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Zooming *IN* [actually: signal reconstruction]

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Fungus:

--- Quote from: Bud on October 21, 2022, 09:47:23 pm ---Based on the  rules quoted above, a 20GHz s scope would not be able to perfectly reconstruct my 1kHz square wave because the spectrum of my square wave is infinite.

--- End quote ---

Correct, although whether or not you'd be able to see the difference on screen is another matter.

PS: If it's an 8-bit ADC then you can only ever show 128 harmonics of a square wave so a 20Mhz, 8-bit DSO would show the same as a 20GHz, 8-bit DSO.  :)

Fungus:

--- Quote from: tautech on October 21, 2022, 10:07:11 pm ---
--- Quote from: Fungus on October 21, 2022, 06:53:46 pm ---This is where (eg.) the Rigol MSO5000s 8GHz sample rate is a big advantage. It's 6x Nyquist where most scopes aim for 2.5x.

It's not always about "noise". Other numbers matter, too.

--- End quote ---
They do.
Turn on all 4 channels and look what happens to your 8GSa/s.

--- End quote ---

Umm... I get 2Ghz per channel?

ie. 6x Nyquist, the number I already posted above.

switchabl:

--- Quote from: Fungus on October 21, 2022, 10:18:13 pm ---PS: If it's an 8-bit ADC then you can only ever show 128 harmonics of a square wave so a 20Mhz, 8-bit DSO would show the same as a 20GHz, 8-bit DSO.  :)

--- End quote ---

I would suggest that you measure the rise-time with both scopes and then you may want to reconsider this statement...

balnazzar:

--- Quote from: Bud on October 21, 2022, 09:47:23 pm ---
--- Quote from: balnazzar on October 21, 2022, 05:43:12 pm ---Nyquist’s most famous theoremproposes that a signal can be reconstructed perfectly from discrete samples if the following two rules are observed:
1. The highest frequency component sampled must be less than half the sampling frequency and
2. Samples must be acquired in equally spaced intervals

--- End quote ---
There is a flaw in this statement because it gives no guidellines as to how low the amplitude of a high frequency component is considered practically sufficient for satisfactory visual reproduction of the waveform on the oscilloscope screen. Based on the  rules quoted above, a 20GHz s scope would not be able to perfectly reconstruct my 1kHz square wave because the spectrum of my square wave is infinite.

--- End quote ---

Good point.. I'm looking forward to reading more people's comments on that...

bdunham7:

--- Quote from: Fungus on October 21, 2022, 10:18:13 pm ---PS: If it's an 8-bit ADC then you can only ever show 128 harmonics of a square wave so a 20Mhz, 8-bit DSO would show the same as a 20GHz, 8-bit DSO.  :)

--- End quote ---

I don't think that makes sense mathematically.  At the rising and falling edges all of the (odd) harmonics add, although they don't add at their peaks so it gets complicated and hard to visualize or make up simple hand-wavy explanations..  You don't see the individual harmonics, you see their sum.  So harmonics 127 + 129 add up to more than 1 LSB, as do harmonics 247-265, even though individually they wouldn't. 

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