Sampling frequency to use with this signal to avoid aliasing

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Discussion Overview

The discussion revolves around determining an appropriate sampling frequency to avoid aliasing for a given signal, specifically a triangle wave. Participants explore the implications of the Nyquist frequency and rate, as well as the challenges of systematic approaches to sampling in relation to the signal's frequency components.

Discussion Character

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

Main Points Raised

  • Some participants express familiarity with the Nyquist frequency and rate, noting that the maximum angular frequency is 8 rad/s and suggesting that the sampling frequency should be twice this maximum frequency.
  • One participant questions the systematic approach to using the Nyquist rate, prompting further exploration of how to determine what frequencies are aliased.
  • Another participant suggests that for a triangle wave, the sampling frequency theoretically needs to be infinite due to the presence of frequency components extending to infinity, and proposes using Fourier decomposition to assess reconstruction accuracy.
  • There is a mention that sampling at twice the frequency of the triangle wave will lead to reconstructing a sine wave, while emphasizing the importance of considering the finite bandwidth of a real triangle wave.
  • One participant corrects themselves regarding the interpretation of a figure, clarifying that it represents the frequency spectrum rather than the wave itself.

Areas of Agreement / Disagreement

Participants generally agree on the relevance of the Nyquist frequency and the challenges posed by the triangle wave's infinite frequency components. However, there is no consensus on the specific systematic approach to take or the implications of sampling frequency choices.

Contextual Notes

Participants note that the simple Nyquist sampling theorem assumes infinite sampling time, and there are more complex considerations for limited sampling times that are not commonly referenced.

Electgineer
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Homework Statement
The figure below shows the frequency spectrum of a CTS.
Relevant Equations
The sampling frequency without aliasing. I am posting this here because I don't even know how to approach the question. Is there a way to approach this kind of question. Thank you.
IMG_20210530_205602.jpg
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Are you familiar with the Nyquist frequency and the Nyquist rate?
 
FactChecker said:
Are you familiar with the Nyquist frequency and the Nyquist rate?
Yes. I am familiar till the part that the maximum angular frequency is 8 rad/s. So using the Nyquist rate, the sampling frequency should be twice the max frequency. But I don't understand how to systematically approach this kind of question. Thank you.
 
Do you mean that you don't know how to systematically use the Nyquist rate or what?
You have answered the first question.
For the second question, assume point-by-point that anything below 1/2 the sampling frequency is not aliased and that everything above it is aliased to a lower frequency. What would the aliased spectrum graph look like?

PS. The simple Nyquist sample rate theorem (with perfect suppression of aliasing) assumes that the sample is taken for an infinite time. There is a more complicated version for a limited sample time, but it is rarely used and I can not find a reference for it.
 
Last edited:
Electgineer said:
Yes. I am familiar till the part that the maximum angular frequency is 8 rad/s. So using the Nyquist rate, the sampling frequency should be twice the max frequency. But I don't understand how to systematically approach this kind of question. Thank you.
Sort of. You have a triangle wave, so theoretically, your sampling frequency needs to be infinite since there are frequency components going out to infinity. The way to approach it is to do a Fourier decomposition of the triangle wave and decide how well you need the reconstruction to resemble the triangle wave. Sampling at twice the frequency of the triangle wave will result in reconstructing a sine wave with the frequency of the triangle wave (ignoring aliasing). You need to sample at twice the highest frequency in the Fourier series you want to use to reconstruct the reconstruction of the triangle wave with the precision you require. Since any "real" triangle wave will have a finite bandwidth and not be a perfect triangle wave, you might start there.
 
bobob said:
Sort of. You have a triangle wave, so theoretically, your sampling frequency needs to be infinite since there are frequency components going out to infinity.
The figure shown is the frequency spectrum, not the wave. So it is much simpler.
 
FactChecker said:
The figure shown is the frequency spectrum, not the wave. So it is much simpler.
Oops... My mistake for not looking at the figure more closely...
 

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