A mechanism for determining an error vector magnitude EVMTD for a signal transmitted by a device under test (DUT). A receiver (typically an RF signal analyzer) produces a baseband signal in response to the signal transmission. An OFDM input signal (derived from the baseband signal) is accessed from memory. The OFDM input signal includes a sequence of time-domain OFDM input symbols. A reference signal is accessed from the memory. The reference signal includes a sequence of time-domain OFDM reference symbols. EVMTD is computed in the time domain based on a time-domain difference signal, i.e., a time-domain difference between the sequence of time-domain OFDM input symbols and the sequence of time-domain OFDM reference symbols. The error vector magnitude EVMTD is determined without transforming the sequence of time-domain OFDM input symbols to the frequency domain. The error vector magnitude EVMTD is related to a standard-defined composite EVM by a scalar multiple.