Measuring Digital TV Signals in the Broadband Cable - Digital Video and Audio Broadcasting Technology - page 300

diagram, phase jitter produces smear distortion of greater or lesser magni-
tude (Fig. 18.11.). The constellation diagram 'totters' in rotation around
the center point.
To find the phase jitter, the smear distortions of the outermost constella-
tion points are measured which is where the phase jitter has the greatest ef-
fect. Then the frequency distribution within the decision field is considered
along the circular path whose center point is at the origin of the state dia-
gram. Again, the standard deviation which is still affected by additional
noise can be calculated here. This noise effect must then still be calculated
out.
18.2.3 Sinusoidal Interferer
A sinusoidal interferer (Fig. 18.12.) produces circular distortions of the
constellation points. These circles are the result of the interference vector
rotating around the center of the constellation point. The diameter of the
circles corresponds to the amplitude of the sinusoidal interferer.
Fig. 18.12. Effect of a sinusoidal interferer
18.2.4 Effects of the I/Q Modulator
In the first generation of DVB-C modulators, analog I/Q modulators were
used. Errors in the I/Q modulator (Fig. 18.13.) then resulted in I/Q errors
in the QAM-modulated signal. If, e.g., the I branch has a different gain
than the Q branch of the I/Q modulator, I/Q amplitude imbalance is pro-