Question E4A04:
How is compensation of an oscilloscope probe performed?  

Choices:  
A: A square wave is displayed, and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible  
B: A high frequency sine wave is displayed, and the probe is adjusted for maximum amplitude  
C: A frequency standard is displayed, and the probe is adjusted until the deflection time is accurate  
D: A DC voltage standard is displayed, and the probe is adjusted until the displayed voltage is accurate    

Correct Answer is  A: A square wave is displayed, and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible  

To compensate an oscilloscope probe, you feed a known square wave (often the scope’s built‑in 1 kHz calibrator) and adjust the probe’s trimmer capacitor until the displayed square wave has flat, level tops and bottoms with clean corners. That’s why A is correct.

Why A is correct
- Probe compensation matches the probe’s capacitive divider to the scope’s input so the attenuation is flat with frequency.
- A square wave contains many harmonics; correct compensation makes all those components pass with the right ratio, producing flat horizontal portions without tilt, rounding, or peaking.

Why the others are incorrect
- B: Adjusting a sine wave for maximum amplitude can produce peaking (overcompensation) at that frequency and will not ensure flat response across the bandwidth.
- C: A frequency standard/time markers are for calibrating the timebase, not probe compensation.
- D: A DC voltage standard won’t reveal compensation errors because compensation corrects AC (frequency-dependent) behavior; DC attenuation is set by the resistors.

Useful formulas and rules of thumb
- For a compensated attenuator (typical 10× probe):
  - Match time constants: $$R_1 C_1 = R_2 C_2$$
  - Equivalent ratio condition: $$\frac{R_1}{R_2} = \frac{C_2}{C_1}$$
  - For 10×: $R_1 \approx 9\,\text{M}\Omega$, $R_2 \approx 1\,\text{M}\Omega$, and $C_1 \approx C_2/9$.
- Visual cues when adjusting:
  - Undercompensated: rounded leading edge and droop on the top (tilted downward).
  - Overcompensated: overshoot/undershoot and a peaked or wavy top.
  - Properly compensated: flat tops/bottoms and square corners (within bandwidth limits).
- Procedure: Connect to the scope’s probe cal output, set the scope to reasonable time/div and volts/div, and adjust the probe’s trimmer with a nonmetallic tool until the square wave is flat.