# 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

Q.2. Thermal noise is independent of

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

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

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

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

Q.9. A narrow-band noise shows

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

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

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

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

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

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

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

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

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

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

• Shot noise
• Random noise
• Impulse noise
• 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