[time-nuts] Nortel GPSDO osc age alarm

Fri Jan 24 11:49:42 EST 2014

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.

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?

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?

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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