Analog Communication Test Questions - Set 9

ANSWER: Band stop filter

Explanation:
A notch filter allows to pass almost whole range of frequencies through it, except a specific range. The notch filter attenuates a narrow set of frequencies or the stop band to pass through. Electrical equivalent of the stop filter is given by

ANSWER: In the channel

Explanation:
Noise is an unwanted electrical signal that is added with the transmitted signal while passing through the communication channel. The noise interferes with the signal and may produce distortions to the signal.

ANSWER: Unaffected by the value of the resistor

Explanation:
The Noise power is given by
P_n = KTB
Where
K = 1.381 × 10^-23 J/K, joules per Kelvin, the Boltzmann constant
B is the bandwidth at which the power P_n is delivered.
T noise temperature,
which is unaffected by change in the value of the resistor.

ANSWER: Flicker noise

Explanation:
With decrease in the frequency, the power spectral density of the flicker noise increases. It is generated due to fluctuations in the density of the carrier that increases the conductivity of the material. Therefore, there is a voltage drop during the flow of current. The flicker noise is considerable at low frequencies (below a few KHz).

ANSWER: All of the above

Explanation:
Hilbert transform may be obtained by first taking the Fourier Transform of the signal g(t), multiplying it by jsgn(f), then taking the inverse fourier transform and therefore obtaining g(t). jsgn(f) is j for the positive frequency f, and therefore the Hilbert Transform shifts the signal by -900 for a linear system whose input is g(t) and output obtained is g(t). Hilbert transform may be used in generation of SSB signals, representation of band pass signals, and designing of minimum phase type filters.

ANSWER: 4.071 * 10^-6 V, 5.757 * 10^-6 V

Explanation:
Noise voltage V_n = √(4R KTB)
Where, K = 1.381×10^-23 J/K, joules per Kelvin, the Boltzmann constant
B is the bandwidth at which the power P_n is delivered.
T noise temperature
R is the resistance
Noise voltage by individual resistors
V_n1 = √(4R1 KTB)
= √(4 * 10 * 10³ * 1.381 * 10^-23 * 3000 * 10 * 10³)
= √16.572 * 10^-12
= 4.071 * 10^-6 V
V_n2 = √(4R₂ KTB)
= √(4 * 20 * 10³ * 1.381 * 10^-23 * 3000 * 10 * 10³)
= √33.144 * 10^-12
= 5.757 * 10^-6 V

ANSWER: 284.48 * 10^-7

Explanation:
Noise voltage V_n = √(4R KTB)
Where, K = 1.381 × 10^-23 J/K, joules per Kelvin, the Boltzmann constant
B is the bandwidth at which the power P_n is delivered.
T noise temperature
R is the resistance
Noise voltage by resistors when connected in series is
V_n = √{4(R₁ + R₂) KTB}
= √{4(20 * 10³ + 30 * 10³) * 1.381 × 10^-23 * 293 * 10 * 10³ }
= 284.48 * 10^-7

ANSWER: 11.15 * 10^-7

Explanation:
Noise voltage V_n = √(4R KTB)
Where, K = 1.381 × 10^-23 J/K, joules per Kelvin, the Boltzmann constant
B is the bandwidth at which the power P_n is delivered.
T noise temperature
R is the resistance
Noise voltage by resistors when connected in parallel is
V_n = √{4R KTB}
Here for resistors to be in parallel,
1/R = 1/R₁ + 1/R₂
= 1/10K + 1/30K
= 0.1333
R = 7.502KΩ
V_n = √{4 * 7.502 * 10³ * 1.381×10^-23 * 300 * 10 * 10³}
= √124.323 * 10^-14
= 11.15 * 10^-7

ANSWER: All of the above

Explanation:
A periodic function repeats its values in regular intervals or periods. A periodic signal g(t) is a function of time that satisfies the equation
g(t)= g(t + T₀) for all the values of t.
t is time
T₀ is a constant that denotes the complete cycle of g(t).
The signals that do not satisfy the equation are known as non periodic or aperiodic signals.

ANSWER: Periodic signal

Explanation:
A periodic function repeats its values in regular intervals or periods. The signal sine wave is a periodic function. Its value may be determined at any point of time as it repeats itself at regular intervals.

ANSWER: Both a and b are correct

Explanation:
Properties of Hilbert transform are:
1. The signal and its Hilbert transform have same energy density spectrum
2. The signal and its Hilbert transform are mutually diagonal
3. The signal and its Hilbert transform have same auto correlation function.

ANSWER: f(x) = f(-x)

Explanation:
If f(x) is a real-valued function of a real variable then f is even if the following equation holds for all the values of x and x.
f(x) = f(-x)
Or
f(x) - f(-x) = 0

ANSWER: Occurrence is random

Explanation:
Random signals have their occurrence random in nature. These signals cannot be specified in time. The signals are irregular in nature, that is, they do not repeat with time. Thermal noise is a type of random signal.

ANSWER: All of the above

Explanation:
Unit step function is a type of function that exists only for positive side and is zero for negative side. The unit step function is discontinuous at time t=0.
The unit step function u(t) is defined as
u(t) = 0 for t<0
= 1 for t>0

ANSWER: All of the above

Explanation:
In Unit impulse function
1. The width of the pulse is zero.
2. Area of pulse curve is always unity.
3. Height of pulse goes to infinity.
4. The height of the impulse is same as the area of the impulse.

ANSWER: Starts at time t=0 and linearly increases with t

Explanation:


A Unit ramp function is a continuous time function that starts at time t=0 and linearly increases with t. Denoted by r (t), the unit ramp function is represented by
r(t) = 0 for t<0
= t for t>0

ANSWER: Johnson noise

Explanation:
Thermal noise is also known as Johnson noise or White noise. It is the random noise generated in resistive components due to rapid and random motion of atoms or electrons. Johnson noise is named after the scientist J.B. Johnson. Thermal noise contains all frequency components in equal amount. The white noise has infinite average power and is not physically attainable but it has suitable mathematical properties so it is used for system analysis in most of the systems.

ANSWER: All of the above

Explanation:
It is defined as the value of input signal to noise ratio below which the output signal to noise ratio decreases much rapidly than the input signal to noise ratio. It is the property of envelope detectors used for the demodulation of modulated signals. It occurs due to presence of large noise and therefore causes loss in the message signal. When the noise is very large as compared to the input at envelope detector, the message signal at the output is mixed with noise.

ANSWER: V_n is Directly proportional to √k

Explanation:
Noise voltage V_n is given by
√(4R KTB)

Where, K = 1.381 × 10^-23 J/K, joules per Kelvin, the Boltzmann constant

B is the bandwidth at which the power P_n is delivered.

T noise temperature

R is the resistance

If K is the Boltzmann's constant, the rms value of thermal noise voltage is proportional to √k.

ANSWER: f_s ≥ 2f_m

Explanation:
If the signal of frequency f_m is sampled at the rate f_s ≥ 2f_m only then the spectrum of the sampled signal is obtained without overlapping. The spectrum obtained is repetitive in nature and is completely without overlapping.