Products > Test Equipment
Measuring Distortions with the Scope:What you see is not what you really have..
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pdenisowski:

--- Quote from: elecdonia on January 05, 2023, 07:18:50 pm ---I'll post images from actual measurements if anyone here is interested.

--- End quote ---

I'm interested :)  Always amazed at the different ways people find to use XY mode
nctnico:

--- Quote from: Performa01 on January 05, 2023, 08:21:21 pm ---
--- Quote from: elecdonia on January 05, 2023, 07:52:28 pm ---Several posts in this topic display the FFT harmonics from the output signal of a "function generator." Typically the ratio of fundamental/harmonics shown is no better than -50dB for function generators. ...

--- End quote ---

Thank you so much for giving us valuable insight in the obscure field of audio engineering.  We learn something new every day!

In case you did not understand Tautechs response: This thread originally was (and somehow still is) about the distortion in AWGs (Arbitrary Waveform Generators), which have been shown to provide decent distortion levels down to -80 dBc.

In case you did not read the posts within this thread: unsurprisingly, it has been found that the linearity of the frontend of an average general purpose DSO cannot keep up with the modern AWGs, hence we get much worse distortion figures than what the generators are actually capable of.

--- End quote ---
After doing some testing (with a Tektronix AFG31000 ) the question is more like: how useful is long FFT where it comes to showing valid results or not. I did some testing with the RTM3004 which is limited to 128kpoints. As a result the FFT noise floor is about -80dB and shows no distortion products. On my GW Instek with 1Mpts FFT (resulting in a lower noise floor) I can see the front-end distortion products just fine.

gf:

--- Quote from: nctnico on January 06, 2023, 11:32:44 am ---After doing some testing (with a Tektronix AFG31000 ) the question is more like: how useful is long FFT where it comes to showing valid results or not. I did some testing with the RTM3004 which is limited to 128kpoints. As a result the FFT noise floor is about -80dB and shows no distortion products. On my GW Instek with 1Mpts FFT (resulting in a lower noise floor) I can see the front-end distortion products just fine.

--- End quote ---

Even if you consider anything below (say) -50dBc as potentially invalid, a long FFT is still useful if you need to separate closely spaced frequencies.
2N3055:

--- Quote from: nctnico on January 06, 2023, 11:32:44 am ---
--- Quote from: Performa01 on January 05, 2023, 08:21:21 pm ---
--- Quote from: elecdonia on January 05, 2023, 07:52:28 pm ---Several posts in this topic display the FFT harmonics from the output signal of a "function generator." Typically the ratio of fundamental/harmonics shown is no better than -50dB for function generators. ...

--- End quote ---

Thank you so much for giving us valuable insight in the obscure field of audio engineering.  We learn something new every day!

In case you did not understand Tautechs response: This thread originally was (and somehow still is) about the distortion in AWGs (Arbitrary Waveform Generators), which have been shown to provide decent distortion levels down to -80 dBc.

In case you did not read the posts within this thread: unsurprisingly, it has been found that the linearity of the frontend of an average general purpose DSO cannot keep up with the modern AWGs, hence we get much worse distortion figures than what the generators are actually capable of.

--- End quote ---
After doing some testing (with a Tektronix AFG31000 ) the question is more like: how useful is long FFT where it comes to showing valid results or not. I did some testing with the RTM3004 which is limited to 128kpoints. As a result the FFT noise floor is about -80dB and shows no distortion products. On my GW Instek with 1Mpts FFT (resulting in a lower noise floor) I can see the front-end distortion products just fine.

--- End quote ---

-80dB noise floor? You mean dynamic range?
At what sensitivity and signal level?

On SDS2000X HD noise floor is approaching -140dBm..

On following capture dynamic range aproaches 100 dB (-5dBV minus -103dBV)
Same number of points..
No wonder nonlinearities are seen...
If dynamic range were 80 dB they also would not be seen..
JeremyC:

--- Quote from: Martin72 on December 26, 2022, 08:22:32 pm ---Back to topic, comparison...

FFT of SDG1062X and 2122X, sinewave 1khz, appx 0dBm @50Ohm:

SDG2122X:



SDG1062X:



--- End quote ---

I’m new to the forum, however I watch it since ~2019.
In my opinion many people don’t try to understand FFT backgrounds and they do misinterpreting the results. Somebody should create new topic like “FFT good practices / primer”, independent of the equipment vendor. FFT can be confusing… FFT windows may sound confusing, almost each vendor has own interpretation of FFT, etc… But all are base on the same math formulas.
For beginners:
  - Check your DSO ADC bits, and if:
   - it’s 8 bit, ignore everything below 48dB in respect to dbV should be ignored.
   - it’s 10 bit, ignore  everything below 60dB in respect to dbV should be ignored.
   - it’s 12 bit, ignore  everything below 72dB in respect to dbV should be ignored.
   - it’s 14 bit, ignore  everything below 84dB in respect to dbV should be ignored.
   - it’s 16 bit, ignore  everything below 96dB in respect to dbV should be ignored.
… and so on.

- Use the vernier and adjust that your wave to covers 90 - 95% of the p-2-p signal on the screen.
- Don’t use any kind of averaging, ERES, highres, etc.
- Use AC coupling (most scopes will be fine with frequencies > 30Hz in AC coupling, check yours).
- Use plenty of samples… it will depend on your scope.
- Ignore myths...

As good start I suggest to check THD + Scope published by W2AEW about 3 years ago:



If anybody is in high-end audio, you should ignore oscilloscope and use good quality ADC/DAC combined with software like Audio Rightmark, ARTA, or REW suite. ...Or buy specialized equipment, however it may come pricey…



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