This paper presents a cycle efficient software frequency response analyzer (SFRA), which is integrated into the firmware of the digitally controller power converter to measure the frequency response of the plant and the open-loop gain of the closed-loop system. The algorithm uses the readily available measurements taken for the power stage control and does not require additional hardware. This enables periodic measurement of the frequency response which can be used to monitor changes in the power converter. The proposed technique can run on a conventional microcontroller without any host intervention, and adds no cost to the system, which is important for the cost-sensitive power converters. Both floating point and fixed point implementation are compared enabling widespread applicability of the technique. In order to validate the efficacy of the analyzer, a synchronous buck power converter is designed and its control transfer function is compared with the measured online frequency response. An algorithm is outlined to fit the measured SFRA data to a pole-zero format through intelligent cost function estimation. The correlation of zeros and poles identified using this method with the modeled plant transfer function is analyzed. A dual core approach is discussed to run SFRA on a high-switching-frequency converter, thus showing applicability of the technique in high-switching-frequency converters.