[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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