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

[Bob kb8tq]

Hi

Ideally in a digital control loop you want at least 2 “extra” bits past the 
minimum required to hit your target accuracy. Things like noise, LSB
non-linearity, calculation roundoff, all drive that need. Indeed for a 
low noise loop, you would push the margin out even further. 

If your OCXO’s EFC is not perfectly linear (they never are) then it’s 
“slope ratio” gets into the act. If the highest slope is 2X the lowest 
(which is not uncommon), that will add another bit to the total. 


Bob

> On Mar 3, 2020, at 12:14 PM, Jim Harman <j99harman at gmail.com> wrote:
> 
> Attila,
> 
> In your post from October you say,
> 
> What, I think, you haven't had a look at is the resolution of your DAC.
>> You get, including your resistive divider a 17bit resolution. But this
>> is not enough. You want to be able to control the OCXO such, that at
>> the loop time constant, you have less than 1LSB offset in frequency.
>> Usualy people aim for something in the order of 1000s as loop time
>> constant,
>> often even longer than that. Assuming your GPS receiver gives you
>> approximately
>> 1ns RMS jitter (probably worse than that, but it makes it easy to
>> calculate)
>> that would mean frequency control of the level of 1e-12 is required.
>> Assuming
>> your OCXO has a tuning range of 1ppm (I've seen 0.1 to 20ppm for OCXOs)
>> that would mean you have to controll the EFC voltage better than parts in
>> 1e-9
>> or 30 bits. Yes, this is kind of unreallistic, but that's what the design
>> should aim for. If you acheive something around 24bits, you will be
>> probably
>> close enough. (Note: that's 24bit resolution and stability over the
>> loop time constant. It is not 24bit absolute resolution)
> 
> 
> 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.
> 
> 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?
> 
> Thanks for any explanation you can provide.
> 
> 
>> 
> --Jim Harman
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