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USER'S MANUAL. PROMAX-27
APPENDIXES
APPENDIX A.-
MEASUREMENT OF THE MODULATION ERROR RATIO
(MER).
Analogue and digital carriers are very different in terms of the signal content and
the power distribution over the channel. Therefore need to be measured differently.
The amount of distortion in a system is related to the total power of all of the
carriers making accurate power measurements critical for optimum performance.
Instruments such as signal level meters that are designed to measure only
analogue carriers will not accurately measure digital carriers.
Modulation error ratio (MER), used in digital systems is analogous to Signal/Noise
(S/N) measurement used in analogue systems. MER represents the ratio of the error
power to the average power in an ideal QAM signal. Ideally you should have at least 4
or 5 dB of margin from the MER where significant errors occur, to allow for system
degradation. MER measurements are useful for early detection of non-transient (noise)
impairments, such as system noise and the second and third order beats (CSO and
CTB). This measurement takes in account not only amplitude noise, but also phase
noise.
To determine the MER of a signal is a key part of determining the margin from
failure of the digital system. Unlike analogue systems where you can see degradations
in Carrier/Noise (C/N) performance, a poor MER is not noticeable on the picture right up
to the point of system failure.
MER is defined as follows, expressed in dB:
20
07/2008
RMS
error
magnitude
log
average
symbol
Figure 33.- Modulation error ratio (MER).
(
dB
)
magnitude
RMS Error Magnitude
Ideal Symbol
Average Symbol Magnitude
Page 61
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