[time-nuts] 60 Hz frequency and phase measurement

[jimlux]

There's some designs on the list (using a PICPET, for instance) to 
measure the local line frequency and phase..

but the schemes we've discussed require connecting to the power line in 
some way.

What about a non-contact sensing approach?  Something you could put in a 
box and it would pick up the electric or magnetic field as the input?

Just how strong is the field anyway?  I've always been trying to cancel 
or shield it or reduce it in some way, so I've never actually measured 
it in a calibrated way.  A 10cm antenna on a 1Meg scope probe looks like 
about 40 mV peak to peak (for the 60 Hz component) along with lots of 
other high frequency stuff (40 kHz and a few hundred kHz in my office) 
from switching power supplies.

I realize that in a office/industrial area you'll probably pick up all 
three phases in some way.

What about using a small loop? or a magnetoresistive sensor (like the 
compasses in phones)?

Has anyone tried any of these?