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This paper presents algorithms for fast measurement and the nonparametric estimation of the unknown changing frequency, amplitude, and phase difference of the signals from two channels with the same frequency, as well as other power quantities, such as the apparent, the active, and the reactive power. The possibilities for systematic error reduction through use of the interpolated discrete Fourier transform using the Rife-Vincent windows class I (RV-I) are described. RV-I windows are designed for maximization of the window spectrum side-lobes fall-off and owing to their minimal leakage, minimal systematic bias curves can be evaluated as a function of the measurement interval duration expressed in signal cycles. Parameters are calculated from the discrete Fourier transform coefficients around the component peaks by summation to reduce the leakage effects. The optimum for reducing the time of measurement and for reducing systematic errors under non-coherent conditions of sampling real noisy signals could be the estimation with the three cycles window using the three-point interpolation and the RV-I window order 3.