# Homework Help: Discretion and harmonic amplitude

1. Nov 9, 2013

### MissP.25_5

What's the relationship between DFT and harmonic amplitude? How do I find the harmonic amplitude using discrete fourier transform? Here's what I have done so far. "harm.freq" is harmonic frequency here.
I have done the DFT calculation and now what? Aftet I have performed DFT, how do I find the amplitude of the harmonic wave? Below on post #6 is the formula and I have to plot the graph using excel based on the table given.

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2. Nov 9, 2013

### njoysci

http://www.fpv.umb.sk/kat/kf/Integrovana_prirodoveda/TG/Zvuk/Zvuk_kmitanie/www.me.psu.edu/me82/Learning/FFT/FFT.html [Broken]

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3. Nov 9, 2013

### MissP.25_5

I have read the whole thing but it doesn't show how to calculate the amplitude.

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4. Nov 9, 2013

### Staff: Mentor

Where did you do the DFT?
What is the relation between t and frequency?
The DFT gives two amplitude components for the cos and sin contribution, the total amplitude is just the root of the sum of squares of both.

5. Nov 9, 2013

### MissP.25_5

That's the formula I used and the calculation is done with excel. Look at the cell O6, that's the value after DFT. I am required to use this formula and I have to plot a graph based on this table given.

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6. Nov 9, 2013

### MissP.25_5

That's why I did, in the cell where I wrote "discretized" in cell O6, is the root of the sum of squares of both cos and sin contribution . The DFT is 3.030972661. Now how can I plot the amplitude? If that number is the amplitude, then how come I can plot a graph if it's only ONE number? Please look at my reply above this post.

7. Nov 9, 2013

### Staff: Mentor

The amplitude for a specific frequency is a single number. For every frequency, you should get such a number.

8. Nov 9, 2013

### MissP.25_5

Ok, so the 3.030972661 is the amplitude for what frequency? 50hz I suppose? And what is the table with α,β and k for?

9. Nov 10, 2013