[time-nuts] Simple AC mains zero-cross detector

[Charles Steinmetz]

Hal wrote:

>What sort of interference do you see?

There is a general "grass" on the entire waveform.  At our location, 
the tops of the sine wave are clipped off (as the power is 
delivered).  See attached image (the orange trace is the AC we 
receive from the grid; cyan is the distortion residual from a 
distortion analyzer -- there are rich harmonics out to the 20th or so 
within -50dB of the fundamental).  The image is not properly scaled 
to show the high-frequency "grass."

>What does an interesting transient event look like?

It all depends.  They are not all that fast (unless your house feed 
gets struck by lightning, in which case "what does it look like in 
the data collection" is the least of your worries), and usually 
comparable in amplitude to the power signal +/- 10dB (again, unless 
there is a very close lightning strike), so they generate extra zero 
crossings spaced anywhere from low mS to tens of mS.

>If you are going to post-process the data anyway, why not collect raw data
>and let the post-processing take care of the local interference?  That lets
>you defer decisions about the appropriate filtering.

There is enough high-frequency "grass" to reduce the precision of 
your zero crossing determinations.  Since there is no useful 
information on grid behavior at these frequencies, it is better to 
remove it to improve your zero-cross precision.  You can do a lot 
with post-processing, but you can't fix EVERYTHING in the mix.  You 
have to start with the best data collection you can get, which in 
this case means filtering out the low-amplitude stuff above 1kHz or so.

>Is there any database of events that I can check when I see something
>interesting?  Or turn things around and pick an event and see what it looks
>like when it gets here?

Not that I'm aware of.  (But as I noted previously, I'm not 
personally a grid-nut).

Best regards,

Charles


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