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Old 30th April 2011, 11:39 PM   #1
rjm is offline rjm  Japan
Richard Murdey
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Default Low distortion oscillator?

There was an application note I read once that dealt with measuring the distortion of op amps, possibly it was part of a datasheet. Linear Tech or Burr Brown most likely. Anyone remember it? I can't find it anymore.

My existing tools are fine for frequency sweeps, but none of my function generators will not cut it when trying to determine distortion below 0.01%.

I'm thinking I just need a fixed frequency oscillator circuit for around 1kHz and around 0dB (1 V rms) output. For what I intend to do that would work fine ... but I'm unsure about the best approach.

Advice appreciated.
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Old 1st May 2011, 12:22 AM   #2
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Soundcard?

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Old 1st May 2011, 01:52 AM   #3
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One of the most interesting articles is in Jim Williams book Analog Circuit Design: Art, Science and Personalities. If you go to Google Books, the chapter on Hewlett Packards oscillator and JW's efforts to do a solid state version is available to read, or at least most of it. I think it starts around page 50.

There's also a similar application note, AN-43 that covers a lot. That should be available on the LT site.

I'd also read about the Cordell distortion analyzer, as the signal generator portion of that is extremely good. You'll find that on Bob Cordell's site.
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Old 1st May 2011, 06:10 AM   #4
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Quote:
Originally Posted by wakibaki View Post
Soundcard?
Nice. I was forgetting I was sitting on a 24bit 192kHz Onkyo soundcard. That should do the trick.
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Old 1st May 2011, 06:54 AM   #5
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Hello rjm,

You can look also this thread that speak about THD measurement and ultra low thd oscillator, and this wiki page dedicated of the ultra low thd oscillator based on the LT AN 67 app note.
Good reading !


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Old 1st May 2011, 11:03 AM   #6
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My soundcard has a THD of about 0.003% at 2496 - round trip, output looped to input.

I found the datasheet I was thinking of earlier about low distortion sinewave generator: it was the Linear Tech LT1007/1037. (Thanks Frex for jogging my memory.)

LT1007 - Low Noise, High Speed Precision Operational Amplifiers - Linear Technology

So the question is it realistic to imagine I might be able to better my soundcard sinewave distortion by about 10dB using the osicallator circuit? Anything less and it wouldn't be worth the trouble...

(..OK datasheet says "parts per billion" - but real world performance?)
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Old 1st May 2011, 11:14 AM   #7
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With many sound cards I believe it's the case that you can actually get lower distortion if you don't go for full scale output e.g. 50% amplitude. It's certainly true of my M Audio card.
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Old 1st May 2011, 11:44 AM   #8
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The LT AN67 oscillator perform better THD performance of any sound card available. At my knowledge, no commercial hardware are capable to reach this level of THD at this frequency .
Remember that it's frequency is 10kHz, not at 1kHz as you find in all THD tests.


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Old 1st May 2011, 12:54 PM   #9
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Go to ebay and pick up an HP 206a or 207a or 209a. They are still today among the finest generators. Very low distortion, and constant amplitude.

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Old 2nd May 2011, 01:47 AM   #10
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I don't understand how sound cards are specified. My Emu 0202 has great specs and looks great when fed back into the input for spectral analysis, but if I put it on the scope (or use a conventional THD analyzer) it's absurdly noisy. I'd never use it as a signal source, except in conjunction with PC based measurements. What am I missing?
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