Quiz Electronics Engineering 21 July 2020

Quiz Electronics Engineering
Exam: NIC
Topic: Miscellaneous
Date: 21/07/2020

Each Question carries 1 Mark
Negative Marking: 1/4
Time: 10 Minutes

Q1. Which of the following is used as a data selector?
(a) Encoder
(b) Decoder
(c) Multiplexer
(d) De-multiplexer

Q2. An EMF is induced in the coil whenever flux passed through it
(a) is decreased
(b) is increased
(c) is abruptly reduced to zero
(d) all of the above

Q3. A signal has frequency components from 300 Hz to 1.8 KHz. The minimum possible rate at which the signal has to be sampled is
(a) 3600 samples/sec
(b) 1800 samples/sec
(c) 600 samples/sec
(d) 1500 samples/sec

Q4. The impulse response h[n] of a linear time-invariant system is given by h[n + 3] + u[ n – 2] – 2u[n – 7] where u[n] is the unit step sequence. The above system is
(a) stable but not causal
(b) stable and causal
(c) causal but unstable
(d) unstable and not causal

Q5. Complement of the expression A’B + CD’ is
(a) (A’ + B)(C’ + D)
(b) (A + B’)(C’ + D)
(c) (A’ + B)(C’ + D)
(d) (A + B’)(C + D’)

Q6. The power formula for three phase circuit is
(a) 3VL IL cosϕ
(b) √3 VP IP cosϕ
(c) 3 V_P I_P
(d) √3 V_L I_L

Q7. The equation ∇.J ⃗ = 0, is known as
(a) Poisson’s equation
(b) Laplace equation
(c) Continuity equation
(d) Maxwell equation

Q8. Holding current for an SCR is best described as:
(a) The minimum current required for turn off
(b) The current required before an SCR will turn on
(c) The amount of current required to maintain conduction
(d) The gate current required to maintain conduction

Q9. Convolution of x(t+10) with impulse function δ(t-2) is equal to
(a) x(t+8)
(b) x(t-2)
(c) x(t+12)
(d) x(t+2)

Q10. Flat top sampling of low pass signals
(a) gives rise to aperture effect
(b) implies oversampling
(c) leads to aliasing
(d) introducing delay distortion

SOLUTIONS

S1. Ans.(c)
Sol. Multiplexer has many inputs and one output. We select any one input at a time. So, it is also called Data Selector.

S2. Ans.(d)

S3. Ans.(a)
Sol. Given:
fH = 1800 Hz
fL = 300 Hz
We know that,
NR = (2 f_H)/K where, K = f_(H )/(f_(H- ) f_L )
Bandwidth = fH – fL = 1800 – 300 = 1500 Hz
Now, K = f_(H )/(f_(H- ) f_L ) = 1800/1500 = 1.2 i.e., Integer value of K = 1
So, NR = (2 f_H)/K = (2 x 1800)/1 = 3600 samples/sec

S4. Ans.(a)
Sol. The system is said to be Causal if the output depends upon present and past input and The system is said to be stable only when the output is bounded for bounded input.
Here, System depends upon future input so it is not Causal and input is Unit step which is bounded so output will also be bounded because output is combination of unit step function. So, It is Stable.

Therefore, System is Stable but not Causal.

S5. Ans.(b)
Sol. Applying De Morgan’s Theorem, we will solve this expression.
(A’B + CD’)’ = (A’B)'(CD’)’
= [(A’)’ + (B)’].[(C)’ + (D’)’]
= (A + B’)(C’ + D).

S6. Ans.(b)
Sol. For three phase circuit,
Power =√3 V_P I_P cosϕ
where,
V_P = phase voltage
I_P = phase current
cosϕ = power factor

S7. Ans.(c)
Sol. Continuity equation is
∇.J ⃗ = -(dρ_v)/dt
For steady current or lossless region, (dρ_v)/dt=0
So, ∇.J ⃗ = 0

S8. Ans.(c)
Sol. Holding current of SCR or thyristor is the minimum value of current which must flow through circuit so that SCR can remain in ‘ON’ state and conducts.

S9. Ans.(a)
Sol. According to convolution property,
x(t+a)* δ(t+b) = x(t+a+b)
Here,
x(t+10)* δ(t-2) = x(t+10-2)
= x(t+8)

S10. Ans.(a)
Sol. Flat top sampling of low pass signal gives the spectrum of sinc function with attenuated amplitude of high frequency components. This affect is called Aperture effect. An aperture refers to finite time width window(aperture) through which signal passes. This can be reduced by either increasing the sampling frequency or reducing sampling aperture width.