VeEX VEPAL CX350S-D3.1, Руководство пользователя

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7.1.4.2 Graph Mode
The graph mode displays the
Adaptive Equalizer Stress
. The adaptive equalizer graph is a useful tool for troubleshooting linear distortion.
Overview: Adaptive Equalization
Modern Cable Modems, Set-top-boxes and Cable Modem Termination Systems (CMTS) use advanced Adaptive Equalizer technology to
compensate for complex in-channel frequency response impairments caused by micro-reflections, amplitude ripple and group delay
occurring in the cable network. Signals in a cable system can arrive at the receiver before or after the desired signal. Signals normally
arriving afterwards are either reflections or micro-reflections.
Rather than equalizing the entire upstream or downstream RF frequency spectrum, an adaptive equalizer adjusts its characteristics based
on a single digitally modulated QAM carrier only as channel conditions change. This process maximizes or greatly improves the Modulation
Error Ratio (MER) in the forward or reverse path.
Impairment Graph (Adaptive Equalizer)
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.
Interpreting the Adaptive Equalizer Display