[time-nuts] Building a mains frequency monitor

[Tim Shoppa]

IMHO the transients and harmonic content, are more interesting than the
60Hz frequency!

But if the 60Hz frequency with superb rejection of transients is the only
goal:

The 60Hz line clock chips of the 1970's used the LV transformer secondary,
a single pole of RC low-pass filtering to get rid of spikes, and a Schmitt
trigger.

See e.g. MM5402N datasheet:
http://electrosurplus.online.fr/data/MM5402N%20NATIONAL.pdf

I believe there are some good tube circuits for extracting 60Hz from line
frequency, in the MIT Radiation Lab series and follow on books. I think at
least one example is in the phantastron section.