Linear and Non Linear Circuits

Discussion in 'General Electronics Chat' started by Sparky49, Nov 15, 2012.

  1. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Hi all.:)

    I've been doing some reading on linear and non-linear circuits, however I've come across something which I need some more help with.

    As I understand it, if I were to construct the black circuit below and pass a sine wave through it, I would get the spectrum shown (both the red a blue lines). Lots of harmonics.

    However, if I add the green diode, the red lines will disappear leaving fewer harmonics.

    Why is this?

    I'm guessing that it's something to do with 'balancing' the circuit, but what exactly is going on? I realise that the diodes will 'clip' the tops and bottoms of the sine waves, but I don't see how this will affect the harmonics.

    Many thanks for your time,

    Sparky
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    It is hard to tell from the drawing without knowing about the input signal. If it is a sinewave then the single diode will clip the positive peaks and the other diode will clip the negative peaks. The output starts to look like a square wave with only even(odd) harmonics.

    BTW - what does balancing the circuit mean?!!?
     
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  3. Ron H

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    The harmonics of a square wave are all odd.
     
    Last edited: Nov 15, 2012
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  4. Papabravo

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    Had to be one or the other and I couldn't remember off the top of my head.
     
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  5. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Thanks guys, that makes sense.

    So if one were to keep 'squaring' the sine wave, the odd harmonics would become greater and greater?

    Apologies for improper terminology, 'balancing' was kind of describing what was happening to the sine wave, with one diode, only the bottoms are clipped, but adding the second clips both. I sometimes use a poor choice of word.

    Sparky.
     
  6. Ron H

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    Yep. Here's a simulation with odd harmonics 3,5,7, and 9, scaled and summed. If you took a square wave and applied a brick wall filter that eliminated all the higher harmonics, it would look like this.
     
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  7. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Nice, thanks Ron!

    What program is that?
     
  8. Ron H

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    It's good ol' LTspice, known and loved by dozens(?) of AAC members.:D
     
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  9. Papabravo

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    It gets my vote for answering numerous question without running the risk of letting the magic smoke out. It is well know that electronics depends on magic smoke for proper operation.
     
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  10. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    I have found the best piece of test equipment I own, is my nose. :D
     
  11. nigelwright7557

    Senior Member

    May 10, 2008
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    That circuit is often used in guitar fuzz boxes to create lots of harmonics.

    I found a better solution was a soft limiter which gave a rounded off square wave.
     
  12. Wendy

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    Ahh, there is nothing to wake you up like a mirror image brand of a part number on a finger tip. No permanent scars yet, but I'm working on it.
     
  13. Ron H

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    Yeah, I once had a nice red D on the tip of my index finger for a few hours.
     
  14. Jony130

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    Feb 17, 2009
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    What I know is that :

    - Even harmonics (2nd, 4th, etc.) result from asymmetrical non-linearities of a transfer function.
    - Odd harmonics (3rd, 5th, etc.) result from symmetrical nonlinearities of a transfer function

    But I also don't understand why.
     
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  15. Ron H

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    One way of understanding it is to work backwards. Sum a fundamental with an attenuated second harmonic, and observe the asymmetry (in the Y axis) of the result.
     
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