Fourier Transfer of sawtooth function

  • Thread starter Thread starter hotvette
  • Start date Start date
  • Tags Tags
    Fourier Function
hotvette
Homework Helper
Messages
1,001
Reaction score
11

Homework Statement


Find the magnitude and phase of the Fourier transform of h(t)=t over the interval 0,1

Homework Equations


H(s) = \int^{1}_{0} h(t) e^{-i 2 \pi f t} dt

The Attempt at a Solution


I found this thread: https://www.physicsforums.com/threads/fourier-transform-of-the-sawtooth-function.502113/ but can't seem to come close to the same result. Letting a = -i 2 \pi f = -i \omega we get:
##H(s) = \int^{1}_{0} t e^{a t} dt \\
= \frac{e^{at}}{a^2}(at-1)|^{1}_{0} \\
= \frac{1}{a^2} \left( a t e^{at} - e^{at}\right)|^{1}_{0} \\
= \frac{1}{a^2}(ae^{a} - e^{a} + 1) \\
= \frac{1}{a^2}\left[(a-1) e^{a} + 1 \right] \\
= \frac{1}{a^2}(ae^{a} - e^{a} + 1) \\
= \frac{1}{(-i \omega)^2}\left[(-i \omega-1) e^{-i \omega} + 1 \right] \\
= \frac{-1}{\omega^2}\left[(-i \omega-1) e^{-i \omega} + 1 \right] \\
= \frac{1}{\omega^2}\left[(i \omega+1) e^{-i \omega} - 1 \right] \\
= \frac{1}{\omega^2}\left[(i \omega+1) (\cos \omega - i \sin \omega) - 1 \right] \\
\vdots \\
= \frac{1}{\omega^2} \left[ i (\omega\cos\omega-\sin \omega) + (\omega \sin \omega + \cos \omega -1)\right]##
I just don't see how this is converging to the solution in the referenced thread. Have I gone wrong somewhere?
 
Last edited:
Physics news on Phys.org
hotvette said:

Homework Statement


Find the magnitude and phase of the Fourier transform of h(t)=t over the interval 0,1

Homework Equations


H(s) = \int^{1}_{0} h(t) e^{-i 2 \pi f t} dt

The Attempt at a Solution


I found this thread: https://www.physicsforums.com/threads/fourier-transform-of-the-sawtooth-function.502113/ but can't seem to come close to the same result. Letting a = -i 2 \pi f = -i \omega we get:
##H(s) = \int^{1}_{0} t e^{a t} dt \\
= \frac{e^{at}}{a^2}(at-1)|^{1}_{0} \\
= \frac{1}{a^2} \left( a t e^{at} - e^{at}\right)|^{1}_{0} \\
= \frac{1}{a^2}(ae^{a} - e^{a} + 1) \\
= \frac{1}{a^2}\left[(a-1) e^{a} + 1 \right] \\
= \frac{1}{a^2}(ae^{a} - e^{a} + 1) \\
= \frac{1}{(-i \omega)^2}\left[(-i \omega-1) e^{-i \omega} + 1 \right] \\
= \frac{-1}{\omega^2}\left[(-i \omega-1) e^{-i \omega} + 1 \right] \\
= \frac{1}{\omega^2}\left[(i \omega+1) e^{-i \omega} - 1 \right] \\
= \frac{1}{\omega^2}\left[(i \omega+1) (\cos \omega - i \sin \omega) - 1 \right] \\
\vdots \\
= \frac{1}{\omega^2} \left[ i (\omega\cos\omega-\sin \omega) + (\omega \sin \omega + \cos \omega -1)\right]##
I just don't see how this is converging to the solution in the referenced thread. Have I gone wrong somewhere?

Your answer is correct; you have not gone wrong anywhere that I can see.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
Back
Top