[Math] fourier transform of sinc function

calculusdefinite integralsfourier analysisindefinite-integralsintegration

let us consider fourier transform of sinc function,as i know it is equal to rectangular function in frequency domain and i want to get it myself,i know there is a lot of material about this,but i want to learn it by my self,we have sinc function whihc is defined as

$sinc(\omega_0*t)=sin(\omega_0*t)/(\omega_0*t)$

$\int(sin(\omega_0*t)*e^{-j*\omega *t}/(\omega_0*t))dt $

we can represent as

$\int(sin(\omega_0*t)*(cos(\omega*t)-j*sin(\omega*t))/(\omega_0*t))dt$

because we can distribute in brackets and consider that integral of difference is equal differences of integrals,we get

$\int(sin(\omega_0*t)*cos(\omega*t)/(\omega_o*t))$ -$\int(sin(\omega_0*t)*sin(\omega*t)/(\omega_o*t))$

but first product is zero right?because sin and cosine are orthogonal,so how could continue ?please help me

Best Answer

In terms of deriving the Fourier Transform, I will make some use of techniques highlighted in http://www.claysturner.com/dsp/FTofSync.pdf

Let us start with your expression

$$\int_{-\infty}^{\infty}\frac{\sin(\omega_0t)\cos(\omega t)}{(\omega_ot)}dt-\int_{-\infty}^{\infty}\frac{j\sin(\omega_0t)\sin(\omega t)}{(\omega_ot)}dt$$

Examining the integrals, the term $\omega_0t$ in the denominator makes evaluating the integral more involved.

We can use some relevant trigonometric identities so that we can express $$\sin(\omega_0t)\cos(\omega t)=\frac{1}{2}[\sin((\omega+\omega_0)t)-\sin((\omega-\omega_0)t)]$$

$$\sin(\omega_0t)\sin(\omega t)=\frac{1}{2}[\cos((\omega-\omega_0)t)-\cos((\omega+\omega_0)t)]$$

To deal with the awkward $\omega_0t$ term, we can use the following identity to convert the single integral into a double integral that is far nicer to evaluate: $$\frac{1}{\omega_0t}=\int_0^{\infty}e^{-\omega_0ts}ds$$

Thus the integral to evaluate is $$\int_0^{\infty}\int_{-\infty}^{\infty}\sin(\omega_0t)\cos(\omega t)e^{-\omega_0tx}dtdx-j\int_0^{\infty}\int_{-\infty}^{\infty}\sin(\omega_0t)\sin(\omega t)e^{-\omega_0tx}dtdx$$

which expands to $$\frac{1}{2}\int_0^{\infty}\int_{-\infty}^{\infty}\sin((\omega+\omega_0)t)e^{-\omega_0tx}dtdx-\frac{1}{2}\int_0^{\infty}\int_{-\infty}^{\infty}\sin((\omega-\omega_0)t)e^{-\omega_0tx}dtdx\\-\frac{j}{2}\int_0^{\infty}\int_{-\infty}^{\infty}\cos((\omega-\omega_0)t)e^{-\omega_0tx}dtdx+\frac{j}{2}\int_0^{\infty}\int_{-\infty}^{\infty}\cos((\omega+\omega_0)t)e^{-\omega_0tx}dtdx$$

We can exploit Fubini's theorem to rewrite the integral as

$$\frac{1}{2}\int_{-\infty}^{\infty}\left[\int_0^{\infty}\sin((\omega+\omega_0)t)e^{-\omega_0xt}dt\right]dx-\frac{1}{2}\int_{-\infty}^{\infty}\left[\int_0^{\infty}\sin((\omega-\omega_0)t)e^{-\omega_0xt}dt\right]dx\\-\frac{j}{2}\int_{-\infty}^{\infty}\left[\int_0^{\infty}\cos((\omega-\omega_0)t)e^{-\omega_0xt}dt\right]dx+\frac{j}{2}\int_{-\infty}^{\infty}\left[\int_0^{\infty}\cos((\omega+\omega_0)t)e^{-\omega_0xt}dt\right]dx$$

We shall use the following integral identities to calculate the above integrals :- $$\int_0^{\infty}\sin(at)e^{-st}dt=\frac{a}{a^2+s^2}$$ $$\int_0^{\infty}\cos(at)e^{-st}dt=\frac{s}{a^2+s^2}$$

This results in $$\frac{1}{2}\int_{-\infty}^{\infty}\left[\frac{(\omega+\omega_0)}{(\omega+\omega_0)^2+\omega_0^2x^2}\right]dx-\frac{1}{2}\int_{-\infty}^{\infty}\left[\frac{(\omega-\omega_0)}{(\omega-\omega_0)^2+\omega_0^2x^2}\right]dx\\-\frac{j}{2}\int_{-\infty}^{\infty}\left[\frac{\omega_0x}{(\omega-\omega_0)^2+\omega_0^2x^2}\right]dx+\frac{j}{2}\int_{-\infty}^{\infty}\left[\frac{\omega_0x}{(\omega+\omega_0)^2+\omega_0^2x^2}\right]dx\text{ (Eq. 1)}$$

To evaluate the real component integrals in (Eq. $1$), we use the following result:- $$\int_{-\infty}^{\infty}\frac{a}{a^2+s^s}ds=\frac{|a|}{a}\int_{-\infty}^{\infty}\frac{1}{1+y^2}dy=sgn(a)\left[\arctan y\right]^{\infty}_{-\infty}=sgn(a)\pi$$

This leads to $$\int_{-\infty}^{\infty}\left[\frac{(\omega+\omega_0)}{(\omega+\omega_0)^2+\omega_0^2x^2}\right]dx=sgn(\omega+\omega_0)\pi$$ $$\int_{-\infty}^{\infty}\left[\frac{(\omega-\omega_0)}{(\omega-\omega_0)^2+\omega_0^2x^2}\right]dx=sgn(\omega-\omega_0)\pi$$

As regards the imaginary component integrals, note that the numerator is simply a constant times the derivative of the denominator, so we have

$$\int_{-\infty}^{\infty}\left[\frac{\omega_0x}{(\omega-\omega_0)^2+\omega_0^2x^2}\right]dx=\frac{1}{2\omega_0}\left[\ln\{(\omega-\omega_0)^2+\omega_0^2x^2\}\right]_{-\infty}^{\infty}$$

$$\int_{-\infty}^{\infty}\left[\frac{\omega_0x}{(\omega+\omega_0)^2+\omega_0^2x^2}\right]dx=\frac{1}{2\omega_0}\left[\ln\{(\omega+\omega_0)^2+\omega_0^2x^2\}\right]_{-\infty}^{\infty}$$

Combining the imaginary component integrals as per the integral in (Eq. $1$) we wish to evaluate and noting that they have opposite signs, we have $$-\int_{-\infty}^{\infty}\left[\frac{\omega_0x}{(\omega-\omega_0)^2+\omega_0^2x^2}\right]dx+\int_{-\infty}^{\infty}\left[\frac{\omega_0x}{(\omega+\omega_0)^2+\omega_0^2x^2}\right]dx\\=\lim_{x\rightarrow\infty}\frac{1}{2\omega_0}ln\left[\frac{(\omega+\omega_0)^2+\omega_0^2x^2}{(\omega-\omega_0)^2+\omega_0^2x^2}\right]+\lim_{x\rightarrow -\infty}\frac{1}{2\omega_0}ln\left[\frac{(\omega-\omega_0)^2+\omega_0^2x^2}{(\omega+\omega_0)^2+\omega_0^2x^2}\right]=\frac{2}{\omega_0}\ln(1)=0$$ Thus the imaginary terms in the integral cancel out, leading to the integral in (Eq. $1$) being a real result, as follows $$\frac{\pi}{2}\left[sgn(\omega+\omega_0)-sgn(\omega-\omega_0)\right]$$

Putting everything together we have: $$\int_{-\infty}^{\infty}\frac{\sin(\omega_0t)\cos(\omega t)}{(\omega_ot)}dt-\int_{-\infty}^{\infty}\frac{j\sin(\omega_0t)\sin(\omega t)}{(\omega_ot)}dt=\frac{\pi}{2}\left[sgn(\omega+\omega_0)-sgn(\omega-\omega_0)\right]$$

The result is a rectangular function that starts from frequency $-\omega_0$ and ends at frequency $\omega_0$.

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