we know that the input to the FSK Generator is a binary sequence 1010…etc.
FSK generator uses two product modulators upper-modulator and lower-modulator with carriers
and
.
A level shifter is there in which the output of the level shifter is
when the input is a binary ‘1’ and ‘0’ volts for the input ‘0’ level shifter.
i.e,
The working of the FSK generator is as follows when the input binary sequence is ‘1’
on the upper modulator 1 has been shifted to a voltage so that the output of product modulator 1 is 
and on the lower modulator input ‘1’ is passed through an inverter and if the output of the inverter is ‘0’ then the output of the level shifter will not change it remains at ‘0’ volt itself.
then the product modulator 2 output is
then the overall output 
similarly, when the input sequence is a binary ‘0’
