Digital Signal Processing Reference
In-Depth Information
tion is then about 38…40 dB. To bring this shoulder attenuation back to a
reasonable order of magnitude, passive band-pass filters tuned to the DVB-
T channel are connected downstream (Fig. 20.16.). This again provides a
shoulder attenuation of better than 50 dB (critical mask). But there is noth-
ing that can be done against the in-band carrier/noise ratio of about 38…40
dB now present. These interference products are the result of the clipping
required for reducing the crest factor and will now determine the perform-
ance of the DVB-T transmitter. I.e., every DVB-T transmitter will exhibit
a C/N ratio of the order of about 38…40 dB.
Today, direct modulation is used in virtually every DVB-T modulator,
i.e. the signal is converted directly from the digital baseband into RF as a
result of which analog I/Q modulators are used. In consequence, this cir-
cuit section, too, which is now no longer operating with theoretical perfec-
tion, has adverse effects on the signal quality, resulting in I/Q errors such
as amplitude imbalance, I/Q phase errors and lack of carrier suppression.
It is the art of the makers of modulators to keep these influences to a
minimum. However, the presence of an analog I/Q modulator in the DVB-
T transmitter is always detectable by measuring instruments as will be seen
later in the chapter on test engineering. As well, the finite quality of the
signal processing in the DVB-T modulator also results in the creation of
noise-like interferers. Further noise occurs on the transmission link in de-
pendence on the conditions of reception. Similarly, multiple echoes and si-
nusoidal or impulse-like interferers can be expected and echoes, in turn,
can lead to frequency- and location-selective fading.
S/N
Fig. 20.16.
Shoulder attenuation after clipping and after bandpass filtering
To calculate the crest factor in COFDM signals:
The crest factor is usually defined as:
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