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| Measuring Distortions with the Scope:What you see is not what you really have.. |
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| rf-loop:
--- Quote from: Performa01 on December 29, 2022, 09:50:57 am ---My bad – In my initial comments I simply forgot that the linearity of the average scope frontend (including SDS2000X HD) is not up to the task of characterizing the harmonic distortion of an SDG1000X or 2000X. --- Quote from: Martin72 on December 28, 2022, 12:03:16 am ---One of the findings one could derive from this would be of a fatalistic nature, namely that one can forget the FFT function with oscilloscopes. Is this really the case, or do you have to try a little harder to get a more credible result? And if so, in which direction one would have to go for it.... --- End quote --- If you look up the datasheet of PGAs that can be used for a scope like this, such as LMH6518, you will find the harmonic distortions specified somewhere between -44 and -50 dBc. Considering this, the SDS2000X HD still does a fairly decent job. The FFT is still useful for many tasks, including distortion measurements within the usual range of interest, which hardly ever exceeds -60 dBc except for high end audio. There is a reason, why specialized audio analyzers exist. You just have to take into account that the SFDR of a general purpose scope frontend might be only about 60 dB in practice – many (if not most) of the more affordable old SA boat anchors haven't been much better btw. For instance, there is no restriction for single tone narrowband measurements, like phase noise or modulation spectra, where especially the SDS2000X HD will give you an exceptional dynamic range that clearly exceeds the ~72 dB that you could expect from the 12 bits. The THD of a general purpose oscilloscope frontend is usually not specified in the datasheet. On the other hand, the Picoscope 4262 has a guaranteed linearity of 16 bits – but that is certainly not a general purpose instrument, with its 16 bit converters and limited bandwidth of just 5 MHz. Yet this is the way to go if you need to measure down to -96 dBc and don't want to be restricted to the audio frequency range like with soundcards. --- End quote --- Referring to the previous one. It is really good to keep in mind the mixing results generated by the oscilloscope itself, especially when working with FFT settings where the processing amplify is high and we are looking at weak signal levels, such as, for example, rather low harmonics and nonharmonics spurs of the signal source being examined. Here is just small part of observations. In the test, I looked at the dual tone signal. Throughout the test, the signal from the signal generator was kept completely unchanged. . The only variable was the CH1 V/div setting on the oscilloscope. I did a test for all possible values between 40mV/div to 100mV/div. Both signals had a level of about -12dBm. Signals freq. 101.3kHz 158mVpp and 103.3kHz 158mVpp. At this level, dual tone peak-peak total is close to full scale (at 40mV/div). I have full set of results but here just few of them. (there is really lot of changes when go through all possible fine steps, just one fine step and change may be over 10dB.. 15dB in 3rd order IM products) img 71 just 100mV/div and img 21 just 50mV/div. In images 17 and 35 there is perhaps most high and most low 3th order im levels (64dBc and 84dBc, 20dB change is quite big) image 40 some kind of one just random medium example. Then Last image without number is just how this dual tone looks in time domain ETA: Added image. With spectrum analyzer exactly same signal what was used with SDS2504X HD images. This somehow proof that dual tone from generator is least not worse than in this SA image. (for avoid SA own generated distortion Attenuator was set for 23dB so that mixer level stay enough low for keep SA's 3rd order IMD as low as possible. SA's performance is not enough for analyze SDG 1000X in this matter. |
| blackdog:
Hi rf-loop Your measurements are a good example of what I often say on forums, know your instruments! In one of my posts I mentioned that I used 2V RMS as the output signal and then terminated with a 50 Ohm termination resistor. This is because with most generators, THD is fairly favorable and then you can then attenuate externally as needed. This level may of course just not good for the particular generator used, therefore know your instrument! I have tested it on five of my function generators where possible at the 2V RMS. And then you get the second step that you will have to perform as a user, this as you did with your last measurements, which area of the input attenuator from here the osciloscope is the most linear. There are different parts in the osciloscope where the gain can be controlled, these include these, relay input attenuator, " Voltage Control Amplifier" and digital. So then you can also have a fairly large difference between the steps when you look at the THD and as in your last measurements at the third order products. It's quite complex to take all those variables into account.... Technicians who do a lot of measurements with a Spectrum Analyser are used to using e.g. a passive attenuator to attenuate their generator signal in steps to see if they do not overload the input of their Spectrum Analyser. I think most users of modern oscilosscopes are not always used to that when doing FFT measurements. I think it's good to bring this to people's attention here. Thanks for the nice measurements. Kind regards, Bram |
| pdenisowski:
--- Quote from: blackdog on January 05, 2023, 01:10:20 pm ---Technicians who do a lot of measurements with a Spectrum Analyser are used to using e.g. a passive attenuator to attenuate their generator signal in steps to see if they do not overload the input of their Spectrum Analyser. I think most users of modern oscilosscopes are not always used to that when doing FFT measurements. --- End quote --- Yes. I worked as a field applications engineer in test and measurement for > 20 years and this was my experience as well. It bears repeating that whenever making any type of "distortion" measurement (IP3, THD, etc.) it's very important to ensure that the measured distortion is not being created within the measuring instrument itself. :) |
| Performa01:
--- Quote from: rf-loop on January 05, 2023, 12:15:00 pm ---I have full set of results but here just few of them. (there is really lot of changes when go through all possible fine steps, just one fine step and change may be over 10dB.. 15dB in 3rd order IM products) --- End quote --- Yes, that was my observation as well. Sometimes I have cheated a little and taken advantage of this when presenting test results in the following thread - especially when I was talking about "sweet spots" ;) https://www.eevblog.com/forum/testgear/two-tone-test-with-scope-and-sa/ |
| mawyatt:
Since the DSO generally isn't designed as a Spectrum Analyzer, think what rf-loop has shown with the Linearity dependance on actual Input Signal Chain Gain/Attenuation Distribution is an important point in our DSO usage as a SA (FFT). These DSOs are really Data Acquisition Systems, and a SA is just input signal data processing, same as with a 'Scope usage. However, a Scope doesn't really need much Display DR or Linearity beyond 1% (~40dB) because we can't really "see" the effects on the display since this is a linear display, however when utilized as a SA in FFT Mode the display is now in dB and thus much higher on screen overall signal amplitude dynamic range, and subtle signal artifacts begin to show as do measurement instrument non-linear channel effects. The quality modern DSO must possess a higher Signal Channel DR for use with the FFT Mode, and this imposes a higher initial design and build cost. Overall Signal Chain "Sweet Spot" is an interesting topic. There was a unique class of amplifiers we employed long ago that had very interesting properties, one of which was the linearity "Sweet Spot" with a theoretical infinite dynamic range. We utilized very fast SiGe 400GHz Ft devices and later planned on >600GHz Ft InP devices for implementing special purpose custom ASICs. One of these had another unique property with gain expansion rather than compression with increasing input signal levels, all of these prior efforts were to try and "squeeze" as much DR from a given supply rail as possible and thus were operated mostly in current domain. So we were trying to place the Input signal within the "Sweet Spot" of the Signal Channel Dynamics to allow further signal processing, similar to what has been shown by changing the DSO Gain/Attenuation Distribution. Best, |
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