Low-distortion Audio-range Oscillator - Page 270 - diyAudio
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Old 14th May 2013, 08:24 PM   #2691
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Happy to say ---> I just got a portable PC upgrade--- Sony S class with i7 Quad core --- 12GB RAM and all SSD. Win 8 Pro (64 bit);

The QA400 latest software down-loaded fine and the software/hardware worked 100% without a hitch. :-)


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Old 14th May 2013, 09:22 PM   #2692
RNMarsh is offline RNMarsh  United States
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Gone back to my second childhood, I guess.... naive and reckless.

Last edited by RNMarsh; 14th May 2013 at 09:25 PM.
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Old 15th May 2013, 06:48 AM   #2693
RNMarsh is offline RNMarsh  United States
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When looking at the output of an 339A oscillator into an ADC at various input levels between -3dB to -20dB, there is a large change between the 2H and 3H.

On the ADC/FFT, at -20dB input, the 2H was below the 3H by 5dB. At -10dB input the 2H and 3H were about the same level; At -3dB input, the 2H was >10dB above the 3H.

This isnt a useful characteristic. I want to know the harmonic structure, not just a THD number and if the test equipement changes the DUT harmonic structure, what good is it?

Wiill Demian's interface or davada's changes solve this?. Does the e-MU models (0202 and 0404) do the same thing... showing variations of large amounts of 2H/3H with input level? At what input level is ADC displaying the correct harmonic levels?

Thx-RNMarsh

Last edited by RNMarsh; 15th May 2013 at 06:51 AM.
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Old 15th May 2013, 08:56 AM   #2694
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At what input level is ADC displaying the correct harmonic levels?
That's an easy one: at no input level.

Samuel
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Old 15th May 2013, 01:32 PM   #2695
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You are of course looking at the resolution limit of the a/d. This is where the cat chases it's tail. What you need is a precision oscillator to test the a/d.

That's why I mentioned using multiple locked and phase adjusted oscillators. It is a reciprocal method for looking at the edge of nowhere.
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Old 15th May 2013, 02:08 PM   #2696
davada is offline davada  Canada
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Originally Posted by Samuel Groner View Post
That's an easy one: at no input level.

Samuel

This is brilliant. Then the Worlds greatest oscillator is the one that puts out nothing.
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Old 15th May 2013, 03:17 PM   #2697
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I was more about to say that there is no input level at which an ordinary ADC can estimate the harmonic distortion of the oscillators we're converned with here with reasonably uncertainity, but indeed at zero input the measurement should be pretty good, usually!

Samuel
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Old 15th May 2013, 04:10 PM   #2698
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Originally Posted by Samuel Groner View Post
but indeed at zero input the measurement should be pretty good, usually!

Samuel
Unfortunately at zero input the snr approaches zero and the errors approach infinity. But it shouldn't be a problem if you know what you are doing.

I think the first step would be to know what the actual performance of the oscillator is. When its distortion is so low it becomes quite hard to know what is contributing what. I have measured pots with higher distortion than these oscillators. Ultimately, below -130 dB having confidence in the distortions is very difficult.

Once the level of harmonic generation becomes so close to nothing there may be little to be gained going further. At this stage looking at other nonlinearities may be much more useful. It is inconceivable that any transducer would have distortions within 40 dB of this level. Perhaps a multitone IM test would be more useful and less technically demanding. Its also a more real world situation. A group of musicians playing horns will have a lot of harmonics but should have no sounds below the fundamental. Or perhaps settling time would show issues. Going much further down this road will lead to madness. . .
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Old 15th May 2013, 04:17 PM   #2699
RNMarsh is offline RNMarsh  United States
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Originally Posted by simon7000 View Post
You are of course looking at the resolution limit of the a/d. This is where the cat chases it's tail. What you need is a precision oscillator to test the a/d.

That's why I mentioned using multiple locked and phase adjusted oscillators. It is a reciprocal method for looking at the edge of nowhere.

This can help with knowing what is what, harmonic-wise. However, it doesnt change the ADC non-linearities which are pretty gross below -100db (or higher). Somewhere in the middle of the ADC operating range it is at its best linearity. Maybe a 40dB range in the middle could be used for measurement purposes. There the changes to the harmonic levels/relationships/structure is fairly constant/stable.

Other-wise, i am inclined to continue to use the swept BP wave - spectrum - analyzer to measure residuals. Even thought the noise floor is more visible. or, other passive notch filter methods though they are not as convenient but are more accurate with various input levels.

This is consistant with Sam G's conclusions when using his Audio-Precision.

Thx-RNMarsh

Last edited by RNMarsh; 15th May 2013 at 04:35 PM.
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Old 15th May 2013, 04:54 PM   #2700
RNMarsh is offline RNMarsh  United States
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Default Why lower noise -

If we go back to #2666 for a moment -- The 339A display shows the affect of a concerted effort to not just lower THD but to lower the noise throughout the osc/analyzer system. the noise floor is now lower than the other analyzers and the 3H can be seen which would have been obliterated with noise on a stock 339A when using a wave/spectrum analyzer to view the residuals. This is why i am interested in lowering the noise of the better AD725D and maybe the VP-7722A (looks like a keeper).


Thx-RNMarsh

Last edited by RNMarsh; 15th May 2013 at 05:18 PM. Reason: Connecting the DOTS ---
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