Ever wondered how scope probes really work? Most textbooks treat scope probes as a combination of a resistive divider in combination with capacitors to provide an extended frequency response. But as will be revealed, the reality is that they are much more complex in principle. Read on.

Conclusions
The morals of this tale are:
• Trim your probe’s compensation capacitor!
• Textbook analyses of probe operation rarely mention transmission line effects but these are fundamental to the design of a probe.
• There IS a difference between low bandwidth and high-bandwidth probes. High-bandwidth probes are designed with carefully tailored transmission-line cable and to minimise the effects of end-to-end transmission-line reflections. Much more attention is paid to stray capacitances and build quality.
• A x10 probe will only exhibit 10MOhm impedance at low frequencies. The impedance at higher frequencies is mainly determined by the probe compensating capacitance.
• Use identical probes with equal rise time and bandwidth when interchannel timing is important (eg, chasing race conditions or clock skew).
• Probe ground-lead inductance can destroy waveform fidelity and bandwidth. Use the kit of adaptors in your probe’s pouch to ensure low inductance probe grounds!
• Don’t let your probes dangle off the test-bench. Even the good ones break when you tread on them or run your office chair over them!

• Don’t let your probes dangle off the test-bench. Even the good ones break when you tread on them or run your office chair over them!