Scoposcopy: Probe Compensation, Input Coupling, Waveform Asymmetry

Here’s an explanation of several things concerning Scope Probe compensation adjustment, AC vs. DC input coupling, and the Waveform Asymmetry from the scope’s Calibrator square-wave output. This demonstration uses the old Tektronix 2213a but applies to most oscilloscopes that use the same system of probe calibration.

A “sloping line” symptom from the scope’s Calibrator trace only occurs, in this scope, when the probe is set to 1x attenuation and the channel input coupling is set to “AC coupled.” Furthermore, the probe compensation capacitor has almost no effect on the trace when the probe is set to 1x. For proper compensation adjustment the probe should be set to 10x and the capacitor adjusted for the flattest possible square wave from the calibrator.

The AC-coupled input setting brings the _average_ of the signal down or up to the zero-volt baseline setting. A slight asymmetry seen in the peak voltages from the calibrator is due, in this case, to a duty-cycle asymmetry: the upward going parts are not exactly the same duration as the downward going parts of the “square wave” signal. This results in the AC-coupled signal being slighty asymmetrical in voltage around the zero baseline. But it is still symmetrical as far as the _areas_ are concerned, so the “average” of the areas is still brought correctly to the baseline.

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