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Signal and systems miscellaneous

Signals and Systems

  1. If x(t) ←F.T→
    2 sin ω
    = X(jω), when
    1 + ω

    x(t) =
    1, |t| < 1
    0, otherwise

    then the Fourier transform of signal y(t)


    1. 4 sin2 ω
      ω2
    2. 8π sin2 ω
      ω2
    3. 2 sin2 ω
      πω2
    4. None of these
Correct Option: B

First approach:
y(t) = x(t) * x(t)
Y(jω) = X(jω) X(jω)

=
2 sin ω
.
2 sin ω
ωω

=
4 sin2 ω
ω2

Second approach
Given, waveform
slope =
2
= 1
2

If
y(t) ←F.T.→ Y(jω)
then
d2
y(t) ←F.T.→ (jω)2 Y(jω)
dt2

y′′(t) = δ(t + 2) – 2δ(t) + δ(t – 2)
F{y′′(t)} = ejω2 – 2 + e–jω2
= 2
e2jω + e–2jω
- 2
2

= 2.cos 2ω – 2
= 2(cos 2ω – 1)
= 2(1 – 2 sin2 ω – 1)
= 2(– 2 sin2 ω)
= – 4 sin2 ω
Now, (– jω)2 Y(jω) = – 4 sin2 ω
or
Y(jω) =
4
sin2 ω
ω2

Hence, alternative (A) is the correct choice.



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