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is it true, oscilloscope must reach at least 4x observed freq?
robert.rozee:
--- Quote from: switchabl on September 13, 2022, 02:08:52 pm ---It is what happens if you just ignore the Nyquist criterion. And then apply sinc-based interpolation anyway. Did you have to turn that on manually or is that actually the default? If so, that would seem to be either a bug or a strange design choice indeed.
--- End quote ---
the default behavior of a Siglent SDS1204X-E. i'd imagine most other similar modern DSOs would default to the same.
--- Quote from: BillyO on September 13, 2022, 01:57:39 pm ---I wonder if the OPs question is well answered here?
--- End quote ---
he asked a simple question, and in return received a whole jumble of complex technical explanation that went straight over his head - and that was, indeed, irrelevant to the context in which his question sat. i'd imagine he is long gone, and will likely not be seen on the forums again.
to answer the OP's question: as the (broadly speaking) frequency of the signal you're sampling heads towards 1/2 of the scope's sampling rate, the image displayed by your DSO will approach a smooth undulation / sine curve. if you wish to observe fine detail in a complex waveform, you need to ensure that your DSO has a sampling rate at least several times the 'frequency' of that detail - as per the earlier posted screenshots.
if you are just wanting to observe a simple RF signal (such as AM modulation on an RF carrier), then you can get away with a sampling rate that is a tad over 2x the carrier frequency. just be wary that a DSO will 'smooth over' signal features that approach the scope's limits.
cheers,
rob :-)
Fungus:
--- Quote from: switchabl on September 13, 2022, 02:08:52 pm ---It is what happens if you just ignore the Nyquist criterion. And then apply sinc-based interpolation anyway. Did you have to turn that on manually or is that actually the default? If so, that would seem to be either a bug or a strange design choice indeed.
--- End quote ---
It's mathematically correct. What else would it do?
switchabl:
If the scope has a 200 MHz front-end and the sampling rate is set to 10 kSa/s like in the example, it is not mathematically correct (unless the user makes sure the signal itself is < 5kHz BW or it does decimation). Keysight scopes e.g. switch to linear interpolation in such a scenario which I think is a sane default. At the very least it gives you an immediate visual cue that you are sub-sampling. I guess the only mathematically correct option would be dot mode?
EDIT: I am not sure if the Siglent behaviour is a Lecroy-ism or if this is actually more widespread. It seems strange to me.
Fungus:
--- Quote from: switchabl on September 13, 2022, 03:32:42 pm ---Keysight scopes e.g. switch to linear interpolation in such a scenario
--- End quote ---
And are therefore less valuable when you're trying to learn signal theory. :)
nctnico:
--- Quote from: switchabl on September 13, 2022, 03:32:42 pm ---If the scope has a 200 MHz front-end and the sampling rate is set to 10 kSa/s like in the example, it is not mathematically correct (unless the user makes sure the signal itself is < 5kHz BW or it does decimation). Keysight scopes e.g. switch to linear interpolation in such a scenario which I think is a sane default. At the very least it gives you an immediate visual cue that you are sub-sampling. I guess the only mathematically correct option would be dot mode?
--- End quote ---
IMHO switching to linear interpolation isn't a good solution either. It is entirely possible that the signal you are looking at still meets Nyquist at a lower sampling rate. The oscilloscope simply doesn't know that. This is a typical case where the user needs to know what he/she is doing.
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