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: [email protected] [mailto:[email protected]]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 -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
