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Zeeweii toyscopes - How do they do it?
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tatel:
I recently got into an argument with fellow forum member radiolistener about Zeeweii toyscopes.

radiolistener's point is these devices doesn't have any antialiasing filter so they can't be trusted above 5 MHz (DSO2512, thread here: https://www.eevblog.com/forum/testgear/new-2ch-pocket-dsosg-sigpeak-dso2512g/msg3547154/#msg3547154)

My point was that even DSO154 seems to show (thread: https://www.eevblog.com/forum/testgear/new-toy()-scope-dso154pro-1ch-claimed-40mss/msg4531274/#msg4531274) accurate frequencies and waveforms.

After reading about Nyquist theorem and aliasing (https://www.asdlib.org/onlineArticles/elabware/Scheeline_ADC/ADC_NumRep_Nyquist.html) I reckon radiolistener's point is valid. But how does Zeeweii achieve to show semingly good results?

Obviously I know fungus has a DSO154 so he will have probably something to say, then hopefully all the usual suspects will also get into the slaughterhouse with their chainsaws 8)
wasedadoc:
It is very simple.  Any frequency components in the input signal that are higher than half the sampling frequency get shifted down.  Folded around the half sampling frequency.  Say sampling at 100MHz. Input 51MHz sine wave.  Reconstructed output will be 49MHz sine wave.  Input 50MHz sine will produce 50MHz sine.  Input 52MHz sine, output 48MHz sine etc. Input 100MHz, get DC out.

If there are multiple input components eg 30 MHz and 60 MHz the 30 MHz one will be reconstructed properly.  The 60 MHz one will appear as 40 MHz. Aliasing does not change the amplitude.  The amplitude of the false 40 MHz out will be the same as the 60 MHz in.  Aliasing does not change the amplitude or frequency of the 30 MHz component.

Still using 100MHz sampling rate example, if you input a 20MHz square wave then the harmonics , 60MHz, 100MHz and up will be aliased down and distort the viewed waveform.  However the 20MHz component in the input has the largest amplitude.  It still has the largest amplitude at the output.  The output will not be a square wave but will have recognisable period of 50nS.

In summary, if there is negligible amplitude of frequency components above the half sampling frequency then the effect on viewed waveforms is negligible.

You have probably seen aliasing many times when watching a cowboy movie on TV.  The wagon wheels rotating backwards has the same cause.  The TV frame rate is too low compared to the forward rotation rate of the wheels. 
tatel:
Thank you, thats a much clear -and fast- answer than anything I could get from the stuff I readed.

So, are the fears induced in us poor little newbies exagerated?
tatel:
Also, if we get an aliased waveform that is, say 48 MHz, how could the toyscope show an accurate frequency measurement of 52 MHz?

I'm going to have quite a lot of fun doing some tests.

Again, thank you very much for giving a simple answer
wasedadoc:

--- Quote from: tatel on April 21, 2023, 07:51:40 pm ---Also, if we get an aliased waveform that is, say 48 MHz, how could the toyscope show an accurate frequency measurement of 52 MHz?

I'm going to have quite a lot of fun doing some tests.

Again, thank you very much for giving a simple answer

--- End quote ---
The toyscope has a frequency counter working on the input signal, not the sampled signal.
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