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How much noise floor and other things matter in oscilloscope usability
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nctnico:

--- Quote from: Fungus on December 23, 2021, 03:03:25 pm ---
--- Quote from: nctnico on December 23, 2021, 02:53:04 pm ---
--- Quote from: Fungus on December 23, 2021, 02:51:59 pm ---
--- Quote from: Fiorenzo on December 23, 2021, 12:00:05 pm ---For example the noise floor:
In what situation really matter to have an oscilloscope with a "low" noise floor?

--- End quote ---
When you're measuring very small signals.

--- End quote ---
Actually not because the relative noise floor remains more or less constant!
I just tried on an R&S RTM3004. At 1V/div I get 17mV stdev. At 10V/div I get 170mv stdev (Stdev= RMS with DC removed). The noise floor scales with the V/div setting.

--- End quote ---

If the signal is large, you can see it, it's just that the traces on screen are thicker.

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Yes, and thick traces just suck. Try to make a cursor measurement on a trace that is 20% of a division even with V/div set to 1V/div. Needless to say that small variations of a signal are also lost in a noisy oscilloscope.

Don't confuse needing a pre-amplifier to look at signals that are outside the V/div range of an oscilloscope. That is a different subject!
Fiorenzo:
Ok, you all are giving me many usefull information, but now give a look to the attached photos.
The first image shows the apparent noise of CH1, It is set in AC mode, 1X attenuation, without bandwith limit and is also sampling in normal mode with the probe attached and grounded to its own tip.
The second image shows the same configuration but with the probe attached to the output of and old transformer just to check its ripple.

I see a lot of noise floor in this photos.
Do you agree? It seem too much to me, but as I said before I am not an expert in analoge stuff so maybe I am doing something wrong... Or maybe this oscilloscope is particularly noisy...
Fiorenzo:

--- Quote from: Fungus on December 23, 2021, 03:06:33 pm ---
--- Quote from: Fiorenzo on December 23, 2021, 03:01:05 pm ---Which kind of signal need an oscilloscope with a low noise frontend?

--- End quote ---

A 1mV signal.

(for example)

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What circuits works with such a low signal?  This is my question from the beginning.
I understand that noise sucks but it is the practical application of a low noise oscilloscope that i cannot figure out.
For example, if I work with on an old valve radio am I going to encounter such kind of low signal? It Is only an example....

I don't want to buy an oscilloscope and find myself after a year that It is going to particularly limit my study in electronics, this are costly instruments, I can spend some more money than that spent for the Rigol but I must do the right choice and now I don't feel confident with my actual knowledge.
bdunham7:
I haven't compared those two models directly, but comparisons with two lower models of Rigol and Siglent were enough to convince me that low noise was an important factor in some cases.  The Rigol was a lot noisier than typical analog scopes, whereas the Siglent is comparable with a good basic CRO.  The Siglent also has a for-real 500uV/div capability and the Rigol actually only went down to something like 2mV/div (or perhaps 5mV/div) and then expanded the signal and decreased the resolution for the lower ranges, a sort of digital zoom.  They had a 500uV/div range if you hacked them, but it was pretty useless.

As for some previous responses, I disagree that the 10X probe will be noisier than the scope, that might apply to a very quiet scope but with either of these the scope front-end noise will be the issue when you are looking at small signals with a 10X probe.  And you'll almost always be using a 10X probe because 1X is very limited bandwidth, so a 'low' signal will be anything below 50mV.  So when I've used a 10X probe with scopes like this on small signals (the lower versions of Siglent and Rigol) the Siglent is clearly superior (3-4X better at least) but sometimes I wish it were even better.  Always remember the bandwidth limiter will help if your signals are below 20MHz.

There was a mention that the noise appears on all scales, not just the lowest volts/div settings.  There's some of each, I suppose you could call them the analog-front-end noise and the ADC noise.  The Siglent clearly has more noise on the lowest volt/div settings and above 2mV/div, the 'ADC' noise is pretty minimal.  The Rigol will be worse in this regard because of the digital expansion for the lowest ranges.

You asked about sample rate and bandwidth.  2GSa/s is just enough for 500MHz, but the Siglent only has that with two channels active and so the BW is limited with 4 channels.  The Rigol has more than adequate samples for its 350MHz bandwidth under any conditions.  I'm not sure how much this matters for your uses.

The memory configuration and lack of zoom-out on the Siglent is a baked in trait that I don't think is going to ever change.  I find it to not be problem, but it has annoyed some people who are used to a different configuration.  I'm also annoyed because I think its usability could be improved with trivial effort by reducing the whole-record display to a bar on the top.  This whole thing is less of a problem on the SDS2000X series because in zoom mode you are wasting 1/4 of a fairly large screen.  Earlier models were wasting 1/2 of a much smaller screen.
bdunham7:

--- Quote from: Fiorenzo on December 23, 2021, 03:36:01 pm ---What circuits works with such a low signal?  This is my question from the beginning.
I understand that noise sucks but it is the practical application of a low noise oscilloscope that i cannot figure out.
For example, if I work with on an old valve radio am I going to encounter such kind of low signal? It Is only an example....

--- End quote ---

On an old valve radio you might want to use a 100X probe, in which case a 'small signal' that would merit using the lowest range of the scope might be hundreds of millivolts.  With a 10X probe, which is what you will almost always use, you might start caring about front-end noise at 50mV.  Low front end noise gives you better FFT performance as well.  There are all sorts of cases where noise is an issue and if you are just starting out,  I can't predict which ones you will run into.
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