Promax PROLINK-4C Manual De Instrucciones página 160

4) Attenuator
It permits to select attenuation between 0 and 30 dB. It is advisable to activate
the 30 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 QAM 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 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, QAM 64 demodulators require an MER greater than 23 dB. 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).
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USER'S MANUAL. PROLINK-4C
02/2003