Test 3 - Signals and System | Electronics and Communication (ECE)

Description: A test for Signals and System of Electronics and Communication (ECE)
Number of Questions: 20
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Tags: Signals and System
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Two discrete time systems with impulse responses h1[n] = $\delta$[n -1] and h2[n] = $\delta$[n - 2] are connected in cascade. The overall impulse response of the cascaded system is

  1. $\delta$[n - 1] + $\delta$[n - 2]

  2. $\delta$[n - 4]

  3. $\delta$[n - 3]

  4. $\delta$[n - 1] $\delta$[n - 2]


Correct Option: C
Explanation:

The 4-point Discrete Fourier Transform (DFT) of a discrete time sequence {1, 0, 2, 3} is

  1. [0, -2 + 2j, 2, - 2 - 2j]

  2. [2, 2 + 2j, 6, 2 - 2j]

  3. [6, 1 - 3j, 2, 1 + 3j]

  4. [6, - 1 + 3j, 0, - 1 - 3j]


Correct Option: D
Explanation:

A sequence x(n) with the z-transform X(z) = z4 + z2 -2z + 2-3z-4 is applied as an input to a linear, time-invariant system with the impulse response h(n) = 2$\delta$(n-3) where $\delta$(n) = $ \begin{cases} 1, n = 0 \\ 0, otherwise \end{cases}

$ The output at n = 4 is

  1. -6

  2. zero

  3. 2

  4. -4


Correct Option: B
Explanation:

The input x(t) and output y(t) of a system are related as y(t) = $ \oint_\infty x(\tau) cos(3 \tau) \ d\tau $the system is

  1. time - invariant and stable

  2. stable and not time-invariant

  3. time -invariant and not stable

  4. not time-invariant and not stable


Correct Option: D

A linear, time - invariant, causal continuous time system has a rational transfer function with simple poles at s = - 2 and s = - 4 and one simple zero at s = - 1. A unit step u (t) is applied at the input of the system. At steady state, the output has constant value of 1. The impulse response of this system is

  1. [exp (- 2t) + exp (- 4t)] u (t)

  2. [- 4 exp (- 2t) - 12 exp (- 4t) - exp (- t)] u (t)

  3. [- 4 exp (- 2t) + 12 exp (- 4t) u (t)

  4. [- 0.5 exp (- 2t) + 1.5 exp (- 4t)] u (t)


Correct Option: C
Explanation:

For a signal x(t), the Fourier transform is X(f). Then the inverse Fourier transform of X(3f + 2) is given by

  1. $ \dfrac{1}{2} \times (\dfrac{1}{2}) e^{j3\pi t} $

  2. $ \dfrac{1}{3} \times (\dfrac{1}{3}) e^{-j4\pi t} $

  3. $ 3 \times (3t) e^{-j4\pi t}$

  4. x(3t + 2)


Correct Option: B
Explanation:

Consider the sequence| x[n] = [– 4 – j51 + j25]. The conjugate anti-symmetric part of the sequence is

  1. [– 4 – j2.5,j2, 4 – |j25]

  2. [– j2.5, 1, j25]

  3. [– j2.5, j2, 0]

  4. [– 4, 1, 4]


Correct Option: A
Explanation:

The system under consideration is an RC low-pass filter (RC-LPF) with R = 1.0 k$\Omega$and C = 1.0 $\delta$F.

Let tg(f) be the group delay function of the given RC-LPF and f2 = 100 Hz. Then tg(f2) in ms, is

  1. 0.717

  2. 7.17

  3. 71.7

  4. 4.505


Correct Option: A
Explanation:

The power in the signal s(t) = 8 cos $\left( 20\pi t - \dfrac{\pi}{2} \right)$ + 4 sin $(15 \pi t)$ is

  1. 40

  2. 41

  3. 42

  4. 82


Correct Option: A
Explanation:

A 5-point sequence x (n) is given as X [–3] = 1, x [–2] = 1, x [–1] = 0, x [0] = 5, x [1] = 1. If x($e^{j\omega}$) denotes the discrete – time fourier transform of x [n], what is the value of $\displaystyle \int_{-\pi}^\pi x(e^{j\omega})$$d\omega$?

  1. 5

  2. 10$\pi$

  3. 16$\pi$

  4. 5 + j10$\pi$


Correct Option: B
Explanation:

If the unit step response of a network is $(1 - e^{-\omega t})$, then its unit impulse response is

  1. $\alpha e^{-\omega t}$

  2. $\alpha^{-t} e^{-\omega t}$

  3. $(1 - \alpha^{-1}) e^{-\omega t}$

  4. $(1 - \alpha) e^{-\omega t}$


Correct Option: A
Explanation:

Choose the function $f(t); -\infty \lt 1 \lt +\infty$for which a Fourier series cannot be defined.

  1. 3 sin (25t)

  2. 4 cos (20t + 3) + 2sin (10t)

  3. exp (-|t|) sin(25t)

  4. 1


Correct Option: C
Explanation:

The system under consideration is an RC low-pass filter (RC-LPF) with R = 1.0 k$\Omega$and C = 1.0$\mu$F.

Let H(f) denote the frequency response of the RC-LPF. Let f1 be the highest frequency such that 0$\le$|f| $\le$f1, $\dfrac{ | H(f_1) | }{H(0)}$$\ge$ 0.95. Then f1 (in Hz) is

  1. 327.8

  2. 163.9

  3. 52.2

  4. 104.4


Correct Option: C
Explanation:

The trigonometric Fourier series for the waveform f(t) shown below contains

  1. only cosine terms and zero value for the dc component

  2. only cosine terms and a positive value for the dc component

  3. only cosine terms and a negative value for the dc component

  4. only sine terms and a negative for the dc component


Correct Option: C
Explanation:

The impulse response h [n] of a linear time-invariant system is given by h[n]= u[n+3] + u [n-2)-2n[n-7] where u[n] is the unit step sequence. The above system is

  1. stable but not causal

  2. stable and causal

  3. causal but unstable

  4. unstable and not causal


Correct Option: A
Explanation:

If the region of convergence of x1 [n] + x2 [n] is $\dfrac{1}{3} \lt |z| \lt \dfrac{2}{3}$, then the region of convergence of xn [n] - x2 [n] includes

  1. $\dfrac{1}{3} \lt |z| \lt 3$

  2. $\dfrac{2}{3} \lt |z| \lt 3$

  3. $\dfrac{3}{2} \lt |z| \lt 3$

  4. $\dfrac{1}{3} \lt |z| \lt \dfrac{2}{3}$


Correct Option: D
Explanation:

The ROC of addition or subtraction of two functions $x_1(n) \ and \ x_2(n)$ is $R_1 \cap R_2$. We have been given ROC of addition of two function and has been asked ROC of subtraction of two function. It will be same.

Let x(t) be the input to a linear, time-invariant system. The required output is 4x (t-2). The transfer function of the system should be

  1. 4 ej4$\pi$f

  2. 2 e-j8$\pi$f

  3. 4 e-j4$\pi$f

  4. 2 ej8$\pi$f


Correct Option: C
Explanation:

A function is given by f (t) = sin2 t + cos 2t. Which of the following is true?

  1. f has frequency components at 0 and $\dfrac{1}{2\pi}$Hz.

  2. f has frequency components at 0 and $\dfrac{1}{\pi}$Hz.

  3. f has frequency components at $\dfrac{1}{2\pi}$ and $\dfrac{1}{\pi}$Hz.

  4. f has frequency components at $\dfrac{0.1}{2\pi}$ and $\dfrac{1}{\pi}$Hz.


Correct Option: B
Explanation:

The differential equation 100$\dfrac{d^2 y}{dt^2}$- 20$\dfrac{dy}{dt}$ + y = x(t) describes a system with an input x(t) and an output y(t). The system, which is initially relaxed, is excited by a unit step input. The output y(t) can be represented by the waveform


Correct Option: A
Explanation:

In the system shown below, x (t ) = (sin t)u (t). In steady-sate, the response y (t) will be

  1. $\dfrac{1}{\sqrt 2} sin \left( t - \dfrac{\pi}{4} \right)$

  2. $\dfrac{1}{\sqrt 2} sin \left( t - \dfrac{\pi}{4} \right)$

  3. $\dfrac{1}{\sqrt 2} e^{-t} sint$

  4. sin t - cos t


Correct Option: A
Explanation:

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