Frequency-shift keying (FSK) is a type of frequency modulation scheme in which the digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK). BFSK transmits only the binary information of a message signal, zeros or ones.
In Binary Frequency Shift Keying (BFSK) system, the carrier frequency is switched between two frequencies according to the binary input. The 555 timer functions in astable mode. The frequency of operation is given by the expression
Fo = 1.45/(Ra+2Rb)
If the resistor Ra varies, the frequency varies. When the input is at logic 1, the transistor turn ON and effectively resistance of Ra decreases, because transistor offers a resistance of a few ohms in parallel to Ra. This makes the frequency of operation less. The potentiometer Ra is used to fine tune the frequency. When the input is at logic 1, frequency of operation is given by the expression
Fo = 1.45/(Ra//R + 2Rb)