MHB Find \(H(s)\) for Causal LTI System: Region of Convergence

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The transfer function \(H(s)\) for the causal LTI system is derived as \(H(s) = \frac{1}{s^2 + s + 1}\). The region of convergence (ROC) for this system is specified as \(\text{Re} \{s\} < -\frac{1}{2}\), ensuring stability. The inverse Laplace transform of \(H(s)\) results in a time-domain response of \(\frac{2}{\sqrt{3}}e^{-\frac{1}{2}t}\sin\Big(\frac{\sqrt{3}}{2}t\Big)\). A causal system is characterized by the condition \(\lim_{z\to\infty}H(z) < \infty\). This analysis confirms that the system is both causal and stable.
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Determine \(H(s)\) and specify its region of convergence. Your answer should be consistent with the fact that the system is causal and stable.

In order to find \(H(s)\), we need to find \(X(s)\) and \(Y(s)\).
\begin{align*}
x(t) &= Ri + L\frac{di}{dt} + \frac{1}{C}\int i(t)dt\\
X(s) &= \mathcal{L}\bigg\{i + \frac{di}{dt} + \int i(t)dt\bigg\}\\
&= I(s) + sI(s) - I(0) + \frac{1}{s}I(s)\\
&= I(s)\bigg(1 + s + \frac{1}{s}\bigg)\\
y(t) &= \frac{1}{C}\int i(t)dt\\
&= \mathcal{L}\bigg\{\int i(t)dt\bigg\}\\
&= \frac{1}{s}I(s)\\
H(s) &= \frac{\frac{1}{s}}{1 + s + \frac{1}{s}}\\
&= \frac{1}{s^2 + s + 1}
\end{align*}
View attachment 2097

What is a causal system?
For convergece, \(\text{Re} \ \{s\} < -\frac{1}{2}\) since the inverse Laplace of H is
\[
\frac{2}{\sqrt{3}}e^{-\frac{1}{2}t}\sin\Big(\frac{\sqrt{3}}{2}t\Big).
\]
 

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So a causal system is when
\[
\lim_{z\to\infty}H(z) < \infty
\]
 
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