Digital Analysis (QAM View OPT4): Constellation Mode
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QAM Modulation and Symbol Rates
The SDA analyzes two Quadrature Amplitude Modulation formats, 64 QAM
and 256 QAM.
The Symbol Rate corresponds to the size and shape (bandwidth) of the
digital signal. This rate is expressed in “millions of symbols per second”
(Msym/s) and may be edited in the Digital Summary Mode. The default for
64 QAM is 5.057. The default for 256 QAM is 5.360.
Modulation Error Ratio (MER)
A 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.
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
of bits in the data transmission. This ratio is expressed in scientific notation,
where 1e-3 equals one error in every 1000 bits transmitted. One error in
every 1,000,000 bits transmitted is expressed as 1e-6. A more negative
exponent value indicates a lower error rate.
BER is another good indicator of overall system performance. The SDA has
Pass/Fail BER default thresholds of 1e-8 before Forward Error Correction
and 1e-9 after Forward Error Correction.
Basic Operation
To activate CONSTELLATION Mode, press the green Function key and
then press the Modulation Measurement Mode key.
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