Digital Signal Processing Reference
In-Depth Information
decreases by the same amount. Fig. 21.22. shows the conditions for the
signal/noise ratio S/N in the presence of amplitude imbalance and of a
phase error, respectively, which have now been derived as formulae. In the
practical implementation of a DVB-T modulator, the aim is an amplitude
imbalance of less than 0.5% and a phase error of less than 0.5 degrees.
S/N
[dB]
AI
AI[%]
[°]
Fig. 21.23.
DVB-T signal/noise ratios with amplitude imbalance (AI) and phase
error (PE) of the IQ modulator
Central DVBT carrier
8K: Nr. 3408 = continual pilot
2K: Nr. 852 = scattered pilot / payload
Fig. 21.24.
Distortions in the vicinity of the center carrier due to the channel cor-
rection in the DVB-T receiver
Thus, I/Q errors of the DVB-T modulator can only be identified by ob-
serving the center carrier but may interfere with the entire DVB-T signal.
In addition, it will be found that in each case at least the two upper and
lower carriers adjacent to the center carrier are also distorted. This is
caused by the channel correction in the DVB-T receiver where channel es-
timation and correction is performed on the basis of the evaluation of the
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