An implantable medical device system (12, 14, 32, 34) configured for iterative analysis of cardiac signals comprising: a plurality of electrodes (16, 18, 20, 36, 38, 40, 42) for sensing cardiac signals; self-correlation means for generating a self-correlation function from the sensed cardiac signals, the self-correlation function having amplitudes as a function of lag depth; and peak selector means for identifying amplitude peaks in the self-correlation function and finding a first estimate of cardiac rate and first affiliated confidence having ratings of at least low or high confidence; wherein: the peak selector means comprises picket test means for determining whether, for a selected peak having a first lag depth in the self-correlation function of a given iteration, at least one additional peak appears at a second lag depth that is a multiple of the first lag depth; and the peak selector means further comprises candidate selection means for selecting candidate peaks to determine suitability of the candidate peaks for calculating an estimated cardiac rate operable to: identify a quantity of candidate peaks using peaks of the self-correlation function; select as a first candidate peak one of the following: a candidate peak having the least lag depth of the identified candidate peaks, or a candidate peak having an amplitude that is larger than that of the candidate peak with the least lag depth by at least a first margin and which corresponds to a cardiac rate exceeding a rate threshold; wherein the peak selector means is configured to use the candidate selection means to identify one or more candidate peaks, and the picket test means to determine whether any candidate peaks are suitable to estimate cardiac rate and, if so, to report an estimated cardiac rate. a plurality of electrodes (16, 18, 20, 36, 38, 40, 42) for sensing cardiac signals; self-correlation means for generating a self-correlation function from the sensed cardiac signals, the self-correlation function having amplitudes as a function of lag depth; and peak selector means for identifying amplitude peaks in the self-correlation function and finding a first estimate of cardiac rate and first affiliated confidence having ratings of at least low or high confidence; wherein: the peak selector means comprises picket test means for determining whether, for a selected peak having a first lag depth in the self-correlation function of a given iteration, at least one additional peak appears at a second lag depth that is a multiple of the first lag depth; and the peak selector means further comprises candidate selection means for selecting candidate peaks to determine suitability of the candidate peaks for calculating an estimated cardiac rate operable to: the peak selector means comprises picket test means for determining whether, for a selected peak having a first lag depth in the self-correlation function of a given iteration, at least one additional peak appears at a second lag depth that is a multiple of the first lag depth; and the peak selector means further comprises candidate selection means for selecting candidate peaks to determine suitability of the candidate peaks for calculating an estimated cardiac rate operable to: identify a quantity of candidate peaks using peaks of the self-correlation function; select as a first candidate peak one of the following: a candidate peak having the least lag depth of the identified candidate peaks, or a candidate peak having an amplitude that is larger than that of the candidate peak with the least lag depth by at least a first margin and which corresponds to a cardiac rate exceeding a rate threshold; a candidate peak having the least lag depth of the identified candidate peaks, or a candidate peak having an amplitude that is larger than that of the candidate peak with the least lag depth by at least a first margin and which corresponds to a cardiac rate exceeding a rate threshold; wherein the peak selector means is configured to use the candidate selection means to identify one or more candidate peaks, and the picket test means to determine whether any candidate peaks are suitable to estimate cardiac rate and, if so, to report an estimated cardiac rate.