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| is it true, oscilloscope must reach at least 4x observed freq? |
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| coppice:
--- Quote from: tggzzz on September 12, 2022, 08:41:01 pm --- --- Quote from: oxy on September 12, 2022, 06:32:08 pm ---Lets take the example of the picoScope Series 6000E: It has several models with bandwidths ranging between 500MHz and 1 GHz, yet all of them with sampling rate of 2.5Gs/s. As I understand the sampling rate sets over Nyquist the max. frequency that I can observe. Thus what hardware specification differentiates those oscilloscopes on the bandwidth? Thanks for ur very nice inputs! :clap: --- End quote --- For digital signals the period is irrelevant; only rise time is important. For a little theory and some practical measurements, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/ If you have a repetitive signal, the sampling frequency is separate to the signal frequency. I have a scope that measures 50ps rise times with ~40kSa/s. No, that does not violate Shannon/Nyquist! Various manufacturers have different names for the techniques, e.g. equivalent time sampling. --- End quote --- I think you mean "for repetitive digital signals". |
| switchabl:
--- Quote from: tautech on September 12, 2022, 08:28:49 pm ---This with a shared ADC design provides 2.5x BW sampling which comfortably meets Nyquist. --- End quote --- Yes, but the sampling theorem refers to the total bandwidth of a (band-limited) signal, not the 3dB bandwidth of the front-end. So realistically, there is nothing comfortable about it. At 2.5x you will either have to live with some non-negligible aliasing or with some non-negligible overshoot (steep filter). |
| tautech:
--- Quote from: switchabl on September 12, 2022, 09:11:53 pm --- --- Quote from: tautech on September 12, 2022, 08:28:49 pm ---This with a shared ADC design provides 2.5x BW sampling which comfortably meets Nyquist. --- End quote --- Yes, but the sampling theorem refers to the total bandwidth of a (band-limited) signal, not the 3dB bandwidth of the front-end. So realistically, there is nothing comfortable about it. At 2.5x you will either have to live with some non-negligible aliasing or with some non-negligible overshoot (steep filter). --- End quote --- Sure, however those that understand their instruments and could push them near their limits will use a single channel on each ADC so to be back near 5x sampling to minimize the chance of aliasing and also provide for greater accuracy measurements. |
| tggzzz:
--- Quote from: coppice on September 12, 2022, 08:47:18 pm --- --- Quote from: tggzzz on September 12, 2022, 08:41:01 pm --- --- Quote from: oxy on September 12, 2022, 06:32:08 pm ---Lets take the example of the picoScope Series 6000E: It has several models with bandwidths ranging between 500MHz and 1 GHz, yet all of them with sampling rate of 2.5Gs/s. As I understand the sampling rate sets over Nyquist the max. frequency that I can observe. Thus what hardware specification differentiates those oscilloscopes on the bandwidth? Thanks for ur very nice inputs! :clap: --- End quote --- For digital signals the period is irrelevant; only rise time is important. For a little theory and some practical measurements, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/ If you have a repetitive signal, the sampling frequency is separate to the signal frequency. I have a scope that measures 50ps rise times with ~40kSa/s. No, that does not violate Shannon/Nyquist! Various manufacturers have different names for the techniques, e.g. equivalent time sampling. --- End quote --- I think you mean "for repetitive digital signals". --- End quote --- No, for repetitive signals. To put it anthropmorphically, the scope neither knows nor cares how you interpret the voltage waveform. |
| coppice:
--- Quote from: tggzzz on September 12, 2022, 10:03:10 pm --- --- Quote from: coppice on September 12, 2022, 08:47:18 pm --- --- Quote from: tggzzz on September 12, 2022, 08:41:01 pm --- --- Quote from: oxy on September 12, 2022, 06:32:08 pm ---Lets take the example of the picoScope Series 6000E: It has several models with bandwidths ranging between 500MHz and 1 GHz, yet all of them with sampling rate of 2.5Gs/s. As I understand the sampling rate sets over Nyquist the max. frequency that I can observe. Thus what hardware specification differentiates those oscilloscopes on the bandwidth? Thanks for ur very nice inputs! :clap: --- End quote --- For digital signals the period is irrelevant; only rise time is important. For a little theory and some practical measurements, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/ If you have a repetitive signal, the sampling frequency is separate to the signal frequency. I have a scope that measures 50ps rise times with ~40kSa/s. No, that does not violate Shannon/Nyquist! Various manufacturers have different names for the techniques, e.g. equivalent time sampling. --- End quote --- I think you mean "for repetitive digital signals". --- End quote --- No, for repetitive signals. To put it anthropmorphically, the scope neither knows nor cares how you interpret the voltage waveform. --- End quote --- OK. I have read the article now. What you said is confusing. You replied to something talking about sampling rates, and gave a reply including reference to a slow sample interval. I assumed by "period" you were referring to the sampling interval. From the article you are referring to the period of a waveform to be sampled. Most digital waveforms don't really even have a period. They don't repeat, so they aren't periodic. |
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