EVM (Digital Demod)

EVM (Error Vector Magnitude) is the Root Mean Square (RMS) of the error vectors computed and expressed as a percentage of the EVM Normalization Reference.

The error vector magnitude is the length of the vector—at the detected symbol location—which connects the I/Q reference-signal vector to the I/Q measured-signal vector. The following graphic shows the calculation of the EVM metric as well as a diagram showing how a single error vector is calculated.

EVM is calculated from the symbol points (the instant in time when symbols are detected). The computation does not include points between symbols. Therefore Points / Symbol does not affect the value. The Syms/Errs table also shows the location of the symbol that has the largest EVM.

For constellations with constant magnitude (QPSK, BPSK, 8PSK, etc.), the EVMs are always normalized to the constellation maximum. For constellations with multiple possible magnitudes (APSK, StarQAM, 16QAM, 32QAM, etc.), the EVMs are normalized to the EVM Normalization Reference.

Shaped OQPSK and Offset QPSK use two points-per-symbol (symbols and midpoints between symbols) to compute EVM and peak EVM due to the offset between I and Q.

For Offset QPSK, when the Half Sine Filter is selected, the OQPSK reference constellation points fall on a circle with a magnitude of sqrt(2)/2, but the EVM is still expressed as a percentage of the magnitude of a QPSK symbol point (magnitude = 1).

For the EDGE demodulation format, the EVM, Phase, and Magnitude Error data results may vary for different Points / Symbol settings. When Points / Symbol is set to 1 (default), the trace data results are compensated for ISI (inter-symbol interference). For Points / Symbol greater than one, the trace data results are not compensated for the effects of ISI.

If the EVM is larger than expected, see the troubleshooting tips in Troubleshooting.

See Also

Mag Err (Digital Demod)

       

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