Boonton 1120/1121 Distortion Analyzer tweaks - diyAudio
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Old 2nd July 2012, 05:05 AM   #1
1audio is online now 1audio  United States
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Default Boonton 1120/1121 Distortion Analyzer tweaks

I posted on another board my adventures tweaking the 1121 to get a lower distortion floor. The notes are here: http://www.diytube.com/phpBB2/viewtopic.php?t=5440

The analyzer now reliably nulls to .0005% THD. This is a substantial improvement.
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Old 2nd July 2012, 05:37 AM   #2
davada is offline davada  Canada
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Hi 1Audio,

What is the typical operating level at the outputs of A2-U1-U2-U3?

David.
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Old 2nd July 2012, 03:37 PM   #3
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I have another pressing project on the bench right now. I'll measure those numbers and a few others when I get it back on the bench soon. I think I may be able to get still better results.
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Old 2nd July 2012, 05:10 PM   #4
davada is offline davada  Canada
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Hi 1Audio,

The reason I'm asking is I've found, for lower levels 5Vrms down, the LT1468 to out perform the LME series and 5534 op amp(s). They are are a bit fussy to use, (parasitic oscillation), but once cleaned up.... LT's 1468 data sheet and design notes show how to handle this.

Might be worth a try.

Cheers,

David.
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Old 2nd July 2012, 09:41 PM   #5
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That is a really interesting IC. I'll get some to play with. Cheaper as well. Unfortunately the 1121 uses duals in those locations. Either will probably pass the signal with harmonics well below -120. The bigger issue I think may be common mode induced distortion, something missing from all the datasheets. . .

I would like to find a chip (dual) with really low common mode induced distortion. But that all pales before using a Tantalum coupling cap with no DC bias.
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Old 2nd July 2012, 10:57 PM   #6
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Hi Demian -- the LT1468 comes in a dual, the 1469, and they really are good. RE common-mode distortion, I've found the summing amps in most notch filter designs to be a big generator of 2nd harmonics -- which I'm sure is CM distortion.

Jim Williams floated the idea of an amp to provide a floating, driven common-ground point in his Wien bridge oscillator in Fig. 47 (write-up on this says it's for fig 48, but somebody swapped the figures) in Linear's App note 43. That may work -- I'd like a fix for this problem in my HP 339 without having to rebuild the entire notch filter.

And Doug Self proposed a way to signal-modulate the power supply used with FET-input amps to minimize input capacitance modulation distortion. Can't at the moment remember where I saw that, but I think it was in a series of articles in EDN or some such trad magazine.
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Old 2nd July 2012, 10:59 PM   #7
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
That is a really interesting IC. I'll get some to play with. Cheaper as well. Unfortunately the 1121 uses duals in those locations. Either will probably pass the signal with harmonics well below -120. The bigger issue I think may be common mode induced distortion, something missing from all the datasheets. . .

I would like to find a chip (dual) with really low common mode induced distortion. But that all pales before using a Tantalum coupling cap with no DC bias.
Ah. dual version is the LT1469.

The LT1468/1469 have a much lower current noise density than the LME. For source resistances in the range of 1k to 20K the voltage noise is the same or higher.
At 3Vrms or lower the distortion at 1KHz approaches the floor of an EMU 0204 fundamental suppressed <= -60dB. Should be similar to your demo board.

For source resistances below 1k I've been looking at the OPA1642/1641 at about the same price of a 1468/1469.

Cheers,

David.
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Old 2nd July 2012, 11:05 PM   #8
davada is offline davada  Canada
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[QUOTE=richiem;3079610]

Jim Williams floated the idea of an amp to provide a floating, driven common-ground point in his Wien bridge oscillator in Fig. 47

Hi Dick,

Is this a virtual ground?

I'll take a look.

David.
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Old 2nd July 2012, 11:12 PM   #9
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Thanks, The LTC data sheet didn't mention the duals. Still more parts to get.

I suspect the National FET switch is the bigger culprit. I need to find an alternative that can be grafted in that would be an improvement.
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Old 1st January 2014, 09:23 PM   #10
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updating my notes-
I can now get reliably -106 dB THD with either input live. The key update is changing U9 to an AD797. It does drop in and work. Expect to do a recalibration and CMRR adjustment. I also replaced U5 and U8 with LME49710's. I have tried the AD797 but something was not happy since there is no series resistor from out to -in. I'll explore after the following.

However the whole input is limited by noise. I'm going to redo an extra input board with new values that should get 10-20 dB lower noise. Its a festival of .1% resistors and the real question is whether the lower values will trade noise for increased harmonic distortion. The other challenge is replacing R13 and R14 with light bulbs. I have found the pieces and I know the lower series resistance will help. Grafting them into the board will be the challenge.
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