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

[Attila Kinali]

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

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