Nanosecond Passive Voltage Probes

Passive voltage probes with megohm input impedance at dc are traditionally understood as RC compensated dividers, but this is inadequate even for system rise times as slow as 100 ns. Qualitative understanding of faster probes is generally in terms of resistive damping of resonances, but this has been notably successful in practice only with the use of resistive conductor probe cable as introduced by Kobbe. This paper demonstrates that nanosecond probes can be understood and designed for a systematic transmission line viewpoint. The properties of the resistive conductor probe cable as a transmission line are an essential feature, and the original motivation for the use of resistive cable as damping of a resonance is seen to be largely irrelevant for nanosecond probes.