Timeline for Oscilloscope design question
Current License: CC BY-SA 4.0
15 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Mar 4, 2021 at 2:46 | comment | added | Kartman | Wow! Did I open a can of worms! Am I to conclude the statement ‘nonsense’ has been watered down???? | |
| Mar 3, 2021 at 13:23 | comment | added | John B | @marcelm if the signal isn't a perfect sine wave, then it's not the highest frequency component in the signal, and therefore your sampling rate doesn't meet the Nyquist criteria for that signal. | |
| Mar 3, 2021 at 13:21 | comment | added | marcelm | @JohnB But what if the signal isn't a perfect sine wave? Or you don't even know the shape of the signal? What if your sine wave has noise? Or amplitude or frequency modulation? Yes, if the signal is a perfect sine wave and you know this, 2 samples/cycle is all you need. But in that case, why use an oscilloscope? A frequency counter would suffice. | |
| Mar 3, 2021 at 13:18 | comment | added | marcelm | @MarcusMüller "We mustn't give in to the naysayers who claim analog scopes are any better." - Oh no, I agree. And in my experience 10 samples/cycle is plenty to match or beat analog scopes. My point was more that "you only need 2 samples/cycle because Shannon-Nyquist" is not true in practice. | |
| Mar 3, 2021 at 13:16 | comment | added | John B | @MarcusMüller Wow that's a bit beyond me! It's more complicated than I thought then. I thought there was only one way to "connect the dots" with a sine curve, as long as you assume the frequency is not higher than the Nyquist threshold. | |
| Mar 3, 2021 at 13:13 | comment | added | Marcus Müller | @JohnB think about continuous signal reconstruction from discrete-time values: in theory, every value between two sample instants requires the sinc sidelobes of all countably infinitely many samples before and after. That's a bit problematic, especially because we don't know the future. Assuming periodic repetition is one of the common tricks to get around that, so that with only a limited amount of past and no future, we can reasonably sinc-interpolate | |
| Mar 3, 2021 at 13:11 | comment | added | John B | @marcelm why would it be limited to whether the signal is repeating? How could that make any difference? | |
| Mar 3, 2021 at 13:10 | comment | added | Marcus Müller | @marcelm that "repetition" explanation is a bit backwards, to be honest, but yeah, from the point of view of a discrete signal having discrete, meaning repeating spectrum, and vice versa, it makes sense. You hit the point big time when you point out sampling imperfections (in addition to bandlimiting imperfections): These together suggest a solid amount of oversampling is desirable. We mustn't give in to the naysayers who claim analog scopes are any better. None of the really high-end devices can be realized in analog, and that should tell people somethinge. | |
| Mar 3, 2021 at 13:08 | comment | added | John B | Chances are with 50Mhz bandwidth and 1Ghz sampling, it has more than one channel. Typically 4. They have to design for the case where you're using all 4 channels at once, and require the max bandwidth. That gives you a 5:1 sampling rate/channel. But that is really the minimum required because there's usually no antialiasing filter. So just above 50Mhz there's still quite a bit of signal coming through. Hopefully the 50Mhz frontend will be attenuating anything above 125Mhz to negligible levels, so 200Ms/s should be good enough to avoid aliasing. | |
| Mar 3, 2021 at 12:29 | answer | added | Neil_UK | timeline score: 5 | |
| Mar 3, 2021 at 12:26 | comment | added | marcelm | @MarcusMüller Shannon-Nyquist only applies when the signal is perfectly repeating. In reality, signals change both due to interference and because they carry information. Furthermore, the scope's hardware frequency response is far from perfect, which affects Shannon-Nyquist's applicability. This paper offers some interesting insights. | |
| Mar 3, 2021 at 11:58 | comment | added | Marcus Müller | @Kartman nonsense; Shannon-Nyquist literally says you can reproduce the exact 50 MHz bandwidth with only >100 MHz sampling rate. What is much better might be the fidelity of operation if bandlimiting to 50 MHz isn't exactly achieved. Also, some people design their oscilloscopes (or configure them) to connect sample values with straight lines, which is mathematically incorrect; but that's nothing to do with analog vs digital, but with "appropriate" and "incorrectly designed". | |
| Mar 3, 2021 at 11:43 | review | First posts | |||
| Mar 3, 2021 at 12:50 | |||||
| Mar 3, 2021 at 11:39 | comment | added | Kartman | 1GS/s is only 20 samples for a 50MHz signal. An analog scope gives you much better fidelity. | |
| Mar 3, 2021 at 11:32 | history | asked | ACBlue | CC BY-SA 4.0 |