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Old 4th February 2008, 01:45 AM   #1
maxpou is offline maxpou  Canada
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Default Linearity

Hi Guys,
how can i check the linearity of my circuits? Thank you! Maxpou
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Old 4th February 2008, 06:36 PM   #2
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A THD analyser.
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Old 4th February 2008, 08:20 PM   #3
AKSA is offline AKSA  Australia
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A triangle waveform generator and a fast CRO for a visual indication
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Old 5th February 2008, 01:39 AM   #4
maxpou is offline maxpou  Canada
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Thank you for answers!
but what's CRO?
Thank you!
Maxpou
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Old 5th February 2008, 04:46 AM   #5
gootee is offline gootee  United States
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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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Old 5th February 2008, 11:45 AM   #6
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Hi,
interesting topic! What is the ballpark of THD-N when the triangle-wave is visible deformend?

Rüdiger
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Old 5th February 2008, 12:30 PM   #7
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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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Old 5th February 2008, 12:38 PM   #8
PMA is offline PMA  Europe
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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.
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Old 5th February 2008, 12:50 PM   #9
Pingrs is offline Pingrs  United Kingdom
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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.
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Old 5th February 2008, 12:51 PM   #10
PMA is offline PMA  Europe
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A "bow" shape (red curve), due to low frequency corner. Nothing in common with non-linearity.
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