This is a simple 555 timer based Frequency modulation. 555 can not generate frequencies at MHz range(max 100KHz); So, practically it can't be used to make normal FM transmitters. But it can be used to make short range 100KHz transmitters. In this diagram green is the input signal ( say audio signal ) and blue is the frequency modulated output. The output is directly fed to demodulator to prove that it can be successfully demodulated. Orange is the demodulated signal which is similar to the green one (input), but it has some phase difference ( obvious ).
