a circuit constructed in accordance with this invention includes a ring oscillator to provide a signal which is dependent on the propagation delays of the inverters comprising the ring oscillator , therefore the frequency of the ring oscillator is inversely dependent upon the propagation delays of the inverter comprising the ring oscillator . means are provided to determine the propagation delay introduced by the components in the ring oscillator by measuring the frequency of the output signal produced by the ring oscillator which provides a signal to a multiplexer which selects among a number of preset delay components those components which are necessary to ensure that the propagation delay caused by the circuitry connected to the input lead of the circuit constructed in accordance with this invention plus the propagation delay introduced by the selectable delay elements is nearly a constant propagation delay .