[time-nuts] using Fury GPSDO with Rb

SAIDJACK at aol.com SAIDJACK at aol.com
Wed Apr 9 02:06:23 EDT 2008


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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