[time-nuts] Nortel GPSDO osc age alarm

Volker Esper ailer2 at t-online.de
Fri Jan 24 21:31:58 UTC 2014


New ways of RF design - see photo ;-)

Ok, I put three 1N4148 in series between DAC out and EFC in and a pull
up of 4k7 up to 12V. It shifts the DAC voltage 1.8 volts up, so the osc
is able to tune to 10MHz+3Hz. Locked!

Now I can go back to the sofa, having a beer or two, being sweet again
to my wife and my dog, :-)

Thanks

Volker


Am 24.01.2014 17:49, schrieb Volker Esper:
> 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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