Don't know if this helps, but many older digital voltmeters used
line-synchronous dual-slope sampling to prevent this king of aliasing.

You may need a multi-pole filter in front of the zero-crossing detector,
though.  Otherwise, noise on the line near zero crossing will cause phase
errors from cycle to cycle, and defeat some of the filtering.

Karl Cunningham


--On Saturday, February 09, 2002 12:50 -0500 ERIC  GUINN
 wrote:

> Hey all.
> 
> I'm trying to get my ducks in a row for my first LP station with computer
> logging, and I'm pulling my hair out trying to find an old piece of data.
> Could someone please help?  Where have I heard of a sampling rate of 60Hz
> (or 30Hz?) which was synchronous and zero-crossing-locked with line
> frequency to allow elimination of 60Hz and aliases without multiple-pole
> filters?  I think I'd like to replicate this bit of Kung-Fu in my design.
> Did I just make this up or did Larry or someone publish an article like
> that?
> 
> Thanks in advance,
> Eric Guinn, AC4LS
> Sevierville, TN

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