Why didn't I see a peak at half the frequency in my FFT analysis of two waves?

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Discussion Overview

The discussion revolves around the expectations and observations related to FFT analysis of two combined sine waves, particularly focusing on the presence or absence of a peak at half the frequency of the main wave. Participants explore the implications of wave characteristics on FFT results, including the effects of modulation and convolution in the frequency domain.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expected to see a peak at half the main frequency in their FFT analysis due to a dip in the wave but later considered that it might result in higher harmonics instead.
  • Another participant inquired about the appearance of the dip in the wave to clarify the description.
  • A participant described the original wave as a combination of two sine waves with opposite phases, suggesting that the dip occurs between them, affecting the overall waveform.
  • One participant explained that the combination of the sine waves, when multiplied by a rectangular wave, leads to convolution in the frequency domain, resulting in a more complex signal that may not clearly show expected peaks.
  • There was a request for images to better understand the described waveforms, indicating that the verbal description was insufficient for clarity.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the expected outcomes of the FFT analysis, with differing views on whether a peak at half frequency should be present and how the wave characteristics influence the results.

Contextual Notes

The discussion highlights the complexity of interpreting FFT results, particularly when dealing with modulated signals and the effects of convolution, but does not resolve the specific expectations regarding frequency peaks.

Sam Smith
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I am carrying out FFT analysis to compare two waves. One looks very much like a sine wave the other has an extra dip occurring at half the frequency of the main wave. I have been thinking around how I might expect this to show up in the FFT analysis. At first i was expecting to see a smaller peak at half the main frequency but after a while I thought that it may just result in a few more higher harmonics due to the fact that the wave is becoming more complex. I did the FFT and I didnt see a peak at half frequency. so I guess the second conclusion is correct btu I still can't reconcile why I would expect to see a peak at half frequency of the main peak?
 
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What does the "dip" look like?
 
Well the original wave is a two sine waves put together each mirroring the other. The dip is in between them so instead of going back to zero the graph dips instead :)
 
OK, so two sine waves mirroring each other have the same frequency but opposite phase. Each of those sine waves is multiplied by a rectangular wave in the time domain, which means that it is convolved with the transform of the rectangular wave in the frequency domain. So you will expect to see a signal that looks much more like the sum of two Fourier transforms of a rectangular wave. Since they are each modulated by the same frequency but opposite phases the resulting sum is not obvious to me. I would expect it to look "complicated".
 
Can you post images? Your description is unclear.
 

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