User's Manual Part 2

Tech-X Flex User Guide - Firmware v06.50 Tech-X Flex
®
(NG2)
8-16
Intro
Overview
Wi-Fi
Ethernet
System
IP/Video
MoCA
RF
Specs
MER
(Modulation error
ratio)
A performance metric for a digitally-modulated signal, typically
used to gauge the reliability of signal transmission. Sometimes
(perhaps erroneously) referred to as a digital carrier-to-noise
measurement, MER is the ratio of the carrier's average symbol
power to the carrier's average error power. A higher number
indicates a better quality of service (that is, less influence from
noise, linear/non-linear distortions, and ingress). Actual
minimums for reliable digital transport may vary according to
architectures and other conditions, but suggested minimums
for an equalized measurement are 23-27 dB for 64-QAM and
28-31 dB for 256-QAM.
The calculation of MER is directly related to the placement of
points on a constellation graph. An “ideal” symbol arrives in the
center of its respective “cell” on the graph, so graphs that
display plots close to their respective centers correspond with
a higher MER. Symbols displayed further from the center
and/or forming “fuzzy” patterns correspond with a lower MER.
Because MER is calculated from sampled bits, the precision
increases with the sample size. Therefore, tests that run
repeated measurements can keep a cumulative data set and
continue to increase the precision of the MER calculation.
For more information about constellation graphs, see About
QAM and the constellation graph on page 8-18.
P/V Ratio or
Peak/Valley
Peak-to-valley ratio, the difference between the highest and
lowest power levels across the frequency band tested, in dB. It
is effectively the highest versus the lowest power level
detected across the bandwidth of a digital channel. On a power
level spectrum graph, a perfect digital signal should appear
“flat” across the frequency band in use by the channel.
However, portions of the bandwidth are normally received at
higher power levels than others due to distortions and
impairments.
Ideally, this number should be as low as possible, with 3 dB
commonly recommended as a maximum. Significant
differences in power levels across the band interfere with the
ability of receiving equipment to decode the modulated signal.
Measurement Description
Preliminary issue - Limited distribution only!