Abstract: A test oscillator circuit separately measures the signal propagation delay for both rising and falling edges through one or more multi-input combinatorial logic circuits. A number of components are configured in a loop so that they together form a free-running ring oscillator. Each synchronous component passes signal edges to a subsequent component in the ring, so the oscillator produces an oscillating test signal in which the period relates to the delays through the components. In some embodiments, the multi-input combinatorial logic circuits emulate tri-state buffers. These embodiments characterize the speed at which these logic circuits enable and disable signal paths.