Resolution determines the shortest impedance discontinuity that a TDR instrument can measure. Because of round trip effects, Resolution = 1/2(System Reflected Rise Time). If a discontinuity, such as a variation in the width of a trace, is small with respect to the system rise time, the reflection will not accurately represent the impedance of the discontinuity. In extreme cases, the discontinuity may effectively disappear. System rise time is the combined rise time of the step generator (TDR), the instrument, and the interconnect between the TDR and the circuit under test. In general, the most significant limitation in impedance testing is the probe. Close attention to probe geometry and probing techniques can greatly enhance resolution.