[time-nuts] New Subscriber, DIY GPSDO project (yes, another one)

[Bob kb8tq]

Hi

As far as I know there is no “closed form” solution to tuning a GPSDO.
It is very much a measure / tweak / measure / tweak sort of thing. 

That said, there are a lot of basic design constraints that are pretty
well known. Your DAC needs to have a pretty small LSB. Updates
are normally done once a second ( = each time the PPS is measured). 

Bob

> On Mar 7, 2020, at 5:07 AM, Gilles Clement <clemgill at gmail.com> wrote:
> 
> Hi,
> How do you optimally tune the control loop time constant ? 
> (Mine gets quite unstable when the update rate is slow - and the amplitude of the change step low - enough not to degrade the OCXO performance )
> Is there a method described somewhere (like the Ziegler–Nichols method for PID) ?
> Thx, 
> Gilles. 
> 
>> Le 3 mars 2020 à 18:28, Attila Kinali <attila at kinali.ch> a écrit :
>> 
>> On Tue, 3 Mar 2020 12:14:37 -0500
>> Jim Harman <j99harman at gmail.com> wrote:
>> 
>>> I don't understand why you say the DAC should have a resolution of 24-30
>>> bits. I can see that the loop time constant affects the precision needed in
>>> the filter calculations, but what does the time constant have to do with
>>> the needed DAC resolution? We don't have to wait for the whole time
>>> constant before changing the DAC, we can update the filter calculations and
>>> look at its output every second and adjust the DAC whenever the PI filtered
>>> phase error is one DAC step or more.
>> 
>> You do wait the whole time before updating the DAC value.. kind of ish.
>> The control loop's time constant is exactly that: The time it takes
>> the control loop for a change in the input to affect the output (very
>> loosely speaking). Yes, the sample rate at which the loop runs is
>> much higher, but that doesn't change the fact that the loop is slow
>> to react. And you want it to be slow to react, as otherwise the high
>> noise of GPS degrades the performance of your OCXO.
>> 
>>> If the OCXO has a tuning range of 1 ppm and we want frequency control
>>> of 1e-12, wouldn't that require a DAC with 1e6 steps or 20 bits,
>>> assuming the DAC covers the full tuning range of the oscillator?
>> 
>> Yes. There is a calculation mistake in there. I corrected it in
>> the next mail: 
>> http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2019-October/097963.html
>> 
>> 			Attila Kinali
>> 
>> -- 
>> <JaberWorky>	The bad part of Zurich is where the degenerates
>>               throw DARK chocolate at you.
>> 
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