Filter transfer functions in the range between f_c/2 and 3f_c/2, centered at the current PM-HLL oscillator frequency (i.e., fundamental frequency estimate) f_c. The amplitude is shown in blue (left scale), and the normalized phase is shown in red curves (right scale). The period-constructive comb filter (solid lines) enhances a component at f_c, whereas the period-suppressive comb filter (dashed lines) cancels it. If the true fundamental frequency f₀ of a signal is lower (indicated as f_0,1) or higher (indicated as f_0,2) than f_c, the phase difference between the two comb filters is +90° or −90°, respectively. This phase difference is used by the PM-HLL algorithm as a control signal to adapt the fundamental frequency estimate f_c towards f₀. Note that, due to the periodicity of the transfer functions involved, the same behavior applies to all harmonics of the signal, i.e., the phase flip between +90° and −90° occurs at the same value of f_c for the fundamental and all its harmonics. Thus, the fundamental and all its harmonics contribute to the control signal in the same way.