Often, it is impossible to obtain a stable trigger with simple edge triggering. The signal in this image consists of a train of pulses. With edge triggering, a trigger event can occur on the rising edge of any pulse, and it is impossible to see what the signal really looks like.

To get a stable trigger, we'd like to always trigger on the first pulse in the train. This sync pulse is 10 µs wide, so we can use pulse-width triggering.

Press the Trigger Menu button, then press the right bezel button to set the trigger type to "Pulse".

Then, use the middle bezel button to set the polarity to "Positive".

Use the ± rocker to set the pulse width to 10 µs.

Use the fourth bezel button to select "Trigger When = Width". This will cause a trigger event on any pulse whose positive width is within 10% of 10 µs. The tolerance can be set (with the ± rocker) to 5%, 10%, 15%, or 20%.

Now, the scope has a stable trigger. You can use the delay timebase to zoom in on a particular part of the signal (e.g., the falling edge of the third pulse).