3.2 System Level Considerations 633.2.10 EVM in Transmitters Including Phase Noise, Linearity, IQ Mismatch, EVM with OFDM Waveforms, and Nonlinearity Error vector magnitude (EVM) is a very important way to measure how accurately a transmitter has reproduced the vectors that correspond to the data being transmit-ted. EVM is another way to measure the signal-to-noise and distortion ratios of the signal. The term EVM is used more often in transmitters compared to receivers as a measure of modulation accuracy instead of SNR. In any system with limited performance, some errors will always be introduced and the vector that is actually transmitted will be different by some amount (an error) from what was intended as shown in Figure 3.18. The instantaneous value of the EVM is defined as the ratio of the magnitude of the reference vector to the magnitude of the error vector iiieEVMa= (3.29)where ei is the ith error vector and ai is the ith reference vector. Normally EVM is averaged over a large number of data samples N to come up with an average or nominal value: 211 i NiiieEVMNa==æö=ç÷èøå (3.30)There are many sources of EVM in a transmitter, and the overall effect of vari-ous sources can be added together as shown here:Figure 3.18 Illustration of EVM using QPSK as an example modulation.