Low-cost EVM measurement of ZigBee transmitters from 1-bit under-sampled acquisition

Error vector magnitude (EVM) or alternately offset EVM (OEVM) is one of the most important performance to verify for RF circuits, such as ZigBee transmitters in order to ensure that the quality of the generated modulated signal complies with the requirements of the wireless communication standard. The conventional solution to measure this performance relies on the use of an automatic test equipment (ATE) equipped with expensive RF channels. This article presents a low-cost solution that permits to realize EVM measurement using only a standard digital ATE. The approach is based on 1-bit undersampled acquisition of the RF modulated signal by a digital tester channel associated with a specifically tailored processing algorithm. This algorithm involves many different steps that are detailed in this article, including phase and amplitude fluctuation extraction, RF signal reconstruction, symbol clock recovery, symbol bits detection, and synthesis of reference data for EVM calculation. The proposed digital test solution is first evaluated through lab hardware experiments and then validated with EVM measurements on an industrial ATE.