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Rigol DS1054z mystery

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evava:
First: I am sorry if this belongs in the beginners section
Secondly: I am sorry for the somewhat clickbait title

Namely, I recently came across an interesting thing that I would like some more experienced people to explain/clarify to me:

Mem depth is set manually to 120k. Then, I apply 1Mhz to the input of the oscilloscope, successively a sinusoidal waveform, a rectangular waveform and a triangular one.
After pressing Auto on the oscilloscope, the oscilloscope correctly displays the waveform with a time base of 200ns and the frequency meter shows 1Mhz. However, when I manually extend the time base to 50ms, the oscilloscope shows the same waveform, with the frequency meter reporting a frequency of about 7Hz!
All this, of course, only with a manually set mem depth of 120Kbit (or less, on this particular oscilloscope).
The waveform is preserved (sine/rectangle/triangle), including voltage values. ​​

I tried googling this phenomenon, but I guess I'm asking the wrong question.
I believe that this mystery applies and appears apparently on all digital oscilloscopes where we deliberately set a small mem depth.
Please explain it to me - thank you.

RoGeorge:
It is called spectrum folding or aliasing.  Usually it happens when the sampling rate is too small, when it's less than twice the max frequency of the signal.  Turn on the 'Anti-aliasing' on the oscilloscope (last button on the right column in your pics), this should help, though there might be some tricky frequency in sync with the sweep frequency that might still appear wrong.


Image from:  https://en.wikipedia.org/wiki/Aliasing

For example, if the signal is similar with the blue sinusoid, but the sampling rate of the oscilloscope is much rare (like the black dots in the image), apparently you'll see on the oscilloscope a much lower frequency (you'll see on the oscilloscope only the orange line determined by the sampling black dots).

evava:
Thank you very much for your answer!

I thought it was something with aliasing, but I can't imagine how it is possible to keep exactly the same waveform?
Every waveform I tried, I got exactly the same waveform, during this aliasing.. :-//
Even pulse, etc.

RoGeorge:
The waveform is preserved.  To convince yourself, you can try drawing by hand on a paper, or write a few lines of code to simulate it.

The signal is repeating itself, so it doesn't matter if you sample now, or if you sample now + 100 periods in between, you'll see the same amplitude.

evava:
Yes, it sounds plausible. You are right.

Now I tried on my new ZEEWEII DSO2512G, and the same phenomenon. Only, it cannot be suppressed by increasing mem depth  :--

How to get around this with digital oscilloscopes with really little memory?  Each time to check if there is not the same waveform with a much higher frequency? Or what else?
How function AUTO works that it always finds the right frequency waveform?

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