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This paper presents a method for evaluating the performance of demodulation-based frequency measurement algorithms in the presence of additive interfering sinusoids. Determination of the performance of amplitude measurement schemes under such conditions is straightforward once the frequency responses of the filters involved in the process are known, since the error induced by a single interfering tone is easily computed using the cascade algorithm's frequency response magnitude. This paper presents a similar method for predicting the worst error of frequency measurement schemes with respect to sinusoidal interference. Once acquainted with the proposed error prediction formula, the only difficulty in designing effective frequency measurement algorithms is the appropriate selection of output filters to achieve the specified performance. The method has been used successfully in designing frequency measurement algorithms currently used in Hydro-Que´bec's special protection schemes.