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| Measuring Distortions with the Scope:What you see is not what you really have.. |
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| nctnico:
--- Quote from: mawyatt on January 08, 2023, 04:53:56 am --- --- Quote from: nctnico on January 08, 2023, 02:31:25 am ---Think of it this way: if you can make extra information appear magically from an 8 bit ADC, then why aren't we all using scopes with 1 bit ADCs? Even cheaper to make! --- End quote --- There is no "magic" in these concepts and in fact many of the slower Delta Sigma types are simply 1 bit core ADC types (1 bit comparator) with massive Oversampling and Multi-Order Modulators followed by high order Decimation Filters and easily achieve well beyond 20 ENOB, these are the common 24 bit Delta Sigma ADC chips that only cost a few $!! Many of the higher speed ADC chips employ techniques similar to the 1 bit core Delta Sigma ADC, except they utilize more than 1 bit (usually 3 ~ 4) in the "Comparator" and DAC "Feedback" path to speed the overall conversions up, and digitize the Signal - Feedback difference instead of integrating such as the DS ADC do (if they utilize integration then some of the benefits of Modulator induced "Noise Pushing" can be employed, but this slows things down). --- End quote --- Yes, but these ADC cores are designed to meet the specifications of the end goal which is to have and ADC with X bits and not X+'magic number' bits. Anything beyond is just luck. If you buy a whole bunch of 3.5 digit DMMs and stick 3 extra digits to them, a few may seem to be accurate beyond 3.5 digits but that is more due to luck and cirumstances (right temperature for example) rather than solid engineering. In the end measuring is about having a certain confidence level in what is being shown on screen. The same goes for measuring signal levels using an oscilloscope. The numbers Performa is listing in a posting above, have a 0.5dB error margin. IOW: a different unit may show different results within the specified error margin. |
| nctnico:
--- Quote from: _Wim_ on January 08, 2023, 06:47:20 am ---@nctnico, do you now of a signal that can be generated easily with an AWG where artifacts would appear within the effective dynamic range. Would be nice to do a side by side comparison, and also useful to see what to watch out for... --- End quote --- The two tone test mentioned earlier on is a good one (and also industry 'standard'). |
| switchabl:
From a DSP perspective, if you are only interested in a low frequency (relative to fs/2) sub-band, you could either do a full FFT and throw part of it away or decimate first and do a shorter FFT. The latter is of course more efficient and this is basically what happens with ERES on. So you can get more frequency resolution (and processing gain) with the same resources. Of course, caveats mentioned by gf apply (only works if ERES implementation actually does decimation, ERES filter less than perfect). |
| mawyatt:
--- Quote from: nctnico on January 08, 2023, 11:48:52 am --- Yes, but these ADC cores are designed to meet the specifications of the end goal which is to have and ADC with X bits and not X+'magic number' bits. Anything beyond is just luck. If you buy a whole bunch of 3.5 digit DMMs and stick 3 extra digits to them, a few may seem to be accurate beyond 3.5 digits but that is more due to luck and cirumstances (right temperature for example) rather than solid engineering. In the end measuring is about having a certain confidence level in what is being shown on screen. The same goes for measuring signal levels using an oscilloscope. The numbers Performa is listing in a posting above, have a 0.5dB error margin. IOW: a different unit may show different results within the specified error margin. --- End quote --- This IS the way things work, whether you want to believe it's due to "magic" or "luck" is of course your prerogative. The mentioned 24 bit DS ADC just uses a 1 bit comparator (ADC if you will), yet achieves better than 20 Effective Bits (ENOB). This is achieved, even specified in the data sheets, using Oversampling and Decimation, basic Signal Processing 101 methods. The idea of "dithering" to achieve higher resolution and improve linearity is not new, this was utilized in the early 70s in the Ring Laser Gyro (RLG) to "unlock" the counter rotating photon (laser) beams at low rotation rates (dead zone), and likely utilized well before this time. Don't think folks back then would rely on "luck" or "magic" to guide/navigate an airplane, ship, submarine, satellite, rocket or missile. So the techniques being discussed are real and repeatable, not conjured up non-sense as we often see. As much as many like the Siglent DSOs (we have a couple), the results shown are not unique to these instruments, but achievable and repeatable by any quality DSO with the necessary features to achieve such, and would expect the Rigol, GW, Keysight, LeCroy, Tek and so on, would respond similarly to these techniques. Anyway, hope you'll realize this isn't "magic" nor "luck", just good old Engineering/Science Signal Processing being put to use for our (and others) benefits. Best, |
| 2N3055:
--- Quote from: nctnico on January 08, 2023, 11:48:52 am ---...The same goes for measuring signal levels using an oscilloscope. The numbers Performa is listing in a posting above, have a 0.5dB error margin. IOW: a different unit may show different results within the specified error margin. --- End quote --- Every measurement ever will have result within it's certainty interval. On any instrument. 0.5dB error margin for a scope is not bad at all... And scope Performa used has specified BW flatness.. So I don't see any problems... |
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