Interference at Odd vs Even Harmonics: Is the Square Wave Really Square?

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Discussion Overview

The discussion revolves around the harmonic content of square wave signals, particularly focusing on the interference patterns observed at odd versus even harmonics. Participants explore the implications of real-world deviations from ideal square waves, including the effects of duty cycle and rise/fall times on harmonic structure.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant notes that the Fourier transform of an ideal square wave shows peaks at odd multiples, suggesting expected interference at odd harmonics.
  • Another participant emphasizes that real-world signals are not ideal, implying that deviations can lead to unexpected harmonic behavior.
  • A participant introduces the concept that the harmonic structure of a pulse signal is influenced by its duty cycle, indicating that variations in pulse width can alter harmonic content.
  • It is mentioned that differences in rise and fall times can also affect whether odd or even harmonics are present in the signal.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which even harmonics may appear in a square wave signal, indicating that the discussion remains unresolved regarding the specific circumstances that lead to this phenomenon.

Contextual Notes

Limitations include the assumption that the square wave is ideal versus real-world variations, as well as the dependence on definitions of harmonic content and signal characteristics.

cyclone24
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If I look at the Fourier transform of the square wave signal, the peaks are at the odd multiples. In the device that I am working on, I would expect the interference at the odd harmonics of the center frequency.

But I see the interference at even harmonics. When is this possible?

(Only if the square wave is not really square?)
 
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cyclone24 said:
(Only if the square wave is not really square?)

Nothing is ideal in the real world.
 
The harmonic structure of a pulse signal depends on the duty-cycle. A real 50% square wave has the odd-harmonic structure you mention. Varying the pulse-width will produce different harmonics.
 
It turns out that the difference in rise and fall times lead to different harmonics (odd/even)...
 

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