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When the BER measuring mode is selected, the monitor will show a picture like
the following:
Figure 12.- BER measuring screen for QAM-modulated signals.
First of all, you will see the modulation error ratio measurement: MER.
Analogue and digital carriers are very different in terms of signal contents and
power distribution over the channel. They, therefore, need to be measured differently.
The modulation error ratio (MER), used in digital systems is similar to the Signal/Noise
(S/N) ratio in analogue systems. The MER represents the proportion of power lost
through wrong data with respect to the mean power of an ideal QAM signal.
To operate,
Though it is preferable to have at least a 3 or 4 dB margin to compensate for any
possible degradation of the system. While QAM 256 demodulators require an MER
greater than 28 dB with margins of al least 3 dB. Normally, the maximum MER value
seen in portable analysers is of approximately 34 dB.
Secondly, it is shown the BER before FEC (Forward Error Correction).
In a digital reception system for cable signals, after the QAM demodulator an
error correction method called Reed-Solomon is applied (see figure 13). Obviously, the
error rate after the corrector is lower to the error rate at the QAM decoder output. This is
the reason because this screen provides the BER measurement before FEC (Forward
Error Correction) and the number of non-correctable errors (W.P., Wrong Packets)
received after Reed-Solomon in the measuring time.
Page 40
QAM 64 demodulators require an MER greater than 23 dB.
Figure 13.- Digital reception system via cable.
USER'S MANUAL. PROLINK-4
02/2003
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