This paper deals with an adaptive observer methodology for estimating the frequency of an unknown periodic signal with non-zero fundamental component corrupted by unstructured and bounded uncertainties. The proposed estimator is characterized by Input-to-State Stability (ISS) with respect to the bounds on the higher order harmonics and on the unstructured disturbance. The influence of each tuning parameter is analyzed through a specified ISS asymptotic bound. The effectiveness of the proposed technique is shown through numerical simulations.
