[time-nuts] DAC for OCXO disciplining

[Mike Ingle]

agreed.
I should have looked closer, I was remembering a control loop I worked with
which implemented the DAC by either adding a quanta of charge to a cap or
removing a quanta of charge, and hand very high effective bits and worked
well.  It only "glitched" when updating.  I had thought that was what the
circuit I linked was doing.

--mike



On Tue, Jan 14, 2020 at 1:51 PM Magnus Danielson <magnus at rubidium.se> wrote:

> Hi,
>
> On 2020-01-14 02:00, Hal Murray wrote:
> > lifespeed at claybuccellato.com said:
> >> Some thoughts that have occurred to me are coarse and fine DACs,
> possibly
> >> sigma-delta or pulse width modulation (PWM).
> > Pulse width modulation has noise at the frequency of the whole
> sequence.  A 10
> > bit DAC running at 1 megahertz will have noise at 1 kilohertz.  As you
> add
> > more bits to get better resolution, the noise frequency gets lower.  For
> > practical values, that noise may be hard to filter.  What doesn't get
> filtered
> > turns into spurs.
> >
> That is why PWM is not a very smart idea at all for these purposes.
>
> I've done a variant where I inverted the spectrum of PWM. I sketched on
> an article but I never finished it.
>
> You want to place your most significant bit energy as high as possible,
> and the same for the second most significant bit. Turns out that this
> becomes fairly simple counter and mux solution which on average achieves
> exactly the same level as the PWM, but with noise which is much easier
> to filter and with about the same level of complexity as the PWM.
>
> But then, you can do first degree sigma-delta and achieve about the same
> thing.
>
> But PWM for this, is not a good idea, it's in fact a terrible idea since
> it has the most significant bit with the lowest frequency which becomes
> hard to filter.
>
> Cheers,
> Magnus
>
>
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