The Role Of EVM Measurements In Characterizing Amplifier Modulation Performance

By Robert Green, Keithley Instruments, Inc.

In wireless communication devices, phase and amplitude distortion created by the power amplifier directly affects the quality of the communication. The most significant measurement for analyzing power amplifier performance in the latest communication system protocols is error vector magnitude (EVM). This is a measure of modulation accuracy, or how well the power amplifier is transmitting information, represented by the varying phase and amplitude of an RF signal. EVM measurements lend insight into the communication link and are the key measure of transmitter performance.

On the receiver side, EVM is a measure of how well the receiver demodulated the transmitted signal. While BER (bit error rate) measurements provide a more direct measure of receiver performance, EVM measurements are faster than BER measurements and give more insight into amplifier performance.

Some design and test engineers may think of EVM as a single number representing modulation accuracy, but factors other than those internal to the modulator (such as gain error, quadrature error, and carrier feed-through) can impact EVM. These can include the transmission frequencies, level of input power to the power amplifier, and power amplifier DC (direct current) bias levels. In addition, EVM measurements get more complex with multicarrier modulation techniques such as orthogonal frequency division multiplexing (OFDM). A complete characterization of either power amplifier or transmitter EVM should include analysis of EVM over the range of transmission frequencies, over the expected input power range, and over an allowable range of power amplifier DC bias levels. It is particularly important for mobile devices to characterize transmission (EVM) performance down to the low-battery shutoff voltage level. For OFDM, EVM analysis vs. subcarrier is essential to understanding transmitter performance.