Abstract: Techniques are provided for monitoring the performance of circuits and replacing low performing circuits with higher performing circuits. A frequency detector compares the frequency of a first periodic signal to the frequency of a second periodic signal. The difference in the frequency between the first periodic signal and the second periodic signal indirectly indicates how much the threshold voltages of the transistors have shifted. The difference in frequency between the two periodic signals can be monitored to determine the speed and performance of circuits on the chip. The output of the frequency detector can also indicate when to replace low performing circuits with higher performing circuits. When the frequency of the second periodic signal differs from the frequency of the first periodic signal by a predefined percentage, a low performing circuit is replaced with a higher performing replica circuit.