# Aliasing (Fourier Transform)

Hi. I have been given a plot for 1 Hz, sampled at 0.2 sec. And, 4 Hz and 11 Hz has also been plotted. So, from the plot, I can see that its really hard to distinguish between the signals after digitalization. My question is how do I find the next higher frequency which, when sampled at 0.2 secs, will look like a 1 Hz wave ?

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Hi. I have been given a plot for 1 Hz, sampled at 0.2 sec. And, 4 Hz and 11 Hz has also been plotted. So, from the plot, I can see that its really hard to distinguish between the signals after digitalization. My question is how do I find the next higher frequency which, when sampled at 0.2 secs, will look like a 1 Hz wave ?
Do you mean 0.2s duration of sampling (with a given sampling frequency), or do you mean one sample every 0.2s (5Hz sampling frequency)?

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I mean one sample every 0.2 seconds

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I mean one sample every 0.2 seconds

In general you need at least two samples per cycle. You have a sampling frequency of 5Hz. Sampling 1Hz with this is not a problem - however very coarse plot. Using the same sampling frequency when you have 4 and 11Hz input would not provide any results at all.
According to Nyquist theorem, the minimum sampling frequency is twice the frequency you are trying to sample. Below you see a link to a PDF explaining this.

http://redwood.berkeley.edu/bruno/npb261/aliasing.pdf

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