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ECE Systems and Signals Homework 6 Solution


1. An LTI causal system is described by the equation:

dy(t)


t
dx(t)


+

y(t − τ)τ2e3−τ dτ =

− x(t), t > 0








dt

0


dt


y(0) = 0, x(t) = y(t) = 0 for t < 0

        (a) Find H(s)

        (b) The periodic signal | cos2(t)| + 1 is applied to the system. Find the Fourier coefficients of the output y˜(t).

        (c) Find the Fourier series coefficients of z(t) = y˜(t − 3) ∗ y˜(2t)

    2. Find the Fourier transform of the following signals:

t2, 0 ≤ t < 1
(a) x1(t) =
0, otherwise

    (b) x2(t) = [u(t + 2) − u(t − 2)] cos(100t) + 1

(c) x3(t) =    −t∞ cos(5(t − σ))δ(σ − 2)dσ

    3. Consider the time-domain real signal x(t) with a Fourier Transform X(ω), where x(t) is shown below.















    (a) Find X(0) and  −∞∞ X(ω)dω

    (b) Compute  −∞∞ |X(ω)|2dω
(c) Compute

X(ω)Y (ω)dω, where Y (ω) =
2 sin(ω)
ej2ω







−∞



ω
Hint: For any real signals f(t) and g(t), we have:



1   ∞


−∞ f(t)g(−t)dt =

−∞ F (ω)G(ω)dω






1

        (d) Sketch the inverse Fourier transform of Re{X(ω)}.

Note: You can answer all parts without explicitly evaluating X(ω)

    4. Consider a signal x(t) with Fourier transform X(jω). Suppose you are given the following facts:

    i. x(t) is real and non-negative.

    ii. F−1{(1 + jω)X(jω)} = Ae−2tu(t), where A is a constant.

    iii. −∞∞ |X(jω)|2dω = 2π

Determine a closed form expression for x(t) and find the constant A.

5. Consider the following signal:

x(t) = e−t sin(2πt)u(t) + δ(t − 2)

    (a) Use the Laplace transform to find the Fourier transform X(Ω) of signal x(t)

    (b) Compute amplitude and phase spectrum.

    (c) Use MATLAB to plot amplitude and phase spectrum for Ω ∈ [−10, 10].













































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