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Old 22nd October 2013, 12:23 PM   #441
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Quote:
Originally Posted by bear View Post
The discussion also swung to how TI and National test the distortion of these ultra-super-low distortion opamps, and this apparently is laid out in some app note. What they do is to compare what is going on at the inverting input with the input (assuming feedback is present), and analyze that residual. Kevin said that in his opinion this is a fairly variable and not totally predictable or accurate method
I think it was Peter Baxandall who showed in one of his WW articles that given a fairly small distortion to begin with, the input signal distortion is pretty much identical to the output distortion multiplied by (I believe) the closed loop gain.

The other way to measure ultra low distortion opamps is to put a smallish impedance between the two input pins to significantly lower the noise gain while keeping the closed loop gain. The output distortion rises with the increase in noise gain and can be referred back to the closed loop gain by dividing by the noise gain.

I'm waiting the arrival of my Boonton 1130 to experiment with a notch before the '400.

Jan
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Old 22nd October 2013, 12:33 PM   #442
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I find it interesting that by using fewer bits in the ADC that the distortion is reduced, compared to a DAC where in effect the converse is true. Well not exactly, but it does seem to run the other way.

Dick, sorry to hear of you health issues, hope that you actually got fixed up and had appropriate patches applied and that ur good for more miles before any scheduled shop time... hospitals suck, imo.

Ok, I did take a quick look at your pages to see what you had done.

One problem on the QA is that the frequency of the oscillator is adjusted to fit the software's bins and also the sinewave is not continuous. So, it's unclear to me that one gets the full performance of the QA using an external oscillator, but perhaps.

Wondering why you would not use the QA's oscillator, and which oscillator do you generally prefer? The QA oscillator is fairly high quality... I hope to test it this weekend on the 7725, assuming I can find the box to check in the menu to make it run continuously.

So the next thing that comes to mind is that since the DUT's signal has to pass through the opamps of the notch, how does that effect the distortion products, or does it? Does one need or want very high performance chips here? In effect the passband, "un-notched" area is just a follower - or is there some gain? Or are we assuming that the added distortion of the notch's active devices is there but adding to a residual signal that is already -1XXdB down so the addition to that signal is essentially meaningless?

Just trying to follow the opera along in the libretto.

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Old 23rd October 2013, 01:42 PM   #443
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Default QA Oscillator alone

I did a very fast check of the QA oscillator.
It is set to the default of 1kHz and 0dB.

The VP-7725 shows almost nil 2nd order! Look at the image.
This was one of the best readings I got, it bounced slowly up and down, going up about 5-6dB.

Important to note this is with both instruments *cold* as in outside temp, which was about 55-60deg F. Thinking the QA works better *cold*.

I also did a loopthrough on the QA. Best readings I have ever gotten thus far. Noise floor happened to be in the -135dB range.

So, I think we can safely conclude that at least the 2nd order spurs in the QA are not from the internal oscillator.

There is some uncertainty however, as setting the level lower than 0dB in the continuous run oscillator dialog box did not produce a signal that the VP-7725 was able to read. I did not put a scope on it to confirm what was coming out of the QA, but loopthrough said it was working fine at that level. (at least I think it did, it may be that I imagined this, as the RUN button probably pulls the continuous feature off) This is part of the uncertainty of operating the VP-7725, it is difficult to know what it wants or what it is really doing at any given point - you hear relays clicking, but it's not clear why or what they are doing. It's unclear that there are not one or more faults either.

I also explored the hitherto unknown "Notch" menu. It shows "AUTO" - so apparently there is an automatic notch applied. If there is a way to take it off AUTO, I could not find it. Another situation where a manual would be nice.

Probably will find one by the time its performance has been equaled by a $99 LSI affair...
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Last edited by bear; 23rd October 2013 at 02:01 PM.
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Old 23rd October 2013, 05:35 PM   #444
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I found there is a threshold that the input must be higher than in order for the internal auto test system to work. Being this is basically for production testing that is OK. Seems to be looking for a 1v input, I recall.

Let me/us know if you figure out what the Notch is used for... or why display it is on auto mode.


It is interesting that the 4 and 5th Harmonics are growing... maybe higher Harmonics continue to grow as well? This is unlike the analog osc which 2-3H dominate and higher orders are much lower.

-RM

Last edited by RNMarsh; 23rd October 2013 at 05:42 PM.
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Old 23rd October 2013, 08:51 PM   #445
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For which unit - the auto ranging threshold?? This with other than the 7725?

Next chance I get I will push a lot of buttons?? See what happens.

I am presuming that the notch is applied similarly to the SR-1...
As of now I can't figure out how to get a whole bunch of stuff to do anything.
There is a keypad for example, with buttons to their right. No idea how to make it do something.

Also one can take the range out of AUTO, and select a manual level, but it doesn't seem to "stick" - there must be another button to push... very mysterious.

Yes, I was surprised to see 2H so low!
But it does serve to indicate that the 2nd harmonic spur in the display of QA is not being generated by the oscillator, so that is almost certainly off the table now.
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Old 23rd October 2013, 09:52 PM   #446
gerhard is offline gerhard  Germany
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Originally Posted by bear View Post
But they were notching the fundamental with about 40-50dB (iirc) of analog notch and (again iirc) about 70dB of digital notch, and then the software "knows" this and stacks that onto the scale for the apparent range. If I have that part reassembled properly.
What is the digital notch good for, other than saving a dozen of bits
for the DSP, if anything?
After the ADC, it's only massaging numbers.



BTW. Can the QA400 do Bode plots (amplitude & phase)?
I don't dare to ask for cross correlation.

regards, Gerhard
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Old 24th October 2013, 01:09 AM   #447
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gerhard, Kevin McKee of SRI claimed that the combination permitted them to measure a much greater dynamic range, get better S/N when measuring harmonics. I am only reporting what he said. You could just email him and see what he says?

When I walked over and actually *looked* at the display it was showing stuff at -240dB!! That is what set off the discussion... as in "is this real"?
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Old 26th October 2013, 12:49 PM   #448
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Default QA400 frontend

Looking for more information about the QA400, I found this project.
Not only found more details about it. But it seems very interesting.

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Old 27th October 2013, 02:30 AM   #449
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Yes, but I have not figured out how to order, if one wanted to. Maybe you have to create an account first? In particular it's unclear how much the board would cost... and they seem to say that they make a "panel" of them to drop the cost, so maybe they are pre-made?

Also in a very fast look, did not see what the power supply might be... and if there is any text documentation...

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Old 27th October 2013, 05:14 AM   #450
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