The new “multilevel” DP-QPSK coherent modulation scheme improves “traditional” approaches to line-side transponder testing. But manufacturers still require a simple way to test transmitters in manufacturing, and simple bit-error-rate (BER) measurements aren’t enough. Time-resolved EVM and masks testing delivers rapid pass/fail validation for efficient compliance testing of vector-modulated signals such as 100G DP-QPSK.