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How much noise floor and other things matter in oscilloscope usability
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Performa01:

--- Quote from: G0HZU on December 28, 2021, 03:39:49 am ---You can see the noise floor is a fairly flat -154dBm/Hz across the AF band.

--- End quote ---
Yeah - bipolar technology, using rf-transistors (with very low intrinsic base resistance) makes for a good noise matching at low impedances like 50 ohms – and a low 1/f corner frequency.

High impedance FET input stages are noisy under such conditions. On the other hand, high impedance inputs are much more versatile. We can adapt them to any impedance we like by means of a pass through terminator (at least at low frequencies).

Meanwhile I’ve experimented a bit further and detected at least two flaws in my previous noise measurement:

1.   The input was AC coupled by accident, which of course increases LF-noise significantly.
2.   The input had a 50 ohm through terminator fitted, but since this scope is sensitive to the source impedance, an additional 50 ohm end terminator should be used to complete the 50 ohms setup.

Now look at the screenshot attached.

Pico4262_Noise_5MHz_D50kHz

I’ve tried to resemble your settings as close as possible but still kept the total FFT bandwidth at 5 MHz in order to keep the high frequency noise out of the LF region. Display units are dBm now for better comparability. Frequency step is 38.15 Hz, which is equivalent to a RBW of 112 Hz with the Flat-Top window – so noise levels will read slightly higher than in your setup.

With a noise level of -134.7 dBm this is very comparable to your RSA 3408A above some 30 kHz.
At 1 kHz, the FET input goes up by 17.8 dB to -116.9 dBm, but obviously stops at -114 dBm with this RBW.

So I’m confident to claim that the Pico 4262 has the same low noise in a 50 ohm system, as long as you keep the input DC-coupled and stay above 30 kHz.

EDIT: I have updated my original posting, where you can also see the updated noise density plot.
Fungus:

--- Quote from: Fiorenzo on December 27, 2021, 08:28:48 pm ---I would like to thank you everybody for the many replies.
You have been very important and educative to convince me in the decision that in my work It would be better an oscilloscope with a low noise front end than one with a very fast ADC like the Rigol.
I am receiving an sds2104x plus in the next two days so I will do a limited comparison with the Rigol mso5000 that I still have.

--- End quote ---

Still waiting for the screenshots of your ripple with averaging turned on...  :popcorn:

gf:

--- Quote from: Fungus on December 28, 2021, 09:50:30 am ---
--- Quote from: Fiorenzo on December 27, 2021, 08:28:48 pm ---I would like to thank you everybody for the many replies.
You have been very important and educative to convince me in the decision that in my work It would be better an oscilloscope with a low noise front end than one with a very fast ADC like the Rigol.
I am receiving an sds2104x plus in the next two days so I will do a limited comparison with the Rigol mso5000 that I still have.

--- End quote ---

Still waiting for the screenshots of your ripple with averaging turned on...  :popcorn:

--- End quote ---

Looking at this post again, I can believe that the first image shows noise from the scope (although it is quite a lot). But I rather cannot believe that the 40mVpp "noise band" on top of the sawtooth in the second image is scope noise as well (apparently scope settings are the same as in the first image). I guess the latter is already present in the input signal. I don't feel able to assess whether it is random noise, or rather a high-frequency oscillation. FFT should help to reveal it.
Fungus:

--- Quote from: gf on December 28, 2021, 10:50:02 am ---
--- Quote from: Fungus on December 28, 2021, 09:50:30 am ---Still waiting for the screenshots of your ripple with averaging turned on...  :popcorn:

--- End quote ---

Looking at this post again, I can believe that the first image shows noise from the scope (although it is quite a lot). But I rather cannot believe that the 40mVpp "noise band" on top of the sawtooth in the second image is scope noise as well (apparently scope settings are the same as in the first image). I guess the latter is already present in the input signal. I don't feel able to assess whether it is random noise, or rather a high-frequency oscillation. FFT should help to reveal it.

--- End quote ---

I'm just interested in what an MSO5000 can do with a signal like that when a user really uses all the provided features.
Fungus:
Also... the color gradient mode as mentioned on the first page. How would the ripple appear if you enable that?


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