[time-nuts] EFC tracking

[John Miles]

> >sound card ADCs, the high end 24 bit ones, are pretty darn linear [...]
>
> That is actually a very debatable proposition, a lot of them are
> tracking types that conveniently cover up any lack of linearity
> on the analog side of the fence.

Can you elaborate on that?  Linearity is linearity - wouldn't it ultimately
show up in the card's IMD specs?

> The major problem with using sound ADC's is that their references
> has absolutely no long term stability, so you will see your EFC
> wiggle and wander all over the place, even when it stays perfectly
> still.

One idea: the sound card gives you at least two input channels to work with,
and it isn't much of a stretch to assume they share a common reference.
Perhaps you could use one channel to digitize the oscillator's supply
voltage or (ideally) EFC reference voltage output, using the same
chopper/mixer/VFC/whatever approach, and have the software take the
difference out.

> I would find one of those cheap-ish DVM's with a serial or USB port...

I'd be surprised if a cheap DVM outperformed a decent sound-card ADC in any
respect, other than perhaps DC baseline drift.

-- john, KE5FX