Promax PROLINK-4/4C Premium Manual De Instrucciones página 185

USER'S MANUAL. PROLINK-4/4C
4) Attenuator
It permits to select attenuation between 0, 20 and 40 dB. It is advisable to activate
the 40 dB attenuator under that measurement conditions where the signal level is
near to the maximum input level (approximately starting from 20 dB under the
maximum level) and it is possible that the tuner becomes saturated. Under no-
saturation conditions, when increasing the attenuation value the BER measure
must to maintain or to increase (insufficient signal level) but never to decrease.
Once you have defined the QPSK signal parameters, it will be possible to
measure the BER. When the BER measuring mode is selected, the monitor will show a
picture like the following:
Figure 19.- BER measuring screen for QPSK modulated signals.
Two BER measurements are shown:
1) BER before FEC (Forward Error Corrections)
2) BER after Viterbi
In a digital reception system for satellite signals, after the QPSK decoder two
different correction methods are applied (see figure 20). Obviously, each time we apply
an error corrector to a digital signal, the error rate changes, therefore if we measure in a
digital satellite television system, for example, the error rate at the output of the QPSK
demodulator, at the output of the Viterbi decoder, and at the output of the Reed-
Solomon decoder, we obtain nothing more than different error rates. This is the reason
because the BER measurement is provided before FEC, after Viterbi.
Figure 20.- Digital reception system via satellite.
06/2005
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