Communication Engineering - Online Test
Q1. The input and output of a continuous time system are respectively denoted by x(t) and y(t). Which of the following descriptions corresponds to a causal system ?
Answer : Option C
Explaination / Solution:
The output of causal system depends only on present and past states only.
In option (A) y(0) depends on x(- 2) and x(4).
In option (B) y(0) depends on x(1).
In option (C) y(0) depends on x(- 1).
In option (D) y(0) depends on x(5).
Thus only in option (C) the value of y(t) at t = 0 depends on x(- 1) past value.
In all other option present value depends on future value.
Q2.
The longitudinal component of the magnetic field inside an air-filled rectangular waveguide made of a perfect electric conductor is given by the following expression 
The cross-sectional dimensions of the waveguide are given as a = 0.08 m and b = 0.033 m. The mode of propagation inside the waveguide is
The cross-sectional dimensions of the waveguide are given as a = 0.08 m and b = 0.033 m. The mode of propagation inside the waveguide is
Answer : Option C
Explaination / Solution:
We have the expression,
and the cross sectional dimension is
a = 0.08 m, b = 0.033m
Now, we compare the equation for TEm,n mode whose Hz is given as
Hence, given mode is TE21 mode
Q3. The impulse response h(t) of a linear time invariant continuous time system is
described by 
where u(-t) denotes the unit
step function, and α and β are real constants. This system is stable if
where u(-t) denotes the unit
step function, and α and β are real constants. This system is stable if
Answer : Option D
Explaination / Solution:
This system is stable only when bounded input has bounded output For stability αt < 0 for t > 0 that implies α < 0 and βt > 0 for t > 0 that implies β > 0. Thus, α is negative and β is positive.
Q4. Step responses of a set of three second-order underdamped systems all have the same percentage overshoot. Which of the following diagrams represents the poles of the three systems ?
Answer : Option C
Explaination / Solution:
Transfer function for the given pole zero plot is:
From the plot Re (P1 and P2)>(Z1 and Z2)
So, these are two lead compensator.
Hence both high pass filters and the system is high pass filter
Q5. Consider a system whose input x and output y are related by the equation 
where h(t) is shown in the graph.
Which of the following four properties are possessed by the system ?
BIBO : Bounded input gives a bounded output.
Causal : The system is causal,
LP : The system is low pass.
LTI : The system is linear and time-invariant.
where h(t) is shown in the graph.Which of the following four properties are possessed by the system ?
BIBO : Bounded input gives a bounded output.
Causal : The system is causal,
LP : The system is low pass.
LTI : The system is linear and time-invariant.
Answer : Option B
Explaination / Solution:
Here h(t) ! 0 for t < 0. Thus system is non causal. Again any bounded input x(t) gives bounded output y(t). Thus it is BIBO stable.
Q6. 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
Answer : Option C
Explaination / Solution:
Q7. For a parallel plate transmission line, let v be the speed of propagation and Z be the characteristic impedance. Neglecting fringe effects, a reduction of the spacing between the plates by a factor of two results in
Answer : Option B
Explaination / Solution:
No Explaination.
Q8. The signal x(t) is described by
Two of the angular frequencies at which its Fourier transform becomes zero are
Answer : Option A
Explaination / Solution:
Q9.
A communication channel with AWGN operating at a signal to noise ration SNR >> 1 and bandwidth B has capacity C1. If the SNR is doubled keeping constant, the resulting capacity C2 is given by
Answer : Option B
Explaination / Solution:
Q10. A discrete time linear shift - invariant system has an impulse response h[n] with h[0] = 1,h[1] =- 1,h[2] = 2, and zero otherwise The system is given an input sequence x[n] with x[0] = x[2] = 1, and zero otherwise. The number of nonzero samples in the output sequence y[n], and the value of y[2] are respectively
Answer : Option D
Explaination / Solution:
