Products > Test Equipment
Rigol DHO800 memory depth / bandwidth upgrades and noise level!
ebastler:
--- Quote from: 2N3055 on November 13, 2023, 07:38:45 am ---[ENOB] is definitely not a ratio of scope AC RMS measurement of noise floor and theoretical RANGE of ADC.
--- End quote ---
Yes, I know. That's why I initially talked about "effective resolution", and then added a disclaimer ("I know that's not quite it") when I mentioned ENOB later. Using the full ADC range was obvioulsy a brain fart, but when using full RMS signal amplitude instead -- how big is the difference between proper ENOB and that simplified "RMS full swing / RMS noise" in practice?
--- Quote ---One another note: DHO800 has 8v P-P range on screen. That equates to 2,8369 V RMS sinewave.
That is your usable full dynamic range. The rest is reserve to prevent clipping.
--- End quote ---
Not sure I agree on this. The oscilloscope has the larger range captured, as witnessed by the ability to zoom out (vertically) on the stored data. You can also use the full range when you want to zoom in on small signal details which sit on top of a large signal swing. And I am pretty sure that the extended vertical range can also be used for measurements.
gf:
--- Quote from: 2N3055 on November 13, 2023, 07:38:45 am ---ENOB is calculated from SINAD (Signal to noise and distortion) ratio. It is measured by inputting low distortion sinewave (better than scope measured) and sweeping across a frequency BW of scope. ENOB is measured as a curve, a graph, not a single number.
It will vary with with frequency.
--- End quote ---
Yes, actually it should be calculated from SINAD, and it is a function of frequency.
Calculating it from SNR is just a first order approximation, assuming distortion is low.
At low frequencies it is not unlikely that this is granted, but at high frequencies I would not be so sure.
EDIT: DC non-linearity could also be measured during factor calibration, and compensated. But of course I don't know if the manufacturer does that.
2N3055:
--- Quote from: Fungus on November 13, 2023, 09:14:05 am ---
--- Quote from: 2N3055 on November 13, 2023, 07:38:45 am ---On scopes it is measured with signal that is 90% of full screen.In this case it would be 7.2V P-P for DHO800 1V/div.
--- End quote ---
That's when you're measuring THD, which we aren't. If you're measuring noise you should use the full range of the ADC for most accurate results.
--- Quote from: 2N3055 on November 13, 2023, 07:38:45 am ---It is definitely not a ratio of scope AC RMS measurement of noise floor and theoretical RANGE of ADC.
--- End quote ---
https://en.wikipedia.org/wiki/Effective_number_of_bits
The SNR tells you the ENOB when distortion is zero.
As noted: Calculating THD requires a lot of fancy gear that most people simply don't have.
OTOH it's a safe bet that THD is small, almost negligible. SNR will give a good approximation. It's the best number we have until more data comes along.
It's also a safe bet that using the same method on different 'scopes is a good way to compare them. We measured the "benchmark" Siglent using the same yardstick and found the new Rigol has two or three more bits more.
I think that's a fair assumption to make.
ie. Rigol's 12 bits are a big advance and this little 'scope has proved itself to be the new reference point.
--- End quote ---
Yes we all know you are smarter than IEEE combined...
SNR tells you what is SNR. Without signal. There is no "zero distortion" in real life. Why do you think ADCs with nominal 12 bit resolution barely break 10bit ENOB?
Nonlinearities. So no it is not negligible and no it is not even close to good approximation. Take a look at IEEE Std 1057™. It is 181 pages or so...
We measure and calculate. As I said there are excellent white papers by R&S, Keysight and Tek on the topic. Also few good ones on EDN.
No fancy anything needed for that. Just will to read and learn.
"An encyclopedia (American English) or encyclopædia (British English) is a reference work or compendium providing summaries of knowledge either general or special
to a particular field or discipline" . Quote from Wikipedia itself.
Wikipedia is encyclopedia. It is just a summary to explain what word means. And to point you in the right direction where can knowledge of the topic be found. It is simply the index to give you starting point on a path to knowledge. It is not THE knowledge.
Fact that you cannot measure it means exactly that. YOU cannot measure it. Live with it. You have no clue what ENOB is on DHO800 and you never will unless either you acquire capability to measure it, or someone measure it for you. Stop inventing fantasy numbers. If you want to gamble go to bookies..
Comparing SNR is not wrong. I sad that. But you keep inventing "your special" ways of doing it. To be able to compare you need to compare at same conditions.
I told you how: full screen is considered full range. By Tektronix, Keysight LeCroy, Siglent ... For your scope that is 8 div vertical. 8 V P-P
At 1 V/div and full BW your scope has cca 55dB od SNR. At 20 MHz BW about 56-57 dB.
55dB at almost 200MHz BW is not bad. Not bad at all.
2N3055:
--- Quote from: gf on November 13, 2023, 09:55:11 am ---
--- Quote from: 2N3055 on November 13, 2023, 07:38:45 am ---ENOB is calculated from SINAD (Signal to noise and distortion) ratio. It is measured by inputting low distortion sinewave (better than scope measured) and sweeping across a frequency BW of scope. ENOB is measured as a curve, a graph, not a single number.
It will vary with with frequency.
--- End quote ---
Yes, actually it should be calculated from SINAD, and it is a function of frequency.
Calculating it from SNR is just a first order approximation, assuming distortion is low.
At low frequencies it is not unlikely that this is granted, but at high frequencies I would not be so sure.
EDIT: DC non-linearity could also be measured during factor calibration, and compensated. But of course I don't know if the manufacturer does that.
--- End quote ---
Yes, as I said we can compare SNR as it is a simple measurement and can be done in consistent manner and give good insight in large part of performance envelope.
If scope SNR is bad , ENOB won't be better.
As a rough estimate, if two scopes have same ranges and similar BW simply putting them in same range makes simple noise AC RMS comparisons meaningful.
If scope has 10 vertical divisions and another one has 8 at 1V/div they won't be the same but simple linear scaling works here.
Some scopes have nonlinearity compensation as a part of ADC calibration of masivelly interleaved ADCs (Keysight for instance. Maybe Rigol does something similar.in its ASIC ADC) Some have digital compensation after ADC, but not in this price range.
2N3055:
--- Quote from: ebastler on November 13, 2023, 09:44:02 am ---
--- Quote from: 2N3055 on November 13, 2023, 07:38:45 am ---[ENOB] is definitely not a ratio of scope AC RMS measurement of noise floor and theoretical RANGE of ADC.
--- End quote ---
Yes, I know. That's why I initially talked about "effective resolution", and then added a disclaimer ("I know that's not quite it") when I mentioned ENOB later. Using the full ADC range was obvioulsy a brain fart, but when using full RMS signal amplitude instead -- how big is the difference between proper ENOB and that simplified "RMS full swing / RMS noise" in practice?
--- Quote ---One another note: DHO800 has 8v P-P range on screen. That equates to 2,8369 V RMS sinewave.
That is your usable full dynamic range. The rest is reserve to prevent clipping.
--- End quote ---
Not sure I agree on this. The oscilloscope has the larger range captured, as witnessed by the ability to zoom out (vertically) on the stored data. You can also use the full range when you want to zoom in on small signal details which sit on top of a large signal swing. And I am pretty sure that the extended vertical range can also be used for measurements.
--- End quote ---
In practice difference will be exactly based on how much distortion is there. Make note that whole spectra of distortion artefacts from full BW will add up to RMS of baseline. Death by thousands cuts type of scenario.
As for vertical range used for calculation, I said what is industry practice. It stems from the fact that scope screen is visual reference and part that manufacturer guarantees specifications for. Not my invention.
By all means, if you use scope as sampler, and pull data somewhere else and postprocess, characterize it fully for full usable dynamic range (by characterizing distortion and clipping behaviour) and enjoy.
But on a scope you are left at mercy of manufacturer provided usage envelope.
Small part of dynamic range is reserve not mapped in normal use for user.
It is same for everyone. It is still fair if you compare like for like..
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