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
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