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

[Gilles Clement]

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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