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DSO Samplerates - From when they´re "enough" and why?
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Martin72:
Hi,
Surely an old question....
Samplerates on a DSO.
You must have "enough" samples to recreate a proper signal.
It also depends on the frequency/bandwith.
Right?
OK..
I´ve got a siglent scope with max. 2Gsa/s and 500Mhz bandwith.
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.
I can not imagine that it was not enough at the time, with each model.
And yet it became more and more in the following years.
Even a cheap Rigol has 8GSa/s at 350Mhz.
In contrast, there are much more expensive models from other manufacturers, which have e.g. 4GSa/s at a bandwidth of 1Ghz.
(always max.)
Or our newer WR9054 which got 20(40) GSa/s and 500Mhz.
So the question:
How many samples do you really need, what does it depend on.
What is the minimum at which bandwidth, when do you need more at the same bandwidth.
Only in special cases or also in general.
Having more than needing is always better but in this case mostly a price decision.
Or not?
Carry on...


nctnico:
If you have the headroom for a Gaussian roll-off: you'd like to have 10 samples per period. If there is a steeper anti-aliasing filter AND a good implementation of sin x/x reconstruction, then 2.5 samples per period is enough. Sin x / x is prone to producing pre-ringing (Gibbs ears) on sharp edges that are not supressed enough by the anti-aliasing filter. Using the selectable bandwidths you typically find on high frequency DSOs you can tune this depending on what you are looking for in a signal.

Likely the 500MHz / 1Gs/s scope you mention has equivalent time sampling. That way you can acquire a repetitive signal using many more samples per second than the samplerate of the ADC. Some older several GHz oscilloscope use samplers that sample at several 10's of kHz. For example the Tektronix TDS820: 6GHz bandwidth using a 50ks/s 14 bit converter.
Fungus:
Pretty much what ntnico said. 2.5x is the minimum to avoid serious artifacts, more than 10x is probably a waste.

What I'd like to see would be dynamic bandwidth switching.

Most modern DSOs have software bandwidth upgrades so internally they have switchable filters, right? It would be nice if they could reduce the bandwidth when you turn all the channels on and avoid some artifacts. Bandwidth is cheaper than sample rate so it could make 'scopes cheaper.

eg. You could have a 4 channel, 1GSample/sec 'scope which has 200MHz bandwidth with one or two channels enabled and 100MHz bandwidth when you turn on three or four channels.
tautech:

--- Quote from: Martin72 on January 17, 2023, 11:05:13 pm ---Hi,
Surely an old question....
Samplerates on a DSO.

How many samples do you really need, what does it depend on.

--- End quote ---
Needs.

Used wisely undersampling can be beneficial to needs.
https://www.eevblog.com/forum/testgear/siglent-sds2000x-hd-12bit-(published-for-chinese-domestic-market-only)/msg4320658/#msg4320658
robert.rozee:
hi,
    this has been a topic of hot debate in the past. the simple answer is, it all depends on what you are looking for in the signal!

have a look at the below thread, link is to a post within the thread i made demonstrating the result of a square wave at a range of different sample rates relative to the fundamental (2x, 5x, 10x, 20x, 50x, 100x, 200x, 10,000x). this shows the results of just the oversampling, with an analog input bandwidth massively higher than the fundamenal frequency. ie, the effect of front-end bandwidth is excluded:
https://www.eevblog.com/forum/testgear/is-it-true-oscilloscope-must-reach-at-least-4x-observed-freq/msg4413487/#msg4413487

btw, the remainder of the thread may also answer some of your questions.


cheers,
rob   :-)
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