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Digital Analysis (QAM View OPT4): Digital Summary Mode

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Features and Definitions

DIGITAL Main Screen Features

The DIGITAL Main Screen (see Fig. 9-3) displays the:

• channel number and modulation format

• Modulation Error Ratio (MER) / Error Vector Magnitude (EVM)

• Bit Error Rate (BER) before Forward Error Correction (FEC)

• Bit Error Rate after Forward Error Correction.

This screen also assigns a Pass or Fail value to these measurements in

relation to user-defined error limits.

See Also

½ Chapter 3, “Channel Plans,” contains more information

about channel plan configuration and editing default limits.

Modulation Error Ratio (MER)

The Modulation Error Ratio measurement is analogous to signal-to-noise

(S/N) or carrier-to-noise (C/N) measurements made in analog systems. It is

the ratio of equalized signal power to total signal degradation. Non-transient

impairments (system noise, CTB, CSO, ingress) are detected early by MER

measurements.

Expressed in dB, a high MER reading means a smaller level of impairment

and a better signal; a low MER reading means a higher degree of

impairment. Because 64 QAM will stop working around 22 dB MER and 256

QAM will stop working around 28 dB MER, the SDA has default thresholds

of 28 dB MER and 32 dB MER respectively to allow a 4-6 dB margin of

safety in the Pass/Fail value (see Fig. 9-3).

Error Vector Magnitude (EVM)

Some system designers prefer to view system performance in terms of Error

Vector Magnitude, which is essentially MER data provided in a percentage

format. The MER format may be changed to the EVM format on the

CONFIGURE MEASUREMENT screen (see Chapter 2).

Bit Error Rate (BER)

With each digital data transmission, some bits will not be received correctly.

The Bit Error Rate is the number of bits in error divided by the total number

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