Efficient EVM Testing of Wireless OFDM Transceivers Using Null Carriers

High-volume manufacturing of current generation orthogonal frequency division multiplexing (OFDM transceivers mandates testing for error-vector-magnitude (EVM) at production testing. During EVM test, a modulated RF input signal is down-converted and demodulated to obtain the output baseband digital data and EVM is computed by processing the baseband digital data. Hence, production testing of OFDM devices would require such modulation- and demodulation-capable automated test equipment (ATE) to perform EVM test. Such capabilities significantly add up to the cost of the ATE, thereby increasing the overall cost of testing. Moreover, test time for EVM can be relatively long compared to other tests due to the need to average over a large number of data bits. In this paper, we propose a methodology for testing EVM using multi-tone signals sourced from inexpensive signal sources that generate standard constellations. Moreover, introducing null carriers in the multi-tone test stimulus enables accurate characterization of system noise with reduced number of data bits. This enables significant speedup in EVM testing. We present the theory to corroborate the proposed approach along with simulation and hardware results. The proposed test method also has the potential to significantly reduce EVM test time under production test conditions.