Ms Ximena

You need to use a high speed mcu (like a 60 Mhz ATMEL) or read the the signal indirectly using logic peripherals like a CMOS 4017 do divide de input signal... so if you divide the high frequency signal to 10 for example you need to measure 100 Khz signal... (10 us)...You can provide several. TO measure low frequency you can put the signal directly on MCU pin.

I suggest to you measure the signal using external interrupt on failing edge of signal.

Remember that to measure high frequency you need to read N times and calculate the media, minimizing errors...

You need 5 Mips or more MCU to do this....

For example you can use a overclock signal (an MCU running a 20 Mhz can runs at 30 ones....without problema...I tested and works fine, with some increasing of temperature....)

I achieve 5 MIPS on MCU, usign a P89C51Rx2 on 6-clock mode and a hibrid cristal at 30 Mhz....I dont tested with 40 Mhz, but I believe that works.....But I dont recommend to comercial products...just laboratory testings....