Frequency measurement methods. PLL?

I use a zero crossing detector circuit which turns 1-120VAC sine wave in to a 24VDC square wave. Square wave goes to the PLC high speed counter input.

This is much better than using a frequency transducer with 4-20mA input as the resolution is determined by the clock in the high speed counter.

I would be interested in seeing suggestions for products which provide this sine to square wave zero-crossing detector functionality as the one we are using is an artisinal product and I don't know what we will do what the artisan retires.

naturephoenix said:

Whats the EL3413 sample speed

Click to expand...

Found this from documentation (link):

"The default setting
for the measuring interval is 10 periods (10 x 20 ms). Experience shows that this is a good compromise
between measuring speed and stability. Deviations from this value are only advisable in the event of
particular measurement requirements (e.g. high measuring speed)
"

Hello,
I have to say thank you all for a lot of help. By reading here and searchig on google and reading again I have simulate PLL method on matlab trial version in discrete mode.
All is working fine. What I mean, it is possible to get correct frequency more than 200ms for sure. It depends about assumed value and of cource PI settings.

Now I would like to get some ideas for Filter, for example If I want to avoid
3rd harmonics, actually I am interedted only on frequencies between 45-55Hz.
When I simulate now noise(3rd harmonic with 5% of my amplitude) my measurements gets wrong.

Do you guys have some ideas how to filter frequencies above some range lets say.