A plurality of phases of sniff operations are performed for detecting wake-up signals being received by an implantable medical device (IMD), including a phase that performs a staged detection of whether the integrated frequency deviation of a received signal is outside of an expected frequency deviation range, where sniff operations are aborted if the integrated frequency deviation of the received signal falls outside of corresponding high and low thresholds in any stage, wherein the difference between the high and low thresholds tightens with each subsequent stage and an average frequency deviation over all completed stages are used in the calculations. One phase of sniff operations includes a dual frequency modulation (FM) detector including a first FM detector that introduces a small delay in a received signal providing an average frequency estimate and a second FM detector having a larger delay that is adjusted based on the estimated average frequency.