[time-nuts] using Fury GPSDO with Rb

SAIDJACK at aol.com SAIDJACK at aol.com
Fri Apr 11 13:30:28 EDT 2008


Hi Scott,
 
interesting, that's a very small control range.
 
That would explain why the Fury had a hard time locking, but seemed to  work 
fine once locked.
 
It's helpful to see how the units' output frequency behaves when  setting the 
dac manually between 0V to 5V using the serv:coarsedac xx  command.
 
Please let us know how you get it to work with a  different Rb,
 
bye,
Said
 
 
In a message dated 4/11/2008 08:44:27 Pacific Daylight Time, smace at intt.net  
writes:

Turns  out this unit has been set so that the EFC was only useful from within 
a .5v  range
centered around 2.5v, also the output was TTL.  Trying with a  different Rb...

Scott

SAIDJACK at aol.com  wrote:
> Hi Scott,
>  
> a good setting is when the  unit recovers in minimal time and with  only 
minor 
> overshoot  from a manual coarsedac change.
>  
> Try dacgain 1000 -  2000.
>  
> TI of 1.2ns / SD 2.2ns is not bad. The M12 errors  can be expected to be  
> larger than this over 8  hours.
>  
> bye,
> Said
>  
>   
> In a message dated 4/9/2008 07:06:45 Pacific Daylight Time,  smace at intt.net 
 
> writes:
> 
> I've  been running  with dacgain 4000, efcscale 1.2, efcdamping 25, 
> phaseco   15.  It has been locked for about 8 hours (since I started the   
> test).  average TI 1.2ns sd 2.2ns.
> 
> I tried  bumping the  coarsedac by 2 increments to see what happens.
> It  tries to recover, but  will oscillate wildly after about 45min or  so.
> I wonder what will happen  after an extended holdover  period.
> 
> It looks like the EFC on this this  FRS-A is  more than twice as sensitive 
> as the LPRO.
> 
>  Scott
> 
> 
> 
> SAIDJACK at aol.com wrote:
>>  Hello  Scott,
>>  
>> forgot to mention: also try  setting the  serv:efcdamping lower. It's an 
IIR 
>   
>> low pass filter, and will  introduce phase-shift that can  cause 
instability 
> if 
>> it  is set  too  high.
>>  
>> bye,
>>  Said
>>   
>>  
>> In a message  dated 4/8/2008 22:59:22 Pacific Daylight  Time,  
>  SAIDJACK at aol.com 
>> writes:
>>
>>  Hello   Scott,
>>
>> this means the gain is set  too high so that there is  not enough  phase  
>>  margin.
>>
>> You can try to  set the serv:dacgain  lower, and  serv:efcs / serv:phaseco 
 
>>   lower.
>>
>> Contrary to OCXO's, Rb's are  quite stable  so  they don't need strong  
>> control, 
>> they  just need a   gentle nudge, so you can try starting with low   
numbers.
>>
>>  Let me  know if that  works,
>> bye,
>> Said
>>
>>   
>>
>>
>> In a message dated  4/8/2008  21:31:50 Pacific  Daylight Time, 
> smace at intt.net    
>> writes:
>>
>>  I've  found that once  the units gets into an  overshoot/oscillation  
cycle
>> it  never  recovers and I have to manually  adjust the  dac to  break the 
> cycle.
>> Is the  notion that you  use   conservative enough settings so that this
>>  never    happens?
>>
>>  Scott
>>
>> SAIDJACK at aol.com  wrote:
>>>  Hi   Scott,
>>>  
>>> let me   reply on this list since other users  are  also using   Rb's.
>>>  
>>> Here is a quick    pointer:
>>>   
>>> 1) determine EFC slope  of Rb  (pos =  frequency goes higher with  higher 
  
>> EFC  
>>> voltage). Set the  slope with  serv:efc:slop   [positive/negative]
>>>   
>>> 2)  Determine maximum  frequency  deviation at  EFC = 0V (serv:coarsedac  
> 0)   
>>> using  a frequency  counter
>>>    
>>>   
>>> 3) Determine maximum  frequency   deviation at EFC = 5V  (serv:coarsedac  
  
> 255)
>>>   
>>> 4) Set   Dacgain  according to table in user manual depending on the    
>>  frequency  
>>> difference between 2)  and  3)
>>>   
>>>  5) Set  Integral part of loop to  zero: serv:phaseco 0.0, this  helps  
for  
>  
>>  now
>>>   
>>> 6) Set Proportional part  of loop to a  number  that  makes the unit lock 
> as  
>>>  quickly  as  desired without oscillation  and overshoot (for  example:   
 
>> serv:efcs 1.0 
>>> to  serv:efcs  6.0
>>>   
>>> 7)  Set  the integral part of the loop so that the   phase is driven to 
0ns  
>  
>> UTC  
>>> offset as quickly   as desired without   overshoot/oscillation, for 
>  example:   
>>> serv:phaseco    20.
>>>  
>>>  Repeat 6) and 7) after  some  hours.
>>>   
>>>  Hope that  helps,
>>> bye,
>>>   Said
>>>   
>>>>  -----Original  Message-----
>>>>  From:  Scott  Mace
>>>>  Sent:  Saturday, April 05,  2008   17:51
>>>> To: Said   Jackson
>>>> Subject:  Re:   Fury    GPSDO
>>>>
>>>>
>>>> Said,   I finally  got  my fury up  and  running, I'm using a  efratom  
>>>> FRS-A and  also a   LPRO-101.
>>>> Do  you  have any recommending  starting  points for  tuning the  servo 
>>>>  for a  rubidium     unit?
>>>>
>>>>   Scott
>>>>   
>>>   
>>>  
>>>
>>>
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