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

[Mete Balci]

A few weeks ago I was working on a prototype containing a MAX5443 single
supply 3V 16-bit DAC in a circuit very similar to the one in its datasheet,
using another MAX voltage ref. and a buffer and as far as I remember the
output was pretty stable.

Mete

On Tue, 22 Mar 2022 at 20:10, Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> > On Mar 22, 2022, at 12:14 PM, Erik Kaashoek <erik at kaashoek.com> wrote:
> >
> > 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.
>
> That’s a very typical result as you dig into the DAC ( = the DAC isn’t
> perfect ).
>
> > 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?
>
> You can go to a stable ADC and let it “servo” the DAC. That’s going to add
> cost and
> (likely) some noise from the ADC. The most common outcome is to find
> another DAC
> and possibly a better reference.
>
> > 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.
>
> There’s *always* a way to spend more money on bench gear :) :) :). In
> terms of this
> project, what you have sounds like it’s doing the job just fine.
>
> Bob
>
> >
> > 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
> >>
> >
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