A system and method for observing threshold voltage variations are provided. A ring oscillator circuit comprises a plurality of inverters arranged in a sequential loop, a plurality of test circuits having devices under test, each coupled between a respective one of the inverters and a power supply. Each test circuit has a bypass field effect transistor (FET) having a first channel coupled between the power supply and a respective one of the inverters responsive to an individual enable signal, and a FET device under test having a second channel arranged in parallel to the first channel. A method is described for determining the threshold voltage of the device under test by disabling, for one of the inverters in the ring oscillator, the first FET device such that the device under test is coupled between the power supply and the respective inverter and affects the operating frequency of the ring oscillator.