[time-nuts] using Fury GPSDO with Rb
Scott Mace
smace at intt.net
Fri Apr 11 11:42:35 EDT 2008
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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