The frequency of an oscillating signal generated by a ring oscillator is used to determine the select-to-output delay of standard cell multiplexers. The ring oscillator has no active logic elements other than an odd or even number of standard cell multiplexers. The signal path of the oscillating signal passes through the select input leads of the multiplexers of the ring oscillator. The ring oscillator can be used to characterize how signal propagation delay varies depending on the voltage supplied to the multiplexers. The lowest supply voltage at which a signal can continue to travel through the most critical circuit path of a test circuit can be modeled. In addition, the ring oscillator can be built into operational circuits to monitor timing and signal propagation delay in real time. Real time monitoring of delay enhances the benefits of adaptive voltage scaling, which is used in signal processing circuits in cell phones.