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DSO Samplerates - From when they´re "enough" and why?

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nctnico:

--- Quote from: alm on January 18, 2023, 03:58:16 pm ---
--- Quote from: vk6zgo on January 18, 2023, 01:27:56 pm ---All the discussion so far seems to be drifting into the problems of higher frequencies, where anti-aliasing filters come into play, but if sampling rates are still reduced at much lower frequencies, any alias components will still be present for those lower frequencies which 'scopes are very commonly required to observe,

--- End quote ---
Unless you are zooming in a lot after acquisition, the "resampling" by the display's limited horizontal resolution, which unless you're using an ancient Tek DSO is many orders of magnitude lower than the sample memory, is probably a bigger worry. But I believe most scopes, including Lecroy scopes, do some sort of peak detect when they resample data for display?

--- End quote ---
Yes and no. In order to make the process fast, the acquired data is decimated using only a subset of the samples (one out of X). If you feed a slow sweep into a DSO with deep memory, you can get all kinds of funny aliasing effects. In practical situations this doesn't really matter though.

alm:

--- Quote from: nctnico on January 18, 2023, 10:06:46 pm ---Yes and no. In order to make the process fast, the acquired data is decimated using only a subset of the samples (one out of X). If you feed a slow sweep into a DSO with deep memory, you can get all kinds of funny aliasing effects. In practical situations this doesn't really matter though.

--- End quote ---
I just tried it on two Lecroy scopes (one VxWorks and one X-Stream) which definitely didn't have peak detect enabled :P, with a signal with a 1 ms period and 2 ns pulse width, and as long as the time base and memory depth were set so the sample rate was at least 500 MS/s, the scopes would reliably display each peak. If the signal was decimated before display, wouldn't you expect this 1 sample peak to disappear intermittently? Of course the amplitude is not very stable with only one sample somewhere within the pulse, but all four pulses are shown consistently.

Do other brands work differently?

nctnico:
There is a bit more to it, but there is definitely some optimisation going on. Try a frequency sweep.

alm:
Here's a logarithmic 100 Hz to 15 MHz sweep. Looks like a decent approximation within the limited screen resolution to me.

markone:

--- Quote from: Martin72 on January 17, 2023, 11:05:13 pm ----snip
At work we have older waverunner lt models from lecroy in addition to the new ones.
200Mhz and 200MSa/s, 500Mhz 500MSa/s, but also a model with 500Mhz and 1GSa/s.

--- End quote ---

I would say that nowadays those specs, only usable with equivalent time acquisition mode, are simply laughable. 

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