Question E4A10:
Which trigger mode is most effective when using an oscilloscope to measure a linear power supply’s output ripple?  

Choices:  
A: Single-shot  
B: Edge  
C: Level  
D: Line    

Correct Answer is  D: Line  

To measure linear power supply ripple, the ripple is synchronized to the AC mains. A full-wave rectified supply produces ripple at twice the line frequency (120 Hz from 60 Hz mains, or 100 Hz from 50 Hz mains). Using Line trigger locks the scope sweep to the power-line phase, giving a rock-steady display of the ripple.

Why D (Line) is correct:
- The ripple is phase-locked to the mains, so triggering from the AC line gives a stable, non-drifting display.
- It avoids the difficulty of triggering on a small ripple riding on a large DC level.

Why the others are not:
- A: Single-shot — for one-time transients, not continuous periodic ripple.
- B: Edge — can work, but the small ripple on a large DC offset makes it hard to get a reliable trigger; more setup fiddling (AC coupling, level, slope) is needed.
- C: Level — similar issue: the ripple may not reliably cross a set threshold, and noise can cause false triggers. Not as robust as Line for mains-synchronous ripple.

Useful formulas and tips:
- Ripple frequency:
  $$f_{\text{ripple}} =
    \begin{cases}
      2 f_{\text{line}} & \text{full-wave rectifier} \\
      f_{\text{line}} & \text{half-wave rectifier}
    \end{cases}$$
- Capacitor-input filter ripple (approx. peak-to-peak):
  $$V_{r(pp)} \approx \frac{I_{\text{load}}}{f_{\text{ripple}} \, C}$$
- Practical scope setup: use AC coupling, bandwidth limit, and Line trigger for a stable, clean view of small ripple on a large DC output.