Linearity
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 4th February 2008, 01:45 AM #1 maxpou   diyAudio Member     Join Date: Apr 2006 Location: Québec Linearity Hi Guys, how can i check the linearity of my circuits? Thank you! Maxpou
 4th February 2008, 06:36 PM #2 myhrrhleine   diyAudio Member     Join Date: Jan 2006 Location: Avalon Island A THD analyser. __________________ [Grasshopper]:Old man, how is it that you hear these things? [master]:Young man, how is it that you do not?
 4th February 2008, 08:20 PM #3 AKSA   diyAudio Member   Join Date: Sep 2001 Location: Melbourne, Australia A triangle waveform generator and a fast CRO for a visual indication __________________ Aspen Amplifiers P/L (Australia) www.aksaonline.com
 5th February 2008, 01:39 AM #4 maxpou   diyAudio Member     Join Date: Apr 2006 Location: Québec Thank you for answers! but what's CRO? Thank you! Maxpou
gootee
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Join Date: Nov 2006
Location: Indiana
Quote:
 Originally posted by maxpou Thank you for answers! but what's CRO? Thank you! Maxpou
A CRO is an oscilloscope (Cathode Ray Oscilloscope).

The triangle waveform generator is a good idea. And it is easy to build a fairly good one. (Do a search at national.com for "tri-wave generator".)

To check linearity, you would want to make sure that vout/vin is constant, for all expected values of vin.

Checking linearity with a triangle waveform for vin is easy: If vout is also a triangle shape, the system is linear versus input voltage. But if the rise or fall portion of the output waveform is curved, or worse, then the system is not linear.

You would probably want to check that that is also true when vin is changing, for all expected rates of change of vin. The triangle generator would also handle that, if it had variable frequency as well as amplitude.
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 5th February 2008, 11:45 AM #6 Onvinyl diyAudio Member     Join Date: Aug 2002 Location: Germany Hi, interesting topic! What is the ballpark of THD-N when the triangle-wave is visible deformend? Rüdiger __________________ "I can feel what's going on inside a piece of electronic equipment. I have a sense that I know what's going on inside the transistors." Robert Moog
jan.didden
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Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
 Originally posted by Onvinyl Hi, interesting topic! What is the ballpark of THD-N when the triangle-wave is visible deformend? Rüdiger

That's a difficult one, as a perfect triangle is a sine wave + 30% 2nd harmonic. Plus 4th, plus 6th, plus.... .

Just as a perfect square wave is a sine wave + 30% 3rd harmonic. Plus 5th, + 7th + ...

That is why a tringle is a very crude way to guesstimate linearity.

Jan Didden
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 5th February 2008, 12:38 PM #8 PMA   diyAudio Member     Join Date: Apr 2002 Location: Prague Should be said that depending on repetition frequency, the triangle may be distorted in shape, as no audio circuit works from DC to infinity. HF corner results in triangle corner rounding, LF corner results in a "bow" shape of triangle. It will work only for repetition freq. in the midband.
 5th February 2008, 12:50 PM #9 Pingrs   diyAudio Member   Join Date: Jul 2005 Location: NW England Is the triangle trace any better than the classical XY trace of I/P - O/P sinewave. Loss of linearity is immediately apparent by curvature of the (ideally) straight line at midband, or lack of ellipticity elsewhere (and shows phase to boot!) This is how I try to detect slew rate limiting. Does that make sense? Brian.
PMA
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Join Date: Apr 2002
Location: Prague
A "bow" shape (red curve), due to low frequency corner. Nothing in common with non-linearity.
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