Promax PROLINK-4/4C Premium Manual De Instrucciones página 190

4) Select the BER (QAM) measurement mode: to do this press key
turn the rotary selector [4] to select the BER (QAM) measuring mode, next, to
activate it press the rotary selector [4] or key
Before visualizing the representation of the Constellation Diagram is necessary to
define some parameters concerning the digital signal (see section '4.9.1.5.1 Measuring
BER of QAM Digital Channels (QAM)' ).
5) Press the rotary selector [4] to access the functions menu in digital mode, and
select the Constellation option to access the Constellation Diagram screen. Turn
the rotary selector [4] to tune the DVB-C (QAM) digital signal to be graphically
shown.
Observing QAM Constellation Diagrams.
In Constellation Diagram mode, the monitor will show a screen like the following
one:
Figure 24.- Constellation Diagram of a DVB-64 QAM fair digital signal.
In the first line is shown the Modulation Type: 64 QAM. Next indicate the
Symbol Rate: 6875 and the frequency of tuning: 400 MHz. Finally appear the values of
MER and BER measurements.
This image corresponds to a correct digital signal. Nevertheless the visualization
of the Constellation Diagram can discover the existence of different deteriorations from
the QAM modulated digital signal, like the noise, phase error and the nonlinearities. All
these can cause to a distortion or error in the location of the points on the Constellation
Diagram. For example, the atmospheric noise can cause a simple location dispersion of
these points as it is shown in figure 25. Also it can affect the transmission oscillator
phase error, causing additional uncertainty in the constellation (figure 28).
Page 54
QAM
64 QAM
6875 kSymb
400.00 Mhz
MER= 32.5
BER= 1.0E-8
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