Communications - Online Test
Q1. The amplitude of a random signal is uniformly distributed between -5 V and 5 V.If the signal to quantization noise ratio required in uniformly quantizing the signal is 43.5 dB, the step of the quantization is approximately
Answer : Option C
Explaination / Solution:
Q2. The maximum value of ᶿ until which the approximation sin ᶿ=ᶿ holds to within 10% error is
Answer : Option B
Explaination / Solution:
Here, as we know
but for 10% error, we can check option (B) first
so, the error is more than 10%. Hence, for error less than 10%, ᶿ = 18º can have the approximation
Q3.
The bit rate of a digital communication system is R kbits/s. The modulation used is 32-QAM. The minimum bandwidth required for ISI free transmission is
Answer : Option B
Explaination / Solution:
In ideal Nyquist Channel, bandwidth required for ISI (Inter Symbol reference) free transmission is
Here, the used modulation is 32 - QAM (Quantum Amplitude modulation
Q4.
A band-limited signal with a maximum frequency of 5 kHz is to be sampled. According to the sampling theorem, the sampling frequency which is not valid
Answer : Option A
Explaination / Solution:
Given, the maximum frequency of the band-limited signal
fm = 5 kHz
According to the Nyquist sampling theorem, the sampling frequency must be greater than the Nyquist frequency which is given as
So, the sampling frequency fs must satisfy
only the option (A) does’nt satisfy the condition therefore, 5K HZ is not a valid sampling frequency.
Q5. 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
Q6. 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
Q7. 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.
Q8. 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:
Q9. 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
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
