A digitally modulated signal is represented by its equivalent complex lowpass or baseband signal The real and imaginary parts of this signal are called the quadrature components and The figure shows the signal constellation in the complex plane and the eye diagrams of the quadrature components for QPSK quadrature phaseshift keying The eye diagram shows how the waveforms used to send multiple bits of data can potentially lead to errors in interpretation You can change the rolloff factor of the baseband pulseshaping filter to see the effect of an amplitude or phase imbalance of the modulator