Error Vector Magnitude (EVM) RMS to dB Calculator

This tool converts the Root Mean Square value of Error Vector Magnitude (EVM) to its deciBel (dB) equivalent. It also computes the dB value from RMS.

How to use the calculator

EVM is a measure of modulation quality of a communication system. Enter a number in either field of the tool. The dB value should be a negative number lower than zero. The RMS value of EVM is expressed as a percentage. Therefore this value should be a number between 0 and 100.

EVM Formula

EVM (%) = SQRT(Perror/Pref) * 100

EVMRMS = 100 * 10^(EVMdB/20)

EVMdB = 20 * LOG10^(EVMRMS/100)

where Pref is the reference power. It’s either chosen to be the RMS power of the ideal signal or the Peak power [1]. Depending on definition, it can make a significant difference in the final value.

EVM example calculations

A value of -40 dB translates to 1% error vector magnitude.

The lower the value of EVM either in percentage or dB, the lower the error. This translates directly to lower Bit error rate and higher quality signals.

EVM of -60 dB is better than -40 dB. It translates to a smaller RMS value of 0.1% and represents less error in the received signal.

Although almost never used in practice, EVM in dB can be converted to a linear quantity.

What is a good value of EVM?

The answer depends on the modulation format and target performance levels. For 802.11 Wi-Fi systems, the IEEE for instance requires -30 dB EVM for 256 QAM modulation and coding rate of 3/4.

As the modulation constellation size increases from 256 to higher, the target EVM is a lower dB value which going back to the definition represents a smaller value of Perror.

EVM RMS to dB table

The following table shows conversion of percentage RMS values to their dB equivalent.

References

[1] R&S Application Note: Do you know your EVM?