# complex waveforms

Discussion in 'Homework Help' started by minius, Jan 20, 2015.

## I attached my attempt if someone can comment on it please.

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1. ### minius Thread Starter New Member

Jan 10, 2015
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Sketch the waveform defined below and explain how you would obtain its Fourier series:
f (wt) = 0 for 0≤wt ≤ pi/2
f (wt) = Vsin(wt) for pi/2≤wt ≤pi
f (wt) = 0 for pi≤wt ≤3pi/2
f (wt) = Vsin(wt) for 3pi/2≤wt ≤2pi
Develop the analysis as far as you are able.

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2. ### MrAl Distinguished Member

Jun 17, 2014
2,562
518
Hi,

Have you done any other Fourier series' ?
You do this the same way but only integrate over the period(s) that are non zero while still using the complete period as the total period.
This looks like a half wave rectified minus sine wave.

3. ### minius Thread Starter New Member

Jan 10, 2015
5
0
This is a new subject for me and I am trying to study and understand the concept. If you can give us a hint it would be appreciated. I believe I need to calculate a0, an, bn coefficients is that correct? I would appreciate any help you can provide.

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
What's Vsin(wt) at wt=pi/2?

What's Vsin(wt) at wt=3pi/2?

Do either of those agree with your sketch?

5. ### minius Thread Starter New Member

Jan 10, 2015
5
0
WBahn thank you for reply. I think they part of sinwave. for the second 3π/2×180º/π = 270º sin is -1 so should be going down on y axis?

6. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
If the voltage is -V at that point in the waveform, then, yah, I think the sketch should reflect that. Don't you?

If necessary, evaluate the function had three or four points within each section and plot those to make sure that your sketch is consistent with the function it's representing. There's no need to guess at this.

Jan 10, 2015
5
0
thank you

8. ### MrAl Distinguished Member

Jun 17, 2014
2,562
518
Hi,

Yes your waveform drawing is not correct yet. It should look like a triac waveform turning on at two places in the cycle.
You are right that you need to calculate a0 and the an, and bn. Some may turn out to be zero.

9. ### minius Thread Starter New Member

Jan 10, 2015
5
0
thanks again MrAl I think I know the sketch and I have a0 = 0 in my calcs but an and bn that looks challenging to me.Any hints on that? this sin(wt) confuses me...

10. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Well, show your work so that we can see if you are even starting out correctly and, if you are, where you are going astray. We are not mind readers.