to feed the EFC
voltage to the oscillator, and that was it.
I hope that makes sense:-)
In this instance I didn't make any attempt to further stabilise the supply
to the op amp or the 2V "reference" but it was only intended to prove the
point and I figured the control loop could take care of any small
variations, and it actually worked quite well.
Since then I've added the SMB connectors to the board and I'm running the
replacement oscillator via a couple of pigtails and no offset for that one.
Next step, assuming I ever get round to it:-), will be a proper enclosure
with connections for an external oscillator, and perhaps even internal
switching via a latching coax relay to allow internal or external options but
will wait and see on that.
Given the control limitations I mentioned earlier though I'm now
considering perhaps leaving the other Nortel board as is and instead putting the
effort into implementing a similar arrangement for a Thunderbolt, which of
course has already been done by John Miles as referenced here....
http://www.ke5fx.com/tbolt.htm
Sooooo, although I didn't feel over happy about having a "faulty"
oscillator to start with, I'm actually very pleased with the way it turned out:-)
Regards
Nigel
GM8PZR
This part is more thrilling than I expected...
Regards
Volker
Am 24.01.2014 12:08, schrieb GandalfG8 at aol.com:
> Hi Volker
>
> One possibility, as I found with one of these recently, is that your
> oscillator has aged such that the required EFC voltage for 10MHz output
is now
> outside the range provided by the board.
>
> The EFC behaviour can be tracked if Lady Heather is enabled from switch
on
> and the DAC voltage monitored.
>
> The EFC voltage should start at 3 volts, in my case reported by LH as
> 3.000002 Volts, and will sit at this level until enough satellites are
being
> tracked for the board to decide it can start the conditioning process,
at
> which point the DAC voltage will ramp upwards over the course of a few
seconds
> until the point is reached where the oscillator frequency crosses 10MHz
and
> the control loop takes over.
>
> All standard stuff of course but, as you've discovered, if the
oscillator
> hasn't reached 10MHz by the time the EFC voltage reaches, in my case
> anyway, approximately 5.6 volts the board's "OSC" report switches to
"BAD" and
> "Normal OSC age" switches to "OSC age alarm" and LH highlights both in
red.
> The EFC voltage finishes its ramp at 6 Volts, reported as 6.000004
Volts,
> and then just sits there, all exactly as you're seeing.
>
> Having removed my "faulty" oscillator, something that needs to be done
> VERY carefully to avoid damage to the board, I found that it required
an EFC
> voltage of just under 6.6 Volts for the the output frequency to reach
10MHz.
>
> The seller, fluke.l, was very helpful as always and offered to supply a
> replacement 34310-T oscillator, this one did have it's required EFC
voltage
> in the correct range and resolved the problem.
>
> However, whilst waiting for the replacement to arrive I left the
original
> oscillator on test and monitoring it for a while suggested that the
only
> thing "wrong" with it was the required EFC voltage, so I reconnected it
to
> the board using a wired lash-up on the bench with a simple 2 Volt level
> shifter inserted into the control loop and did indeed get a locked
condition
> with Lady H reporting the DAC voltage from the board as close to 4.5
volts.
>
> This suggests that whilst the board design requires the oscillator EFC
> Voltage at 10 MHz to be between 3 and 6 Volts an oscillator that falls
outside
> this range shouldn't automaticall be assumed to be "faulty" in more
> general terms, although it's obviously getting a bit long in the tooth
and that
> doesn't help much if it happens to be soldered into your circuit board !
>
> Whilst it is possible to remove and replace the original oscillator
there
> is an alternative to physical replacement, one which I've now
implemented,
> and this uses the mounting positions already available beneath the
> oscillator, not usable though until it's removed, to fit a couple of
SMA or SMB
> connectors.
> These two connectors couple the 10MHz signal from the oscillator into
the
> board (J9) and the EFC Voltage out to the oscillator (J10).
> Although there is a regulated supply available from the oscillator,
which
> might be expected to supply the EFC circuitry, in practice this
connection
> does not seem to be required.
> I have not investigated further as yet to determine whether the board
auto
> senses and uses this supply if it is present, or whether it always just
> ignores it, but it doesn't seem to be an issue either way and it's
interesting
> to note, at the extremes anyway, that LH does report the DAC voltage to
be
> the same in both instances.
> The oscillator can be powered from the board or given its own supply,
in
> which case the only required connections between them are the two coax
leads.
>
> Using this arrangement, and some variation of a positive or negative
level
> shifter if required, it becomes possible to use different oscillators
with
> the Nortel board and it does become an even more interesting toy.
>
> One limitation though when using the Nortel boards in this way is that
> they don't seem to share the versatility of the Thunderbolt when it
comes to
> modifying the oscillator conditioning parameters.
> It's suggested in the LH documentation that such commands are currently
> "undocumented" but that also leaves the possibility that any such
change
> might only be a firmware option.
> If anyone has further information on this that would be much
appreciated.
>
> I've been sufficiently impressed with the modified unit that I'm
seriously
> considering modifying another one, even though it doesn't actually
"need"
> it, although not over enthusiastic about repeating the oscillator
removal.
> However, an added bonus with this configuration is that it's also more
> forgiving when it comes to any damage caused during removal of the
original
> oscillator, since the pads for the external connectors are separate
from the
> internal oscillator pads. Obviously it needs to be ensured that any
> necessary continuity is maintained but that could be easier if a board
mounted
> oscillator is no longer required.
>
> The later single board unit, the NTBW50AA, has a similar external
connector
> arrangement but in this case the connector pads are available without
> removing the original oscillator.
> This hints at the possibility that perhaps a simple track interruption
> might allow fitting of an external oscillator with the original still
onboard
> but examination suggests that the onboard connections route first to
the
> internal oscillator so that would probably still need to be removed.
>
> Regards
>
> Nigel
> GM8PZR
>
>
>
>
>
> In a message dated 23/01/2014 23:59:34 GMT Standard Time,
> ailer2 at t-online.de writes:
>
> Hi!
>
> I bought a Trimble/Nortel GPSDO
>
>
http://www.ebay.de/itm/300933951405?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m
> 1497.l2648
>
> and Lady Heather's now tells me that everything is alright - except
> - DAC 6.000004V
> - OSC BAD
> - osc age alarm
>
> The rectangle "10MHz" output signal (J5) shows a signal at about 9.8MHz
> (a deviation of about 200kHz), wobbling 4Hz up and down. Oddly enough,
> the direct oscillator output (J4) shows a sine wave at a stable
> 10.0000004MHz (a deviation of 0.4Hz).
>
> It seems to me it's not the oscillator that is bad but the servo loop -
> what can I do?
>
> Thank you
>
> Volker
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