A hard disk drive that includes a disk that contains at least one signal and a head that is coupled to the disk. The disk drive also contains a circuit that includes a data sampler that generates a plurality of data samples from the signal, a harmonic sensor coupled to the data sampler and a spectral power accumulator coupled to the harmonic sensor. The harmonic sensor accumulates the data samples. The circuit includes a window generator that determines a window length of the data samples accumulated by the harmonic sensor. The spectral power accumulator accumulates the windows of sample data accumulated by the harmonic sensor. The sample data accumulated by the spectral power accumulator can be accessed by a processor that performs a spectral analysis of the data.