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This design counts from 1Hz-50Mhz, but that is using a old c84 w 4Mhz xtal. With an F84A using 20Mhz it should be able to count up to 250Mhz (depending on the input circuitry)
Sensitivity approximately 100 mV RMS (100 Hz to 2 MHz), 800 mV RMS @ 50 MHz
These PIC based Frequency Meters do not depend on the speed of the MPU too much. It has mainly to do with the speed of the gate at the input of the counter. So a 20Mhz device will not give you any more than the 4MHz component. Here is one that is interesting:
mabusoe, if you check the link you supplied, you will find that it's exactly the same as the one I gave earlier. And you are of couse right about the input gate, thats why I said "depending on the input circuitry" in my post. But you need a fast input circuit AND a fast MCU (or a external prescaler), thats why I suggested to raise the xtal freq.