PWM Signal not visible upon increasing the scope time base

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I am capturing 2 different PWM Signals.

The frequency of first PWM signal is lower when compared to the second one.

Please check the waveforms.

All the waveforms are taken of the same two signals. But upon increasing the time base, the second high frequency PWM Signal vanishes like shown below. Why is this happening? Some settings to be changed in scope?

Oscilloscope : HDO6054A Teledyne Lecroy

 

Hi,

Undersampling artefacts.

Choose an even smaller timebase.

Klaus
 

Could you explain what you mean by "Undersampling artefacts", please
 

Hi,

undersampling causes alias frequencies.
You don´t see the real frequency, you see an alias frequency. --> Nyquist.
There are many documents and videos in the internet. Just do a search.

Did you follow my recommendation and choose a smaller time base setup?

Klaus
 

Yes. I followed a smaller time base and I am able to capture the signal perfectly. I have already attached the images in my question with smaller timebases also
 

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