Communication Engineering - Online Test
Q1. A transmission line of characteristic impedance 50Ω is terminated by a 50 W load. When excited by a sinusoidal voltage source at 10 GHz, the phase difference between two points spaced 2 mm apart on the line is found to be π/4 radians. The phase velocity of the wave along the line is
Answer : Option C
Explaination / Solution:
We have d = 2 mm and f = 10 GHz
Q2. The modes in a rectangular waveguide are denoted by 
where
m n and are the eigen numbers along the larger and smaller dimensions of the
waveguide respectively. Which one of the following statements is TRUE?
where
m n and are the eigen numbers along the larger and smaller dimensions of the
waveguide respectively. Which one of the following statements is TRUE?
Answer : Option A
Explaination / Solution:
TM11 is the lowest order mode of all the TMmn modes
Q3. The impulse response h(t) of linear time - invariant continuous time system is given by h(t) = exp(- 2t)u(t), where u(t) denotes the unit step function.
The output of this system, to the sinusoidal input x(t) = 2 cos 2t for all time t , is
Answer : Option D
Explaination / Solution:
Q4. The Column-I lists the attributes and the Column-II lists the modulation systems. Match the attribute to the modulation system that best meets it
Column-I
P. Power efficient transmission of signals
Q. Most bandwidth efficient transmission of voice signals
R. Simplest receiver structure
S. Bandwidth efficient transmission of signals with Significant dc component
Column-II
1. Conventional AM
2. FM
3. VSB
4. SSB-SC
Answer : Option B
Explaination / Solution:
In FM the amplitude is constant and power is efficient transmitted. No variation in power.
There is most bandwidth efficient transmission in SSB- SC. because we transmit only one side band.
Simple Diode in Non linear region ( Square law ) is used in conventional AM that is simplest receiver structure.
In VSB dc. component exists.
Q5.
Two discrete time system with impulse response h1[n] = 𝛿[n - 1] and h2[n] = 𝛿[n - 2] are connected in cascade. The overall impulse response of the cascaded system is
Answer : Option C
Explaination / Solution:
Q6. The return loss of a device is found to be 20 dB. The voltage standing wave ratio (VSWR) and magnitude of reflection coefficient are respectively
Answer : Option A
Explaination / Solution:
Q7. 
Consider the pulse shape s(t) as shown. The impulse response h(t) of the filter matched to this pulse is
Answer : Option C
Explaination / Solution:
Impulse response of the matched filter is given by
Q8. A transmission line of characteristic impedance 50 W is terminated in a load
impedance ZL. The VSWR of the line is measured as 5 and the first of the
voltage maxima in the line is observed at a distance of λ/4 from the load. The
value of ZL is
Answer : Option A
Explaination / Solution:
Since voltage maxima is observed at a distance of λ/4 from the load and we know
that the separation between one maxima and minima equals to λ/4 so voltage
minima will be observed at the load, Therefore load can not be complex it must
be pure resistive.
also RL = R0/s (since voltage maxima is formed at the load)
RL = (50/5)Ω
Q9. A continuous time LTI system is described by
Assuming zero initial conditions, the response y(t) of the above system for the input 
is given by
is given by
Answer : Option B
Explaination / Solution:
System is described as
Taking laplace transform on both side of given equation
Transfer function of the system
By Partial fraction
Taking inverse laplace transform
Q10. x(t) is a stationary random process with auto-correlation function. 
This process is passed through the system shown below. The
power spectral density of the output process y(t) is
This process is passed through the system shown below. The
power spectral density of the output process y(t) is
Answer : Option A
Explaination / Solution:
The given circuit can be simplified as
Power spectral density of output is
