Hi Nguyen,

You can use AT89S52 to measure the frequency. You can connect the frequency signal to your micro port pin [ of course with micro circuit protection precautions ]. In this case either you can use Timer 2 or say external interrupt polling on ordinary port pin etc. and can determine the frequency. I would prefer this method since the hardware required is much less.

frequency signal---> micro.

You can use frequency to voltage converter that you have. but then to measure the voltage you need an ADC. Thus your circuit hardware gets increased.

frequency signal -->F to V converter ---> ADC --->micro

I dont see any significant advantage in using the second approach.

Regards,
Prahlad Purohit