What is EVM?

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Editorial Team - everything RF

May 28, 2020

Error Vector Magnitude or EVM is a measure of how accurately a wireless system is transmitting symbols within its constellation. A digitally modulated wireless signal can be represented by a constellation diagram. The diagram displays the signal in the form of a scatter diagram in the xy plane. The angle of a point, measured counter-clockwise from the horizontal axis, represents the phase shift of the carrier wave from a reference phase. The distance of a point from the origin represents a measure of the amplitude or power of the signal. Click here to read more about constellation diagrams.

A constellation diagram for rectangular 16-QAM

In an ideal world, a signal sent by a transmitter would have all the constellation points at the ideal locations, but in the real world, various imperfections cause the actual constellation points to deviate from their ideal location. These imperfections / errors include phase noise, low image rejection ratio, carrier leakage, etc.

EVM provides a comprehensive measure of the quality of the radio receiver or transmitter for use in digital communications by identifying sources of signal degradation, such as: filter distortion, amplitude non-linearity, I-Q imbalance, phase noise, etc. EVM is usually represented as a percentage but can also be measured in dB.

Let us consider a system (radio) that is transmitting via a 16-QAM constellation. Suppose, while transmitting the symbol [0000], due to the presence of noise in the system, the received bit deviates from its ideal position. This deviation from the ideal constellation point and the point received by the receiver is the "Error Vector". Whereas, the average amplitude of the error vector, normalized to the peak signal amplitude isthe "Error Vector Magnitude". Hence, the difference between the positions of the ideal / reference phasor and the actual generated / received phasor is the EVM.

 

16 QAM Constellation Diagram (figure above)

IQ Plane Showing the Deviation of Measured Vector from the Ideal Vector (figure above)


To calculate EVM in decibels (dB) we can use the formula below:

To calculate EVM as a percentage (%) we can use the formula below: