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We know that Laplace transform has two parts, \$\sigma \$ and \$ j\omega\$.

\$\sigma\$ is the real part while \$ j\omega\$ is the imaginary part.

Is it okay to say that the term \$\sigma \$ is a damping factor term, while \$ j\omega\$ is a pure sinusoidal (pure frequency) term?

I was reading another question on EE stack exchange at link below:

Relation and difference between Fourier, Laplace and Z transforms

I read a sentence and I highlighted that sentence in attached snap. The sentence is:

Fourier transform F(jω) which is essentially the frequency domain representation of f(t)

So what does that mean? That a Laplace transform is not a frequency domain representation of f(t)?

enter image description here

ocrdu
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s12d
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    When you use \$s\rightarrow j\omega\$ to get the frequency response, you are assuming that the transient, \$e^{\sigma t}\$, has decayed to zero (i.e it's the 'steady state' frequency response). \$\sigma\$ must be negative otherwise the system is unstable. – Chu Aug 22 '22 at 08:59

2 Answers2

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First, what is a transformation? A transform is a mapping of a function in one domain into another domain. To go from one domain to another, you need basis-functions (basis-vectors).

In Fourier transform, the basis-functions are complex exponentials; \$e^{-j\omega t} = \cos(\omega t) - j\sin(\omega t)\$

In Laplace transform, the basis-functions are the product of complex exponentials and decaying exponentials; \$e^{-\sigma t}e^{-j\omega t} = e^{-(\sigma+j\omega) t} = e^{-st}\$.

Fourier transform: \$F(j\omega) = \displaystyle\int_{-\infty}^\infty e^{-j\omega t}f(t) \: \text{d} t \$

Laplace transform: \$F(s) = \displaystyle\int_{-\infty}^\infty e^{-st}f(t) \: \text{d} t \$

Think about the product inside the integral (\$e^{-j\omega t}f(t)) \$ as an inner product (dot product) between two vectors. A dot product is large, if the two vectors are similar to each other. A dot product is small, if the two vectors are dissimilar.

Consider the Fourier transform first. Evaluating \$e^{-j\omega t}f(t)\$ we investigate how similar \$f(t)\$ is to \$\big(\cos(\omega t) - j\sin(\omega t)\big) \$ for a single frequency.

Evaluating \$\displaystyle\int_{-\infty}^\infty e^{-j\omega t}f(t) \: \text{d} t \$ you get a function telling you how similar \$f(t)\$ is to \$\big(\cos(\omega t) - j\sin(\omega t)\big) \$ for any frequency. We call this function \$F(j\omega)\$.

Let's say there is a frequency \$\omega_0\$ and that \$F(j\omega_0)\$ has a very large value. This means that at \$\omega_0\$, \$f(t)\$ and \$\big(\cos(\omega_0 t) - j\sin(\omega_0 t)\big) \$ are very similar to each. This in turn tells you that a sinusoidal component with frequency \$\omega_0\$ is very present in \$f(t)\$. By plotting \$F(j\omega)\$ as a function of \$\omega\$ you see which frequencies are contained in \$f(t)\$ and also how much each frequency component is present. For this reason, the Fourier transform is an excellent tool for frequency analysis of signals.

The same thought process holds for the Laplace transform, however, instead of looking at how similar \$f(t)\$ is to sinusoids, now we look at how similar \$f(t)\$ is to exponentially decaying/growing sinusoids. So \$F(s)\$ is a function telling you how similar \$f(t)\$ is to \$e^{-st}\$ for any \$s\$ and doesn't have the same intuitive usefulness as the Fourier transform does for signal analysis.

However, the Laplace transform is very useful for analysing systems since it allows you to use the initial conditions at \$t=0^-\$ for finding the response of a system. The system response you obtain also contain transient components, and not just the steady state response.

The Fourier transform lacks both of these things for system analysis.

Conclusion

\$F(j\omega)\$ tells you how similar a function \$f(t)\$ is to sinusoids. The Fourier transform is excellent for signal analysis.

\$F(s)\$ tells you how similar a function \$f(t)\$ is to exponentially decaying/growing sinusoids. The Laplace transform is excellent for system analysis.

Carl
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  • I think the limits on your Laplace integation are not right – Clara Diaz Sanchez Aug 22 '22 at 19:37
  • @ClaraDiazSanchez The equation for the Laplace I've written in my answer is known as the **bilateral Laplace transform**. However, there also exists the **unilateral Laplace transform** \$F(s) = \displaystyle\int_{0^-}^\infty e^{-st}f(t) \: \text{d}t \$ which is restricted to causal signals only. It turns out that every physical signal you can create irl is causal. – Carl Aug 23 '22 at 06:47
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So what does that mean? That a Laplace transform is not a frequency domain representation of f(t)?

The Fourier transform is a subset of the Laplace transform. Fourier ignores σ. Laplace is the all-encompassing transform and yes, it encompasses the frequency domain representation of f(t).

Andy aka
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