[time-nuts] Re: Tuning a GPSDO loop for optimal disturbance handling

[Erik Kaashoek]

Hi Bob,
By your advice I went for a 6.5 digit DVM and after logging and plotting 
the DAC output its clear there are some stability issues in the DAC 
output. The voltage is wandering around at about the level of frequency 
wandering observed.
A different supply topology for the DAC and VCXO will have to be 
created.. The DAC resolution is 50 uV (2.048 V / 400000 steps) but the 
random variations are about 4 times p-p larger.
Once this is done I hope longer term logging of frequency and voltage 
will make sense.
Or is it better to have a low pass filter between the DAC and the VCXO 
Vtune input so the loop is fast enough to remove the remaining drift?
If so, what should the time constant of the filter be compared to the 
intersect of the ADEV of the GPS and the VCXO (100 s)?
Having a big time constant (10s?) will be a pain in the initial tuning 
and will require active components.
Does owning a 6.5 digit DVM qualify one as a volt-nut? Or should first 
some voltage references be added?
Erik.

On 5-3-2022 20:50, Bob kb8tq wrote:
> Hi
>
> One suggestion on the frequency plot:
>
> TimeLab as you have it set does a “per pixel” process on its charts. You have
> a limited screen resolution and a ton of data. You have to do something. There
> is no way to display it all.
>
> It’s a good idea to zoom in to some of the “problem regions” and see
> what is really going on. This reduces the number of data points that
> map to one pixel and potentially changes what you see a bit. The closer you
> get to one data point per pixel, the closer you are to “reality” without any
> processing potentially getting in the way.
>
> On the DVM, eBay (with some time spent shopping) will sell you a working
> 6 1/2 digit device for < $100 if you are patient. If you need it now, it’s still
> likely to be below $200. Yes it will be a big clunky box that takes up bench
> space. It will be HPIB and not serial i/o. The display might be a bit fun in
> bright lighting.
>
> While you are shopping, I would get set up for phase noise as well. Often
> it can spot issues that are tough to find with a second to second data stream.
> The setup does not have to be very fancy or terribly expensive.
>
> Bob
>