Math Problem Statement

Convolve the following signals x(t) = e^(-3t)u(t) and x(t) = e^(-2t)u(t-2).

Solution

To solve this convolution problem, we need to find the convolution of two signals given as:

  1. x(t)=e3tu(t)x(t) = e^{-3t} u(t)
  2. h(t)=e2tu(t2)h(t) = e^{-2t} u(t - 2)

where u(t)u(t) is the unit step function.

Step 1: Set Up the Convolution Integral

The convolution of two signals x(t)x(t) and h(t)h(t) is defined as: y(t)=(xh)(t)=x(τ)h(tτ)dτ.y(t) = (x * h)(t) = \int_{-\infty}^{\infty} x(\tau) h(t - \tau) \, d\tau.

Given x(t)=e3tu(t)x(t) = e^{-3t} u(t), this signal is zero for t<0t < 0. So: x(τ)=e3τu(τ).x(\tau) = e^{-3\tau} u(\tau).

The second signal h(t)=e2tu(t2)h(t) = e^{-2t} u(t - 2) also involves a delay by 2, which shifts the function to start at t=2t = 2. Thus: h(tτ)=e2(tτ)u(tτ2).h(t - \tau) = e^{-2(t - \tau)} u(t - \tau - 2).

Step 2: Define the Limits of Integration

Since both functions are causal (involving unit steps), the integral limits will depend on the time tt and the delay introduced by the unit step function in h(t)h(t). This will lead to piecewise definitions based on the ranges of tt where the product x(τ)h(tτ)x(\tau) h(t - \tau) is non-zero.

Step 3: Solve the Convolution Integral

To proceed further, let's apply the convolution in two cases based on time intervals.

  1. For t<2t < 2: Here, h(tτ)=0h(t - \tau) = 0 for all τ\tau since tτ2<0t - \tau - 2 < 0. Hence, y(t)=0y(t) = 0 in this interval.

  2. For t2t \ge 2: Both x(τ)x(\tau) and h(tτ)h(t - \tau) contribute to the integral: y(t)=0te3τe2(tτ)u(τ)u(tτ2)dτ.y(t) = \int_0^t e^{-3\tau} e^{-2(t - \tau)} u(\tau) u(t - \tau - 2) \, d\tau. The step functions imply that τ\tau ranges from 22 to tt: y(t)=2te3τe2(tτ)dτ.y(t) = \int_2^t e^{-3\tau} e^{-2(t - \tau)} \, d\tau. Simplifying inside the integral: y(t)=2te3τe2t+2τdτ=e2t2teτdτ.y(t) = \int_2^t e^{-3\tau} e^{-2t + 2\tau} \, d\tau = e^{-2t} \int_2^t e^{-\tau} \, d\tau. Integrate eτe^{-\tau} from 22 to tt: y(t)=e2t[eτ]2t=e2t(et+e2).y(t) = e^{-2t} \left[ -e^{-\tau} \right]_2^t = e^{-2t} \left( -e^{-t} + e^{-2} \right). Simplifying further: y(t)=e2te2e3t=e2(t+1)e3t.y(t) = e^{-2t} \cdot e^{-2} - e^{-3t} = e^{-2(t+1)} - e^{-3t}.

So the final solution is:

0, & t < 2 \\ e^{-2(t+1)} - e^{-3t}, & t \ge 2. \end{cases}$$ Would you like further details on each step or additional explanations? ### Related Questions: 1. What is the Laplace transform of the convolution of two signals? 2. How does the unit step function affect the limits of integration in convolution problems? 3. What are common methods for finding convolutions without integration? 4. How does time-shifting a function affect its convolution? 5. Can we use Fourier transform properties to solve convolution problems? ### Tip: Remember, convolving with delayed signals often requires adjusting limits carefully due to the unit step function constraints.

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Math Problem Analysis

Mathematical Concepts

Signal Processing
Convolution
Laplace Transforms
Unit Step Function

Formulas

Convolution Integral: y(t) = (x * h)(t) = ∫ x(τ) h(t - τ) dτ
Unit Step Function: u(t)

Theorems

Properties of Convolution with Causal Signals

Suitable Grade Level

Undergraduate