Application Note AN-005: Understanding Constellation ...

90°. Signals in quadrature are orthogonal and do not interfere with one another. Quadrature modulation takes advantage of this principal by combining two signals which are in quadrature for a combined output signal. The advantage of this is that the amplitude and phase of the combined output signal can be modulated independently or

  Modulation, Amplitude, Quadrature, Quadrature modulation