Your ground lead is lying to you

2026-01-24 · 100 MHz scope, standard 10x probes

Measuring a fast edge with the long crocodile-clip ground lead and seeing 30 MHz ringing that is not there in reality is the most common self-inflicted measurement error I know.

why

The ground lead is a loop of wire. A loop of wire is an inductor. Combined with the probe capacitance it forms a resonant circuit, and a fast edge rings it. Twelve centimetres of lead is enough to invent tens of megahertz of noise that exists only in the measurement.

same node, same edge:
6 cm croc lead ...... 180 mV overshoot, ~28 MHz ring, ~6 cycles
spring ground clip .. 40 mV overshoot, ring gone in one cycle

fix

Use the little spring clip that came with the probe and that everyone loses in the first month. Ground it to a point within a centimetre of what you are probing. For anything above about 20 MHz this stops being optional.