1. An implantable cardiac rhythm management device comprising: an input circuit for receiving a sampled signal corresponding to cardiac electrical activity; a controller coupled to the input circuit and adapted to determine a fundamental frequency of the sampled signal by autocorrelating a function of a series of characteristic points, the series of characteristic points determined based on the sampled signal and each characteristic point having a time of a lobe in a curvature series; and a memory coupled to the controller and adapted to store the fundamental frequency.