Looking for guidance in frequency measurement

[turbosupramk3]

Is this what you had in mind?

Looking for guidance in frequency measurement

I would use a 74HC14 with a 100K resistor on its input. If powered from 5V the output will be 5V. If noise is a problem you can add maybe 100nf at the input of the gate. This will not load your existing signal as would an optocoupler would and bandwidth way beyond what you need. You can put a second in series with the first if you want a non inverted version of the signal. This uses the input protection diodes. at 50V you will be applying 500uA.