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How much noise floor and other things matter in oscilloscope usability
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Kleinstein:
For the DSO there are 2 types of noise: noise relative to the input (e.g. input amplifier noise)  and noise relative to the output (ADC noise). For the noise relative to the input the amplifier a 10:1 probe can be a major noise source. The 1 M resistor in the divider has a natural noise of some 130 nV/sqrt(Hz), which is higher than a reasonable JFET based input stage (more like 10 nV/sqrt(hz) range). A low noise for the input is nice, but not relevant when using a 10:1 probe. It can be when using a 1:1 probe.
An 1:1 probe is mainly used when one rellay needs low noise for very small signals (e.g. noise at some point on a circuit, supply ripple). It comes with the price of reduced bandwidth (except with a special, expensive active probe), but with significant lower input noise (e.g. factor 10 from the divider ratio and an additional factor of some 3-10 from not having the resistor noise).

The lower gain settings usually use an internal input divider and if not designed good this may add some noise to 1 or 2 of the higher ranges, which can be a bit annoying as it is avoidable. So ideally a full noise testing would test all ranges, at least with 1 BW setting.

The output related noise, e.g. from the limited resolution  ADC noise, of just ADC noise because of a limited effective ADC resolution that may be lower.
For the ADC noise the sampling rate can make a difference. So the same DSO may look noisy at the higherst sampling rate but looks much better at a lower sampling rate when more samples are averaged. The noise relevant bandwidth is different from the 3 dB bandwith and can be quite a bit higher if the sampling rate is high, or closer to the -3dB BW when the sampling rate is barely sufficient. So it needs some case for the comparison to get comparable condictions (e.g. same sampling rate, relatively close to the maximum, like some 1 Gs/s for the scopes in question).
With the high speed scopes the ADC noise can be a factor.

This is mainly relevant with non repetitve (single trigger) relatively fast signals (so one needs a high sampling rate).
Spectrumanalysis (FFT) is also an example where the noise can matter.  Here also the way the math is done (e.g. does it support averaging, if so the unused time for the math) can make a difference.

Besides the noise of the raw data the methods available for noise reduction can also make a difference. So how good is the averaging over multiple triggers or bandwidth limiting working.

A higher sampling rate can reduce the artifacts from signal parts beyound the Nyquist limit (f_s / 2). With a relatively low sampling rate for a given BW it would need either some extra anti aliasing filtering that can add phase shifts or one has to accept some artifacts if the signal actually contains very fast components. At the same BW, the higher sampling rate scope would usually give a slightly more accurate response.
Noise whise the highest sampling rate often comes with more noise, but after averaging of consecutive samples (the usual way to reduce the data rate - though it can be done worse) the noise improves and thus no panelty from a faster ADC.


edit:  I just remembered: the 10:1 probe is resistive only for low frequencies (e.g. < 10 kHz) but capacitive at higher frequenices. So there is not that much extra noise from the divider and input amplifier noise can still be a factor with the 10:1 probe (at least in the lower ranges).
gf:

--- Quote from: nctnico on December 23, 2021, 01:28:17 pm ---Also note that the noise doesn't only apply to the most sensitive V/div setting, it applies similar to all V/div settings. The noise level is usually specified in Volts using the most sensitive V/div setting but it would be more accurate to specify it as a percentage of a division or full range. In the end the V/div setting adjusts an input divider but the actual noise level (what goes into the ADC and what gets added by the ADC) stays the same; it is just scaled differenty.

--- End quote ---

Eventually it depends on the individual attenuator and front end design. For instance, I'm aware of a low-cost scope model where the relative noise levels are best at 100mV/div, 1V/div and 10V/div; already a bit worse at 50mV/div, 500mV/div and 5V/div; even worse at 20mV/div, 200mV/div and 2V/div; and finally getting more and more worse at 10mV/div, 5mV/div and 2mV/div

Fiorenzo:

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

--- End quote ---

A 1mV signal.

(for example)
[/quote]

What kind of electronics works with such low intensity signals?
Fiorenzo:

--- Quote from: bdunham7 on December 23, 2021, 04:11:46 pm ---
--- 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.

--- End quote ---

Thank you for all your explanations. Very usefull. I am trying to understand: what kind of electronics work with such kind of low signals?
tautech:

--- Quote from: Fiorenzo on December 24, 2021, 12:29:55 pm ---
--- Quote from: bdunham7 on December 23, 2021, 04:11:46 pm ---
--- 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.

--- End quote ---

Thank you for all your explanations. Very usefull. I am trying to understand: what kind of electronics work with such kind of low signals?

--- End quote ---
Few but if ever needing to measure low values indeed noise can get in the way.
When probing high impedance circuits and connection can effect the circuit operation and as 10x probes are most commonly used a 1mV/div scope setting need be used for a 10mV signal however it will only be displayed ~1div high.
To take this to extremes a popular tablet DSO has just 50mv/div max sensitivity which when coupled with a 10x probe permits only 500mV/div sensitivity which is useless for anything but the simplest of tasks.

When we need higher sensitivity 1x is used but at the expense of higher probe capacitance loading on the circuit so such use is often restricted to low impedance measurements like the ripple on a DC rail or that of a PSU.

When most move to a DSO forgetting to set the input attenuation to match the probe is a common newbie error instead of letting the scope display the correct measured value. This is where probe autosense like in the other DSO you are looking at is of substantial value.
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