[time-nuts] Question for my new GPSDO

[Attila Kinali]

On Wed, 16 Oct 2019 15:04:31 +1300 (NZDT)
Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> Consistency of the glitch independent of DAC output is more important than 
> its size. A constant amplitude glitch occurring at the dac update rate is 
> more benign in its effect than a glitch whose amplitude varies with DAC 
> output.

Yes. But it's actually not that bad. The glitch in the DAC is due to
the CMOS switches in the DAC switching. Due to symmetry of the operation
the glitch going from code A to code B will be the (almost) inverse of going
from code B to code A. Which means they should (almost) cancel.

I can't find my notes from when I calculated this, but I remember that
using an external CMOS switch had the same order of magnitude of uncertainty
as using the DAC itself. And a few other effects would be also very close
in magnitude. My conclusion was, considering that the back-of-envelope
calculation of errors would result in a few bits more than necessary,
the added complexity of using an external CMOS switch was probably not
worth it. Of course, to be sure one would need to build both variants
and measure them. Unfortunately, I don't have the means to do that.

Also, my guesstimate would be that using two DACs with metal-foil
resistors for weighting would probably result in lower non-linearity.
But the problem here is that the resistors add quite a bit of noise
which in turn has to be accounted for.

			Attila Kinali

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