Alpha Technologies DSM3 for XM2 - Technical Manual - Manual (Page 36)

36 745-814-B8-001, Rev. C (03/2014)
4.0 Web Interface
4.11.1 QAM Constellation Tool, continued
The tables on the right hand side of the screen provide a summary of common parameters
associated with QAM Constellation analysis. Here’s a breakdown of the parameters listed:
• Frequency – Downstream frequency given in MHz.
• Power – Downstream power given in dBmV.
• Modulation – Modulation type associated with the downstream channel.
• Lock Status – Current cable modem connectivity state.
• Channel ID – CMTS identification of the downstream channel within this particular MAC interface.
• SNR / (RxMER) – Downstream signal quality. Modulation Error Ratio (SNR)
• EVM – Error Vector Magnitude (from hardware MER (Modulation Error Ratio) / software MER).
• Symbol Rate – Msym/sec
• CER Interval – Codeword Error Rate (CER) refresh rate.
• Pre FEC CER – Codeword Error rate (CER) BEFORE forward error correction is applied.
• Post FEC CER – Codeword Error rate (CER) AFTER forward error correction is applied.
4.11.2 QAM Constellation Common Impairments
Several common impairments tend to reveal themselves on the constellation display which can
help determine the cause of the reduced MER levels. Below are examples of several of these
common impairments and their footprints.
Fig. 4-18, Normal - (Good Quality) and Individual Cell Characteristics
Individual cells
and entire QAM
constellation