Learning Library · Signals and oscilloscopes · 4
Probes and probe compensation
A passive 10× probe is a precision voltage divider built into the probe and cable. Its resistors set the 10:1 ratio at DC, and a small adjustable capacitor has to match the scope's input capacitance so the ratio holds at high frequencies too. That adjustment is probe compensation.
The probe tip is on the scope's 1 kHz calibration output. Turn the trimmer until the corners are square.
Why the capacitor matters
Inside a 10× probe, a 9 MΩ tip resistor and the scope's 1 MΩ input form a 10:1 divider. The scope input also has capacitance, from the input circuit and the probe cable, and at high frequencies that capacitance controls the division instead of the resistors. A small capacitor across the tip resistor balances it.
When the two time constants match, the divider is 10:1 at every frequency and a square wave stays square. Too little tip capacitance rounds the corners. Too much makes them overshoot.
Try this
- Leave the trimmer low. The corners roll off: high frequencies are attenuated more than 10:1, and rise times read slow.
- Turn it high. The corners spike: high frequencies get through with less attenuation, and amplitude measurements read high near every edge.
- Switch to 1×. There is no divider to compensate, but the scope now sees the full cable capacitance. Compare the load impedance at 10 MHz with the 10× setting.
1× vs. 10×
| Setting | Best for | Watch out for |
|---|---|---|
| 10× | General-purpose work. Ten times less loading, full probe bandwidth. | Signals are divided by ten, so very small signals sit closer to the scope's noise floor. |
| 1× | Millivolt-level signals at low frequency. | Heavy capacitive loading and a bandwidth that is often only a few megahertz. |
Compensate a probe every time you move it to a different scope or channel, because input capacitance varies from input to input. It takes ten seconds with the small screwdriver that comes with the probe. Set the scope's probe ratio to match the probe too, or every voltage reading is off by a factor of ten.
Start with the right probe. Passive, differential, current, and active oscilloscope probes.
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