i have found my circuitry of frequency meter in microchip.com
h**p://ww1.microchip.com/downloads/en/AppNotes/0037.pdf
now, i would like to determine the input circuit, which is the ECM preamp with TTL output. the output have to match the input of frequency meter as i mentioned above.
do u have any source / reference on ECM preamp circuitry with digital TTL output?