One of the most basic forms of digital modulation is called Amplitude Shift Keying (ASK). In amplitude shift keying, the carrier has two possible states, On and Off, On representing a digital 1, and ...

U1, U2, and U3 form the quadrature oscillator. U4 provides the necessary amplitude gain required for ASK. U5 and U6 are configured as inverters to provide a phase shift of 180°.