Blonder tongue AMM-806, Reference Manual, Page: 227 / 285

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Digital Signal Analysis
The constellation diagram examples shown are based on 64
QAM modulation and use the following basic settings: maximum
possible DVB data transmission rate of 6.92 Msps or 41.73 Mbit/s
(64 QAM); Cosine roll-off filtering with roll-off factor r = 0.15 and
PRBS (pseudo random binary sequence) data stream, no coding.
Constellation Diagram for an Ideal 64 QAM Signal
Amplitude Imbalance describes the different gains of the I and
Q components of a signal. In a constellation diagram, amplitude
imbalance shows by one signal component being expanded and
the other one being compressed. This is due to the fact that the
receiver AGC makes a constant average signal level.
Phase Error is the difference between the phase angles of the I and
Q components referred to 90°. A phase error is caused by an error
of the phase shifter of the I/Q modulator. The I and Q components
are in this case not orthogonal to each other after demodulation.