Second-order notch filters are commonly used for frequency estimation purposes. They are characterized by a single design parameter, the value of which is selected so as to achieve a suitable compromise between tracking performance and bias/variance errors. In this paper a new class of filters (of fourth order) is presented : the funnel-shaped filters, or funnel filters for short. These filters enables one to estimate harmonic signals with enhanced tracking capability, without any deterioration of the steady-state performance.