What happens if you trigger on a very noisy signal for averaging?

You might expect that you will get a flat line, as the noise will average out.

But as you trigger on that signal, something surprising will happen, as you can see in the 🌐video. This is only a simulation considering a bandwidth-limited signal, but you will see a very similar picture on an oscilloscope.

The explanation is surprisingly simple. Triggering on a rising edge implies that the point preceding the trigger point is below the trigger level and the next point must be above the trigger level. Therefore, around the trigger point, the averaged signal must resemble a rising straight line. As it is a noise signal, it must ultimately return to the average value. Therefore, a single peak is left after averaging. The shape is determined by the impulse response of the transfer function.