Quadrature Amplitude Modulation (QAM) [Under Modulation >> Digital modulation]
In QAM, two amplitude modulated "quadrature" carriers are summed - hence the name QAM. As carrier waves are 90° out of phase and amplitude modulated, QAM in a way (amplitude difference between two carriers changes the phase;you really need to visualise this in your mind to grasp better) is a combination of ASK and PSK.
Keep the amplitude same and we have PSK then. In fact, PSK may be designed based on QAM.
Constellation diagram Constellation diagrams useful tools for representing waveforms. A sinusoidal signal/wave becomes a circle on constellation diagram. Refer two examples figures below for clarity:  Above is for BPSK and below for 8PSK  |
Digital cable TV and cable modem use 64-QAM abd 256-QAM. For simplicity, refer below the constellation diagram for 16-QAM.
I and Q stand for "In-phase" and "Quadrature" components
Note that points like in rectangle (rather than circle). Refer tutorial available on compextoreal site for more details on QAM.
© Copyright Samir Amberkar 2010
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