[time-nuts] using Fury GPSDO with Rb

Scott Mace smace at intt.net
Wed Apr 9 10:09:14 EDT 2008


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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