[time-nuts] NCOCXO anyone?

Attila Kinali attila at kinali.ch
Fri Jul 22 05:33:12 EDT 2016


On Thu, 21 Jul 2016 22:22:14 -0500
David <davidwhess at gmail.com> wrote:

> Increased integration has only helped insofar as you can attempt
> designs which would have been prohibitive before.
> 
> I keep trying to come up with a charge balancing design but what about
> Linear Technology's solution from back in 2001?
> 
> A Standards Lab Grade 20-Bit DAC with 0.1ppm/°C Drift
> http://www.linear.com/docs/4177

You can already get 24bit DAC's off the shelf from TI (DAC1282).
I do not know how stable they are in reality. I looked into high
precision DAC's a year or two ago and figured out that once you
cross the 20bit line, all kind of weird stuff happens that is
hard or almost impossible to compensate for. The trick with using
an ADC (which are available up to 32bit) doesn't really work either,
as offset drifts, thermal voltage etc are hard to impossible to
compensate completely. If you go through the volt-nuts mailinglist,
you'll see how much effort it is to even keep 1ppm (~20bit) stability
of a voltage reference... and then to get that performance out of a DAC.

If anything, I think the better approach is to use high resolution DAC
like the DAC1282 or maybe the DAC1280 with a custom modulator and put
it inside the control loop such that the real measurement happens in
the frequency/time domain. The results from Sherman & Jördens[1]
seems to indicate that sub-100fs stability should be possible, though
there are a couple of open questions there.

			Attila Kinali


[1] "Oscillator metrology with software defined radio",
by Jeff Sherman and Robert Jördens, 2016
http://dx.doi.org/10.1063/1.4950898
(it's available from NIST as well)
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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