MOTM

You need a counter with a 'reciprocal time base'. Instead of gating/counting
pulses, it measures the *period* of the waveform and *calculates* the frequency.
In typical MOTM overkill fashion, I use a Philips/Fluke model PM6666 counter
which can measure down to 0.001Hz :) and at 0.1Hz resolution make 1,000
measurements/sec :))

This is a 10yr old model, I paid about $900 new but eBay has them every now and
then for like $350.

Paul S.