MCQ on Noise in Analog Communication (Part-1)

MCQ on Noise in Analog Communication

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Multiple-Choice Questions

Q.1. An amplifier having noise figure of 20 dB and available power gain of 15 dB is followed by a mixer circuit having noise figure of 9 dB. The overall noise figure as referred to input in dB is:

  • 11.07
  • 10.44
  • 21.52
  • 0.63

Answer: 21.52

Q.2. Thermal noise is independent of

  • bandwidth
  • temperature
  • centre frequency
  • Boltzmann’s constant

Answer: centre frequency

Q.3. Transistor T1 operates at 20 kHz and T2 operates at 200 Hz. The flicker noise is

  • more in T1
  • more in T2
  • equal in both
  • depends on bias

Answer: more in T2

Q.4. A triode has transconductance equal to 25 µυ. The equivalent noise resistance is

  • 1 M ohms
  • 1 k ohms
  • 10 k ohms
  • 100 k ohms

Answer: 100 k ohms

Q.5. Johnson noise is

  • always white
  • white for all practical frequencies
  • never white
  • depends on temperature

Answer: white for all practical frequencies

Q.6. The spectrum of the white noise and an impulse function is similar in following respects

  • both have similar magnitude spectrum
  • both have similar phase spectrum
  • both have similar magnitude and phase spectrum
  • they have nothing similar

Answer: both have similar magnitude spectrum

Q.7. Parallel combination of a resistance R and a capacitance C develops a noise voltage source at its common terminal. The rms value of the voltage varies

  • proportional to R
  • inversely proportional to C
  • inversely proportional to square root of C
  • proportional to RC

Answer: inversely proportional to square root of C

Q.8. A noise voltage source has a resistance of 10 ohms. Its power density spectrum is 0.24 x 10-5. The corresponding available power density is

  • 2.6 x 10-5
  • 0.0.25
  • 26 x 10-5
  • 6 x 10-8

Answer: 6 x 10-8

Q.9. A narrow-band noise shows

  • amplitude modulation only
  • frequency modulation only
  • both AM and FM
  • none

Answer: both AM and FM

Q.10. A narrow-band noise source n(t) has symmetrical spectrum and has power density spectrum 0.2 x 10-6. The power density of quadrature component is

  • 0.2 x 10-6
  • 0.1 x 10-6
  • 0.4 x 10-6
  • 0.05 x 10-6

Answer: 0.1 x 10-6

Q.11. In a certain system, the signal power is 13 dBm and noise power is -1 dBm. The SNR will be

  • 14 dB
  • -13 dB
  • 12 dBm
  • 12 dB

Answer: 14 dB

Q.12. If the resistance value is doubled and temperature maintained constant, the available thermal noise power per unit bandwidth will

  • increase two-fold
  • increase four-fold
  • remain unchanged
  • decrease to half

Answer: increase two-fold

Q.13. The co-variance function of a band limited white noise is

  • A Dirac delta function
  • An exponentially decreasing function
  • A sine function
  • Sine2 Function

Answer: A Dirac delta function

Q.14. Noise with uniform power spectral density of N0 W/Hz is passed through a filter H(ω) =  2Exp (- jωtd) followed by an ideal low pass filter of bandwidth B Hz. The output noise power in watts is

  • 2 N0B
  • 4 N0B
  • 8 N0B
  • 16 N0B

Answer: 4 N0B

Q.15. Noise with double-sided power spectral density of K over all frequencies is passed through an RC low pass filter with 3 dB cut-off frequency of fc. The noise power at the filter output is

  • K
  • Kfc
  • fc

Answer: fc

Q.16. A white noise process x(t) with two-sided power spectral density of 1 x 10-10 W/Hz is input to a filter whose magnitude squared response is shown below.

Fig. MCQ 4.16

The power of the output process Y(t) is given by

  • 5 x 10-7 W
  • 1 x 10-6 W
  • 2 x 10-6 W
  • 1 x 10-5 W

Answer: 1 x 10-6 W

Q.17. A system has a receiver noise resistance of 50 Ω. It is connected to an antenna with an input resistance of 50 Ω. The noise figure of the system is

  • 1
  • 2
  • 50
  • 101

Answer: 1

Q.18. Which one of the following types of noise gains importance at high frequency?

  • Shot noise
  • Random noise
  • Impulse noise
  • Transit-time noise

Answer: Transit-time noise

Q.19. Assertion (A): The noise generated by a resistor depends upon its operating temperature. Reason (R): Average noise power generated in a resistor is given by Pn = kT Δf, where k = Boltzmann’s constant T = temperature in degree Kelvin, Δf = bandwidth of interest

  • Both A and R are true and R is the correct explanation of A.
  • Both A and R are true but R is NOT the correct explanation of A.
  • A is true but R is false.
  • A is false but R is true.

Answer: A is true but R is false.

Q.20. Thermal noise is passed through an ideal low-pass filter having cut-off at fc = ω Hz. The autocorrelation value of the noise at the output of the filter is given as

  • a delta function at t = 0
  • Gaussian over the range -∞ ≤ t ≤ ∞.
  • Sinc function over the range -∞ ≤ t ≤ ∞.
  • triangular function over the range -1/2 ωt ≤ t ≤ 1/2 ωt

Answer: Sinc function over the range -∞ ≤ t ≤ ∞.

Q.21. Which one of the following is the correct statement? If the value of a resistor creating thermal noise is doubled, the noise power generated is

  • halved
  • doubled
  • unchanged
  • slightly changed

Answer: doubled

MCQ on Noise in Analog Communication (Part-2)

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