Which trigger mode is most effective when using an oscilloscope to measure a linear power supply's output ripple?
Why Ripple on a linear supply is a direct byproduct of the AC mains, appearing at the line frequency or twice it (60 Hz or 120 Hz in the US). The oscilloscope's line trigger derives its timing from the AC power line itself, so any waveform locked to the mains appears stationary and stable on the screen. That makes the small ripple easy to see and measure even though its amplitude may be too low or too noisy to trigger reliably on its own.
Watch out Edge or level triggering on the ripple signal itself is workable in principle, but the ripple is small and often buried in noise and switching spikes, so the trace tends to jitter; single-shot captures just one sweep and is meant for one-time transients, not a repetitive waveform.
Ripple comes from the power line, so trigger on the line. 60 Hz in, 120 Hz ripple out of a full-wave rectifier.