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Join Date: Jan 2017
Posts: 14

# Phase Spectrum

02/19/2017 5:42 AM

Why a simple positive frequency sine wave shows a phase angle of -90 degrees or π/2 radians in the phase

spectrum?

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Guru

Join Date: Apr 2010
Posts: 6075
#1

### Re: phase spectrum

02/19/2017 7:18 AM

Because the sine wave is pi/2 or 90 degrees out of phase with the cosine wave, which is zero phase.

3
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Join Date: Dec 2008
Location: Long Island NY
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#2

### Re: phase spectrum

02/19/2017 8:40 AM

True but you didn't mention why the cosine function is preferred to the sine function in discrete time base to discrete frequency base conversion.

Obviously the sum of sinusoidal equations could use all sine or all cosine functions with just different values for phase. However the derivation of the Discrete Fourier Transform is less cluttered using the cosine function since the unitary maximum occurs when the phase angle equals zero instead of having to always carrying a transcendental, pi/2 coefficient through all of the equations.

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#3

### Re: phase spectrum

02/19/2017 9:28 AM

But... would the OP have understood that explanation? From the questions he's asking, I doubt it.

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Join Date: Dec 2008
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#4

### Re: phase spectrum

02/19/2017 10:20 AM

You may be right, my explanation maybe beyond the OP. My explanation might be like explaining the mechanics of rayleigh light scattering to a child is the reason why the sky is blue. Then again who knows which answer will solve, inspire, baffle or continue the curiosity to know more.

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"Don't disturb my circles." translation of Archimedes last words
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#5

### Re: phase spectrum

02/19/2017 11:08 AM

That's why I tried to make it simple.

3
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#6

### Re: phase spectrum

02/20/2017 10:05 AM

We need a "rating" page where OP's can click into and select the correct answer:

3. answer left me so baffled I maybe should quit engineering

We almost never get a response on posts like this and are left wondering if any of the answers helped.

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Member

Join Date: Oct 2012
Posts: 8
#7

### Re: phase spectrum

02/20/2017 10:12 AM

I think this is a great suggestion. I 2'nd it!

Guru

Join Date: Apr 2010
Posts: 6075
#8

### Re: phase spectrum

02/20/2017 11:31 AM

That sounds like a good idea.

I try to judge how much understanding the questioner has and match the answer to that level of understanding.

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#9

### Re: phase spectrum

02/20/2017 12:03 PM

4. gave me a good chuckle for the day

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Join Date: Apr 2010
Posts: 6075
#10

### Re: Phase Spectrum

02/20/2017 6:23 PM
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