Uncontrolled characteristic impedance along a spring biased pin probe assembly is avoided by providing a stepped shelf of ground plane that extends outward along the pin and toward the target signal. The length of outward extension is chosen such that even when there is only (or at least) an expected minimum amount of compression of the spring while producing and maintaining contact, the entire exposed portion of the pin is over the shelf, whose depth of step has been selected to produce a selected Z.sub.0 for the exposed pin that matches Z.sub.0 for existing transmission lines already within the probe assembly. The spring biased pin may be a resistor tip spring pin that includes a small resistor in its tip.