What's the reason behind this behaviour?

 

It means that your oscilloscope is triggering multiple times and the shape of the waveform is different at each trigger. Try changing your horizontal sweep rate (TIME/DIV) to a longer time, for example, 500us or 1ms.

 

It means that your oscilloscope is triggering multiple times and the shape of the waveform is different at each trigger. Try changing your horizontal sweep rate (TIME/DIV) to a longer time, for example, 500us or 1ms.

I can't do that because then the signal becomes tight and compressed in the screen and I can not examine its parts

 

hi SAM.
The two waveforms are different in rise time and amplitude, so the trig sync point will alternate between the two waveforms.
What is the source of the scope input signal.?
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Well, I told you what causes it and how to fix it. The rest is up to you.
You can also adjust the sweep holdoff control if your oscilloscope has one.
You can also set the oscilloscope to SINGLE capture.