An electronic apparatus for testing an integrated circuit (IC) that includes a ring oscillator is provided. The apparatus configures the ring oscillator to produce oscillation at a first frequency and configures the ring oscillator to produce oscillation at a second frequency. The apparatus then compares the second frequency with an integer multiple of the first frequency to determine a resistive voltage drop between a voltage applied to the IC and a local voltage at the ring oscillator. The ring oscillator has a chain of inverting elements forming a long ring and a short ring. The ring oscillator also has an oscillation selection circuit that is configured to disable the short ring so that the ring oscillator produces a fundamental oscillation based on signal propagation through the long ring and enable the short ring so that the ring oscillator produces a harmonic oscillation based on a signal propagation through the short ring and the long ring.