Translate signals amplitude into frequency
Both Analog Devices and TI make a variety of voltage to frequency converters - just google "V to F converter" to get several links.

I've used the Burr-Brown (now TI) VFC32 - it does both voltage to frequency and frequency to voltage conversion.

If you are only concerned with the amplitude of the 50 Hz signal, and not its instantaneous value, I would first rectify the signal, to get the peak voltage, then apply that voltage to the VFC.