A method, providing an oscillator output signal to reference inputs of a PLL and an output clock circuit; providing a first divisor value to a control input of the PLL to regulate a closed loop that includes a physics cell, a receiver, and the PLL; providing a second divisor value to a control input of the output clock circuit to control an output frequency of an output clock signal; shifting the first divisor value in a first direction to cause a perturbation in the closed loop; shifting the second divisor value in an opposite second direction to counteract a response of the closed loop to the perturbation and to regulate the output frequency of the output clock signal; and based on the receiver output signal, analyzing the response of the closed loop to the perturbation.