Noise in Analog Communication System MCQ

11. Noise has more effect on Higher frequencies in ______

  1. Frequency Modulation
  2. Amplitude Modulation
  3. Pulse Modulation
  4. Discrete Modulation

Answer.1. Frequency Modulation

Explanation

Frequency Modulation is much more immune to noise than amplitude modulation and is significantly more immune than phase modulation. The effect of noise in FM does not remain constant but it increases with the increase in the frequency of mod s/g.

Figure of Merit (FOM): It is the ratio of output SNR to the input SNR. FOM depends only on the demodulator.

FOMFM = $\frac{{3{\beta ^2}}}{2}$

The figure of merit of FM is far better than the other two. So we can say the noise performance of FM is better than AM and PM.

 

12. Industrial noise exists in a frequency range of

  1. 10 – 700 MHz
  2. 1 – 800 MHz
  3. 1 – 600 MHz
  4. 20 – 50 MHz

Answer.3. 1 – 600 MHz

Explanation

Industrial noise:

  • Industrial noise is external noise.
  • It is generally produced by the industrial working station such that the ignition of the electric motor, starting with the turbine, noise due to the working of automobiles.
  • Industrial noise generally exists in frequency ranges like 1 – 600 MHz.

 

13. The noise figure of a device is 2. If input SNR is 37 dB, what would be output SNR?

  1. 18.5 dB
  2. 34 dB
  3. 40 dB
  4. 74 dB

Answer.2. 34 dB

Explanation

Noise figure (NF) and noise factor (F) are measures of degradation of the signal-to-noise ratio (SNR), caused by components in a signal chain.

(N.F)dB = [(SNR)i/p]dB – [(SNR)o/p]dB

Calculation:

Given:

NF = 2

(SNR)i/p = 37 dB,

N.F(dB) = 10log10(NF)

N.F(dB) = 3 dB

(N.F)dB = [(SNR)i/p]dB – [(SNR)o/p]dB

[(SNR)o/p]dB = 37 dB – 3 dB = 34 dB

 

14. If the channel bandwidth doubles, the S/N ratio becomes

  1. Double of the former S/N ratio
  2. Square root of the former S/N ratio
  3. Half of the former S/N ratio
  4. None of these

Answer.3. Half of the former S/N ratio

Explanation

The noise in the system is directly proportional to the bandwidth, i.e.

Noise ∝ Bandwidth

Signal to noise ratio is defined as:

SNR = S/N = Signal Power/Noise Power

1) As the bandwidth increases, the noise in the system will increase.

2) As the noise in the system increases, the S/N ratio will decrease.

∴  If Bandwidth is doubled then, S/N ratio will be halved.

 

15. Shot noise current is directly proportional to

  1. The bandwidth of the system
  2. Square of the bandwidth
  3. The square root of the bandwidth
  4. 1/(Square root of bandwidth)

Answer.3. The square root of the bandwidth

Explanation

In shot noise, the RMS value of the shot noise current

${I_{n,\;rms}} = \sqrt {2{I_{dc}}qB}$

Where,

Idc = dc current

q = charge of an electron

B = bandwidth over which current is measured.

This relation is given by Schottky.

Shot noise current is directly proportional to the square root of the bandwidth.

 

16. An unwanted signal that enters the transmitted signal which cannot be controlled is called as:

  1. Modulated signal
  2. Carrier Signal
  3. Modulating Signal
  4. Noise Signal

Answer.4. Noise Signal

Explanation

Noise is an unwanted signal which interferes with the original message signal and corrupts the parameters of the message signal. This alteration in the communication process leads to the message getting altered. It is most likely to be entered at the channel or the receiver.

Hence, it is understood that noise is some signal which has no pattern and no constant frequency or amplitude. It is quite random and unpredictable. Measures are usually taken to reduce it, though it can’t be completely eliminated.

The most common examples of noise are −

  • Hiss sound in radio receivers
  • Buzz sound amidst of telephone conversations
  • Flicker in television receivers, etc.

 

17. Voltage Controlled Oscillator is used for

  1. Decoding FM signal
  2. Generating FM Signal
  3. Decoding AM Signal
  4. Generating AM Signal

Answer.2. Generating FM Signal

Explanation

A voltage-controlled oscillator (VCO) is an electronic oscillator whose output frequency is proportional to its input voltage. An oscillator produces a periodic AC signal, and in VCOs, the oscillation frequency is determined by voltage. Depending on the input voltage VCO produces the different frequencies and it is used in the demodulation of Frequency Modulation.

 

18. Two resistors R1 and R2 (in ohms) at temperatures T1K and T2K respectively, are connected in series. Their equivalent noise temperature is

  1. T1 + T2
  2. R1T1 + R2T2
  3. (R1T1 + R2T2) / (R1R2)
  4. (R1T+ R2T2) / (R1 + R2)

Answer.4. (R1T1 + R2T2) / (R1 + R2)

Explanation

Thermal noise voltage:

Due to thermal agitation (rise in temp), atoms in the electrical component will gain energy, moves in random motion, collide with each other, and generate heat this heat produced is called thermal noise.

Thermal noise power (N) = KTB

Where,

K = Boltzman constant

T = temperature in °K

B = Bandwidth

If we consider a Noisy Resistor as an ideal Resistor R connected in series with a noise voltage source and connected to match the load.

P = V2n/4R

Noise Power = KTB

KTB = V2n/4R

${V_n} = \sqrt {4\:KTRB}$

Let equivalent voltage of series combination is $\overline {{V_{{n_1}}}}$

$\overline {V_n^2} = \overline {V_{{n_1}}^2} + \overline {V_{{n_2}}^2}$

Since noise voltage is AC that’s why we are adding its square value.

The equivalent temperature of the series combination is Te

$\begin{array}{l} \overline {V_n^2} = 4\:KTe\:BR\\ \\ = 4\:kTe\:B\:\left( {{R_1} + {R_2}} \right) \end{array}$

$\overline {V_{{n_1}}^2} = 4\:K{T_1}B{R_1} \to$ Voltage across R1

$\overline {{V_{{n_2}}}} = 4\:K{T_2}B{R_2} \to$ noise voltage Across R2

4 KTe B (R1 + R2) = 4KT1 BR1 + 4KT2 BR2

Te(R1 + R2) = T1 R1 + T2 R2

$Te = \frac{{{R_1}{T_1} + {R_2}{T_2}}}{{{R_1} + {R_2}}}$

 

19. The signal power and noise power are denoted by S and N respectively. If the signal power increase to 3S and the noise power reduces by half. The ratio of the old SNR to the new SNR is given by ______.

  1. 6
  2. 2/3
  3. 1.5
  4. 1/6

Answer.4. 1/6

Explanation

The initial signal to noise ratio is S/N —(1)

Now if the signal power increase to 3S and the noise power reduces by half then signal to noise ratio will become:

$\frac{{3S}}{{\frac{N}{2}}} = \frac{{6S}}{N}$ —- 1

Dividing equation (1) by (2) we get:

$\frac{{Old{\rm{\;}}SNR}}{{New{\rm{\;}}SNR}} = \frac{{\frac{S}{N}}}{{\frac{{6S}}{N}}} = \frac{1}{6}$

 

20. In amplitude modulation:

  1. Carrier frequency is changed
  2. Carrier amplitude is changed
  3. Three sidebands are produced
  4. Fidelity is improved

Answer.2. Carrier amplitude is changed

Explanation

A wave has 3 parameters Amplitude, Phase and Frequency. Thus there are 3 types of modulation techniques

Amplitude Modulation:

The amplitude of the carrier is varied according to the amplitude of the message signal.

Frequency Modulation:

The frequency of the carrier is varied according to the amplitude of the message signal.

Phase Modulation:

The Phase of the carrier is varied according to the amplitude of the message signal.

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