The meaning of Frequency Shift Keying (FSK) is transmitting digital data over a signal path (two wires, telephone lines, AM or FM transmitter). Typically, there are only two signals transmitted. One corresponds to a logical “1”. the other to a logical “1”. These two frequency will be generated by a TC9400 V/F converter when connected as shown below.
The V/F converter is set to the lower frequency by the potentiometer. Which frequency is selected is determined by the digital input. A “1” selects the upper frequency and a “0” selects the lower frequency. TC9400, in F/V mode, convert back the digital frequency signal into a digital format. [System’s block diagram source: Microchip Application Note]