[time-nuts] Using cheap sound cards for measurements

John Miles jmiles at pop.net
Fri Aug 21 21:01:22 UTC 2009


> > If isolation turns out to be a problem, I imagine it would be
> practical to
> > use two separate sound cards.  It may not be practical to
> compensate for the
> > frequency-dependent effects of channel leakage, but timing/rate
>  differences
> > between two independent cards should be less important and/or easier to
> > calibrate out.  Unless I'm overlooking something, ADC timing precision
> > and  accuracy would be directly improved by the heterodyne
> ratio, just as
> > effective TIC performance is improved in a classical DMTD
> > configuration.
> >
>
> Wouldn't the cards need to be synchronized, though?  Essentially,
> you're still  comparing the two signals with each other, just
> doing it in software, rather than in hardware, in the classical
> time interval counter scheme counting 1Hz (or 123Hz).  Syncing
> inexpensive cards is a real chore (and the only reason to be
> thinking about using this in the first place is to keep the cost
> to a minimum, otherwise, you might as well build a special
> purpose little box with counters & A/Ds, and an interface)

I'm not sure it's that important (or helpful) for the ADCs to share a common
clock.  Presumably the ~100 Hz beatnotes being digitized are on the order of
1/100000 of the frequencies being measured.  That means that a microsecond
of synchronization error between the ADCs would have an effect similar to a
picosecond-scale error on the DUT/reference sides of the mixers.

Getting microsecond precision out of an audio ADC is going to require
processing multiple successive samples, and IMHO it will also require some
kind of auto-calibration scheme since sound-card clocks probably drift more
than 1 ppm per minute or so anyway.  Given the need for autocalibration --
probably through a high-frequency sidetone sent to both channels in phase --
the difference in complexity between supporting two ADC clock domains and
one is probably not a deal-killer.

Most installations would probably need to use a beatnote frequency closer to
1 Hz, so that would take a lot of pressure off the ADC clocks.  It *might*
be enough to get you out of the autocalibration business, but my guess is
that matching the phase tempco of the (AC-coupled) sound card inputs might
still be necessary for good long-term results.

-- john, KE5FX






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