[time-nuts] Using cheap sound cards for measurements

[John Miles]

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.

-- john, KE5FX

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Lux, Jim (337C)
> Sent: Friday, August 21, 2009 6:45 AM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] Using cheap sound cards for measurements
>
>
> The idea of using a standard stereo sound card interface to do
> Allan deviation measurements has been discussed on the list in
> the past (i.e. Beat the two signals down to some convenient audio
> frequency, digitize, and find zero crossings by curve fits to the
> sampled data).
>
> Several have commented that one needs good isolation between the
> channels of the digitizer to get good results, and inexpensive
> interfaces (e.g. The one that comes on the motherboard) often
> don't have good isolation.
>
> Here's a question.. Is that coupling determinstic and
> "calibrate-out-able"?  Seems that the factors leading to lack of
> isolation are things like layout, capacitive coupling, shared
> ground paths, and the like.  If the card is in a constant
> environment, those shouldn't be changing, so, in theory, one
> could somehow measure it, and apply the inverse transformation.
>
> Jim
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>