Q-Lite Satellite Modem Installation and Operating Handbook
6-96
They are defined as follows:
•
MER (dB) =
Power of ideal signal
Power of error signal
•
EVM (%) =
Power of error signal
Power of ideal signal
A high MER and low EVM indicate a good signal, whereas a low
MER and a high EVM indicate a poor quality signal.
Why are MER and EVM important? Unless you run a BER test then
you do not know what the actual BER for the link is. A ‘good’
Es/No can be misleading since there could be degradation from
interference. Modern FECs have steep BER curves, so the
difference between quasi-error free (QEF) operation and losing the
link can be very small. MER and EVM give an indication of how
close to the ‘cliff edge’ the link is and may allow remedial action to
be taken before the link is lost.
It is good practice when deploying a link to measure these two
values when the link is running well. Then reduce the power level
of the carrier(s) and establish the equivalent values just above the
point at which the carrier unlocks. This will give a good indication
of the range of potential values and can be used to establish an
early warning when the link starts to degrade.
The following is an example of a good signal:
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