On the impact of amplitude variation on interpolated DFT method and how to reduce it

The Interpolated DFT is often used to improve frequency estimation of the spectral lines of DFT’s spectrum. If the measured spectral line is distorted the accuracy of Interpolated DFT is decreased. Such distortion could be a result of amplitude variation during the acquisition of samples. To reduce the impact of amplitude changes a novel algorithm of digital amplitude stabilization was proposed. The simulations have been made to verify proposed algorithm. The amplitude variations were modelled by amplitude modulation of the measured signal. The relationship between systematic error and modulating frequency, phase, modulation index of an amplitude modulation and measured signal frequency were presented for two Interpolated DFT methods, with and without amplitude stabilization. Simulation tests prove that the application of amplitude stabilization (AS) algorithm significantly reduces the systematic error of the frequency estimation using both Interpolated DFT methods.