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

[Matthias Welwarsky]

On Samstag, 7. März 2020 15:35:22 CET Tobias Pluess wrote:
> Hi,
> I am sure it is theoretically possible to find an optimal control loop
> design if you have accurate models of the OCXO and for the behaviour of the
> 1PPS pulse.
> TvB has written a GPSDO simulator which allows to code different PLL and
> control algorithms. I have implemented something similar in Matlab.
> Basically, if you know the VCO tuning characteristics and the phase
> detector constant, then you can use procedures as described in the PLL
> books that are out there (I have ond from R. Best).

Funny, that was also my approach. I have no idea about control theory or PLL 
design so I'm not able to apply any prior knowledge here, but converting 
"gpsim1.c" to Matlab (or Octave, in my case) was pretty much straight forward 
and having a math toolkit under the bonnet is much more versatile than a C 
compiler and runtime.

> Maybe the easiest way is indeed using the GPSDO simulator.
> If there is some interest, I could perhaps post my Matlab code. It plots
> nicely the calculated EFC voltage and simumates the OCXOs frequency
> variation as the EFC is varied.

The GPSDO simulator is fine for sanity-checking certain design parameters, 
like, if the width of your DAC is sufficient for the LO you're targeting. The 
question of "how many bits" you need can be visualized nicely with the 
simulator. However, you'll need model data for the GPS and the LO and you have 
to be aware that whatever optimal filter and control loop design and 
parameters you work out, will have to work outside the limited space your 
model data presents.

BR,
Matthias

> 
> Best
> Tobias
> HB9FSX
> 
> On Sat., 7 Mar. 2020, 12:36 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/0979
> > 63.html> 
> > >                       Attila Kinali
> > > 
> > > --
> > > <JaberWorky>  The bad part of Zurich is where the degenerates
> > > 
> > >                throw DARK chocolate at you.
> > > 
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