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This paper studies the DFT-based algorithm for the phasor measurement application and investigates its performance for off-nominal frequency conditions, harmonics, inter-harmonics, and out-of-band interfering signals. It is mathematically shown that the DFT technique can provide accurate estimation of the phasors in off-nominal frequency conditions when a three-phase balanced system is concerned. The negative-sequence component causes errors to the DFT estimates though. It is also rigorously shown why the DFT cannot accomplish this task in a single-phase system. Moreover, the DFT method is highly robust to the presence of harmonics but it fails to effectively reject inter-harmonics and/or out-of-band interference signals.