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Measuring Distortions with the Scope:What you see is not what you really have..
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mawyatt:
Why we've often requested & performed the classic Two Tone IMD to evaluate DSO linearity. Also why we have the PicoScope 4262, altho don't care much for instruments that require a PC to function (and prefer Macs anyway), and because of this the 4262 rarely sees use!!

Since these modern DSOs are venturing outside the old oscilloscope measurement realm, we are somewhat witnessing the effects of an instrument designed to display a waveform result of maybe 40~50dB on screen range. Whereas a proper Spectrum Analyzer is designed to display a 100~120dB range, big difference with intended usage and on screen Dynamic Range between these instruments.

With this in mind we are seeing the modern DSO becoming a more versatile instrument than a Time Domain waveform display device (O-Scope), and the OEMS are addressing such by providing higher resolution core ADCs, Digital Logic Analyzer capabilities and such.

As Performao1 indicated the DSO input PGA, along with the core ADC are the likely culprits limiting linearity, and for the intended usage (Time Domain Linear Display) provide very respectable performance, however when pressed into the Frequency Domain (Log Display) that begins to show limitations. We must remember that the DSO FFT displays are just a mathematical representation of the Time Domain captured raw data from the ADC and thus limited by the mentioned PGA and ADC channel linearity and noise.

As we see better performing lower cost ADCs and PGAs ICs come available, I'm sure we'll see better FFT performance like the Pico Scope 4262 at much higher frequency capable stand alone bench DSOs, and at affordable prices. Maybe we'll see some new type ADC architectures emerging from the research channels, we know of one (Non-Uniform Sampling), but that's another topic as we've already diverted from the OP topic.

Best,
Detlev:
Hello everyone,

I have here a comparison of three FFT analyses. The SGD6022 is always set identically. Once I measured without an attenuator and then one with 10dB and 20dB attenuator.

I would expect the spectral distribution to be identical for all three FFT measurements since the AWG was not changed. However, we see three different FFTs.

From my point of view, this underlines that a DSO is only conditionally suitable for these low levels of harmonics.

Many greetings
Detlev
mawyatt:

--- Quote from: mawyatt on December 29, 2022, 02:09:10 pm ---Why we've often requested & performed the classic Two Tone IMD to evaluate DSO linearity......

--- End quote ---

Here's using the SDG2000X AWG with the Wave Combine feature producing the Two Tone IMD mentioned above. AWG Ch1 is set to 0dBm at 99.5KHz, Ch2 0dBm at 100.5KHz.

The SDS2000X+ was set to 50 ohm input Z and the FFT performed, see PNG 153.

Then the special purpose PicoScope 4262 was used with an external 50 ohm terminator added, see Pico_AWG4.

Note the added "artifacts" added by the SDS relative to the PS, and also note the difference in the IMD products as shown.

This clearly shows that a typical DSO is not good enough to show the waveform quality of a quality AWG (get similar results with SDG6000X). A special purpose DSO is required to revel the signal quality of these 16 bit DAC based AWGs, even when in this case we were using the Wave Combine feature of the SDG2000X (and 6000) which likely adds a little non-linearity to the resultant waveform. Does anyone know if this combining is done in the digital domain (pre DAC) rather than at the analog post DAC output?

Would expect the HD version of the SDS2000X+ with it's core 12 bit ADC to have a better result, and interested to "See" how well the new Rigol 12 bit ADC DSO behaves (we asked awhile ago for a Two Tone IMD, but no now has offered yet).

Edit: Added a result from our Spectrum Analyzer with a 10dB PAD in front (almost always keep a PAD on front end, saves replacing SA front ends), see PNG18. The SA amplitude hasn't been calibrated, but the relative IMD readings are good. Note the lack of artifacts and the level of the IMD Products. It's interesting that the PicoScope shows even better IMD results than the SA, a tribute to the PicoScope and of course the SDG2000X+ AWG signal source!!

Best,
Martin72:
Maybe I should give this thread a new name and start a new comparison thread.. 8)
Serious, this topic about the limits of a scope is too important IMO..
mawyatt:
Well it is showing just how good these AWGs are  :-+

Agree tho, the DSO FFT issue certainly belongs in it's own thread.

Best
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