Signal Frequency: Are Frequencies Always Present?

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Discussion Overview

The discussion revolves around the nature of signal frequency in the context of a signal with randomly varying amplitude. Participants explore whether all frequencies associated with a signal are present at all times or if only one frequency is present at any given moment.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether all frequencies are present at all times in a signal with random amplitude variations or if only one frequency is present at any specific time.
  • Another participant expresses uncertainty about quantifying randomness in amplitude, suggesting that a "random walk" process might be necessary for meaningful periodicity.
  • A participant argues that frequency is meaningless at a single point in time, emphasizing the need to analyze signals over intervals to determine frequency content.
  • One contributor mentions that a signal can be represented as an infinite series of sine waves, indicating that while certain frequencies may not be prominent at a specific time, they still exist over a broader time frame.
  • A participant discusses the concept of amplitude modulation, noting that the observed bandwidth results from sidebands related to the modulation frequency.
  • Another participant raises a point about the clarity of the time-varying amplitude, suggesting that if it varies over the waveform period, the signal can be expressed as a sum of sinusoidal signals with constant amplitudes, while time-varying amplitudes would affect the presence of sinusoidal components.

Areas of Agreement / Disagreement

Participants express differing views on the nature of frequency presence in signals, with no consensus reached on whether all frequencies are present at all times or only at specific moments. The discussion remains unresolved regarding the implications of amplitude variation on frequency content.

Contextual Notes

Participants highlight the importance of time intervals in analyzing frequency content, indicating that assumptions about the nature of amplitude variation and its impact on frequency representation may not be fully addressed.

Blade_Runner
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signal frequency!

suppose we have a signal varying randomly in amplitude with time...when we plot the frequency responce of this signal , we get a bandwidth of frequencies...My question is ' are all these frequencies present at all times??', or 'at any time , say t=to , only one of these frequencies is present??'...
 
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I'm not sure how you would quantify the randomness of the amplitude. You can't really throw random numbers in for amplitude or you would get junk. You would need to increase or decrease the amplitude with a "random walk" like process to have anything that looks remotely periodic.
 


Blade_Runner said:
My question is ' are all these frequencies present at all times??', or 'at any time , say t=to , only one of these frequencies is present??'...

Well, the very concept of frequency is meaningless for intervals of zero time. If all you're considering is a single point in time, an infinity of different signals (with different frequency content) could result in the point you're seeing.

The best you can do is to study a signal over some interval of time, and describe what frequency content is present in that interval. The next interval of time, however, could include totally different frequencies.

- Warren
 


In my area we deal with discrete functions. Applying the same thinking, I would say that the signal or function can be thought of as an infinite series of sine waves with varying frequencies and amplitudes. While the direct contribution of low or high order sine functions may not be directly prevalent at a specific point in time, t; the function does exist from t0 to t_inf.

Do you have additional information regarding a specific problem?
 


If your signal is a constant-frequency signal with varying amplitudes (AM modulation), the observed bandwidth is sidebands due to the amplitude modulation. The frequency spread of the sidebands is due to the frequency of the amplitude modulation.
 


It doesn't seem clear whether the time-varying amplitude varies over the period of the waveform or over the time the signal is produced. If it's over the period of the waveform, the signal can be expressed as a sum of sinusoidal signals, each having amplitudes that do not change with time. If the main signal varies over the time the signal is produced, all of the sinusoidal components will likely all still be present but their amplitudes will change over time.
 

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