measuring low frequency by cyclone

All possible digital filter solutions are based on a digitized analog signal. Do you already have a digital signal? Otherwise an analog low- or bandpass filter and comparator would be the most simple solution.

I usually won't implement a digital filter for a simple frequency measurement. A multi period measurement with a one-shot filter to supress ringing zero crossings would be sufficient even for a higly distorted power grid.

If you want to exactly reproduce the mains fundamental in presence of strong distortions and other interferences, a (signal processing) PLL can be helpful. A simple second order IIR band pass will be sufficient in many cases, however.