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GATE 2016 EC (EC3) – Question 40

Networks, Signals and Systems · Continuous-time Signals · 2 marks · Multiple choice

The signal $2\cos(2\pi t/3)-\cos(\pi t)$ enters an LTI system with $H(s)=e^s+e^{-s}$. If $C_k$ is the kth exponential Fourier-series coefficient of the output, $C_3$ equals

  1. 0
  2. 1
  3. 2
  4. 3

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Correct answer: (B) 1

Explanation

The input angular frequencies are $2\pi/3$ and π, so their common fundamental is $\omega_0=\pi/3$. The third harmonic is the −cos πt term, whose positive-frequency coefficient is −1/2.

The frequency response at π is $H(j\pi)=e^{j\pi}+e^{-j\pi}=-2$. Multiplying the input coefficient by this value gives $C_3=(-1/2)(-2)=$ **1, B**.

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