1. An implantable cardiac rhythm management device comprising: an input circuit adapted to receive a sampled signal including sampled data points corresponding to cardiac electrical activity; a controller coupled to the input circuit and adapted to compute a curvature series using the sample data points, identify lobes each being an excursion of more than a curvature threshold value from a baseline in the computed curvature series, generate a series of characteristic points each associated with a time of a lobe of the identified lobes in the curvature series, and determine a fundamental frequency of the sampled signal by autocorrelating a function of the series of characteristic points, wherein the curvature series includes curvatures each being a non-linear function of first and second derivatives of the sampled signal at one of the sample data points, and the controller is adapted to compute the first and second derivatives and the curvatures; and a memory coupled to the controller and adapted to store the fundamental frequency.