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| is it true, oscilloscope must reach at least 4x observed freq? |
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| Someone:
--- Quote from: wasedadoc on September 14, 2022, 09:05:02 am --- --- Quote from: Someone on September 14, 2022, 02:07:51 am ---2.5x is a massive gross simplification that looks ok in most cases for a single frequency sine wave. --- End quote --- If the specific combination of sampling rate and reconstruction filter gives an acceptable result on that single frequency sine wave it will also give an acceptable result on any lower frequency single sine wave. Then by the superposition principle it will also give an acceptable result on any combination of such sine waves. That is any waveform which does not contain frequencies above that sampling rate divided by 2.5. --- End quote --- Cool, now go and read what the OP actually wrote. Nothing about single frequency sine waves and ideal sampling/reconstruction. Again I keep saying it, 2.5 is some vague compromise with unspecified criteria. What is actually important? amplitude accuracy? relative phase? waveform shape? I interpret the OP's question as much more toward the balance of fundamental frequency of a clock/signal and the bandwidth limit of the scope, since both are expressed in MHz and nowhere is sampling mentioned. So all this chest beating by people who want to talk about Nyquist (without actually pointing to how it applies to oscilloscopes and their antialiasing + reconstruction) is off topic, and in many points wrong. Band limited signals are pure imagination, they dont exist. Perfect antialiasing filters dont exist, any real sampled signal has errors that should be quantified if you want to start putting some limit on their effect. |
| adam4521:
Yes, sin x/x on Agilent/Keysight only on short time bases/maximum sample rate. If there is any decimation going on (eg you are zoomed in on a longer capture) it will join the dots with straight lines. |
| Fungus:
--- Quote from: Someone on September 14, 2022, 10:21:23 am ---So all this chest beating by people who want to talk about Nyquist (without actually pointing to how it applies to oscilloscopes and their antialiasing + reconstruction) is off topic, and in many points wrong. Band limited signals are pure imagination, they dont exist. Perfect antialiasing filters dont exist, any real sampled signal has errors that should be quantified if you want to start putting some limit on their effect. --- End quote --- Practical experience has found a good balance: Sample at 2.5x the analog bandwidth of the oscilloscope's input circuitry and use sin(x)/x reconstruction to display it on screen. It's good enough for Australians. PS: Most DSOs can get double/quadruple the basic sample rate by limiting the number of enabled channels. |
| EEVblog:
--- Quote from: Fungus on September 14, 2022, 07:33:39 am ---Oscilloscope Sample Rate > 10x highest frequency component of signal (For linear interpolation) So that explains the "x10" rule that the old fogies occasionally mutter around here when they see Rigols/Siglents - they were using 'scopes with linear interpolation(!) :) --- End quote --- I don't think so. The "old fogie x10 rule" started with the Tek TDS210, it was the first "real time" digital scope that looked and acted like an analog scope, and it had 1GS/s for 100MHz bandwidth and also SinX/x. So x10 kinda became the defacto standard that gave everyone the "warm fuzzy" that digital scopes were now fast enough to look and feel like a "real scope" when you turned on the sample dots and could see how many samples it was taking. |
| EEVblog:
--- Quote from: Someone on September 14, 2022, 10:21:23 am ---Cool, now go and read what the OP actually wrote. Nothing about single frequency sine waves and ideal sampling/reconstruction. Again I keep saying it, 2.5 is some vague compromise with unspecified criteria. What is actually important? amplitude accuracy? relative phase? waveform shape? --- End quote --- Remember that the input wave shape gets changed by the input bandwidth response of the scope front end. Different scopes and models have different input antialiasing filters and responses. This is why the shape of the input filters matters in these dicsussions. Gaussian response is usually assumed unless otherwise specificed. Tek mention a x5 rule here: https://www.tek.com/en/documents/primer/evaluating-oscilloscopes |
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