[time-nuts] DAC for OCXO disciplining

[Magnus Danielson]

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