VeEX CX380C, User Manual

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Adaptive Equalizer Measurement
The bar graph shows the performance of the Feed Forward Equalizer (FFE) and Decision Feedback Equalizer (DFE) circuitry.
The vertical axis displays the level in decibels (dB), while the horizontal axis displays units of time typically relative to the
adaptive equalizer tap spacing.
Each bar displays a coefficient corresponding to the Feed-Forward Equalizer (FFE) and Decision Feedback Equalizer (DFE)
taps.
There are eight coefficients for the FFE and 24 coefficients for the DFE. The 8th coefficient, or the tallest vertical bar, is the
incident signal (main signal path) and it is normally close to 0dB representing unity gain.
If any bar appears above the average noise floor to the right of this incident signal, the problem is related to one or several
micro-reflections.
The adaptive equalizer is like a frequency equalizer, however instead of changing the level at a specific frequency, it
changes it at a specific time.
The specific time matches the time at which the reflection occurs - the gain is reduced only at this instant thus reducing the
reflection.
The maximum recommend correction is usually specified by the set top manufacturer and you should allow adequate
margin for degradation.
The taps on the horizontal axis are proportional to the symbol rate.
Interpreting the Adaptive Equalizer Display
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Equalized and Un-equalized MER measurements
Un-equalized MER is typically measured before the adaptive equalizer, and equalized MER is measured after the adaptive
equalizer, but often this circuitry resides in the QAM receiver and cannot be disabled. So while the adaptive equalizer does a great
job of improving the MER of a QAM signal, it makes troubleshooting marginal amplifiers, small ingress, CPD and related
impairments a lot more difficult.
It is important to know how hard the adaptive equalizer is working to determine if there is any margin for system degradation. To
solve this problem, the CX380C adaptive equalizer can be turned off, allowing a technician to easily identify, sectionalize and locate
these difficult linear type distortion problems. Tap the function of “Equalizer On/Off” to enable or disable the adaptive equalizer.
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7.1.5 Timed Stats
For a Digital Channel, run a timed test, where key measurements are averaged over the designated test time. Press Timed Stats
and input the desired test duration. Press Start to initiate the test.
CX380C User Manual RevA00 Page 32 of 105