Noise in Analog Communication System MCQ Quiz in తెలుగు - Objective Question with Answer for Noise in Analog Communication System - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

• Noise factor (F) and Noise Figure (NF) is a figure of merit used to indicate how much the signal to noise ratio deteriorates as a signal passes through a circuit or series of circuits.
• The noise figure is the difference in decibels (dB) between the noise output of the actual receiver to the noise output of an “ideal” receiver with the same overall gain and bandwidth when the receivers are connected to matched sources at the standard noise temperature T₀.
• Noise figure is a useful figure of merit for terrestrial systems, where the antenna effective temperature is usually near the standard 290 K.

The noise figure of an antenna is related to its noise temperature (T_e) as:

\(NF=1 + \left( {\frac{{{T_e}}}{{{T_0}}}} \right)\)

Concept

The effective voltage gain of the n-stage cascaded amplifier is given by:

\(A_v=A_1× A_2------A_n\)

The voltage gain in dB is given by:

A_v(dB) = 20 log(A_v)

Calculation

Given, A₁ = 10, A₂ = 100 and A₃ = 1000

\(A_v=10× 100× 1000\)

A_v = 10⁶

Av(dB) = 20 log( 10⁶)

A v(dB) = 6×20 log(10)

A v(dB) = 120 dB

NOISE:

• Noise is an undesirable product of the electronic devices and components and, more noise results from intentional economies of manufacture and design time.
• The measure of the noise performance of a circuit is the noise figure or noise factor. Noise figure is a comparison between the output signal to noise ratio and the thermal noise of the input signal.

Noise in communication systems is generally divided into two categories-

i) External noise like atmospheric noise, industrial noise solar noise, and cosmic noise.

ii) Internal noise like shot noise, thermal noise, low and high-frequency noise.

Internal and external noises are the main source of noise in the receiver. 

Thermal noise:

• It is also called Johnson-Nyquist noise
• It arises due to thermal agitation of electrons in a conductor material when the temperature is increased above absolute zero and it is also called thermal agitation noise
• It is distributed across the entire frequency spectrum range so it is also called white noise
• Thermal noise is generally observed in the passive component like R, L & C due to the random movement of charge carriers electrons.
• Random noise power is

Pn = KTB

Where,

K = Boltz man's constant

T = temperature

B = Bandwidth over which noise is measured

Concept:

Thermal noise voltage:

Due to thermal agitation (rise in temp), atoms in the electrical component will gain energy, moves in random motion, collides 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.

\(P = \frac{{V_n^2}}{{4R}}\)

Noise Power = KTB

\(KTB = \frac{{V_n^2}}{{4R}}\)

\({V_n} = \sqrt {4\;KTRB} \)

Calculation:

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

\(\overline {V_n^2} = 4\;KTe\;BR \)

\(= 4\;kTe\;B\;\left( {{R_1} + {R_2}} \right)\)

\(\overline {V_{{n_1}}^2} = 4\;K{T_1}B{R_1} \to \) 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

\(Te = \frac{{{R_1}{T_1} + {R_2}{T_2}}}{{{R_1} + {R_2}}}\)

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

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

Where,

I_dc = 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.

So option 3 is correct.

In thermal noise, the RMS value of thermal noise voltage is given by

\({V_{n,\;rms}} = \sqrt {4KTRB}\)

Where,

K = Boltzmann’s constant

T = ambient temperature of resister

R = resistance of the conductor

B = bandwidth over which voltage is measured.

Here,

V_n,rms α T, R, and B

Thermal noise:

• It is also called Johnson-Nyquist noise
• It arises due to thermal agitation of electrons in a conductor material when the temperature is increased above absolute zero and it is also called thermal agitation noise
• It is distributed across the entire frequency spectrum range so it is also called as white noise
• Thermal noise is generally observed in the passive component like R, L & C due to random movement of charge carriers electrons.
• Random noise power is

P_n = KTB

Where,

K = Boltz man's constant

T = temperature

B = Bandwidth over which noise is measured

Shot noise:

• Shot noise occurs by the random variation of charge carriers at the output of the electrode whenever it crosses the potential barrier.
• Major sources of shot noise are PN-junction diode, BJT, and JFETs.

So, based on the above points all options are true except option 2.

Hence the correct answer will be option 2.

The correct option is 4

Concept:

The overall noise figure of cascade devices is \({F_n} = {F_1} + \frac{{{F_2} - 1}}{{{G_1}}} + \frac{{{F_3} - 1}}{{{G_1}{G_2}}} + \ldots + \frac{{{F_n} - 1}}{{{G_1}{G_2} \ldots {G_n}}}\;\)

Here it is asked to find the overall Noise figure of a two-stage cascade amplifier. So here n=2

Gain of each stage  = 10 dB and noise figure F = 10 dB 

Thus, the overall noise figure:

⇒ \(\rm 10+\frac{10-1}{10}\) = 10.9 dB 

BPF:

As, 2 cos(16000πt) and 5 cos(20000πt) both are within the band pass range.

∴ Both of them will pass.

Output signal power: \(2 \left( \dfrac{2^2}{2} + \dfrac{5^2}{2} \right)\)

(the output 2 is due to the BFF gain)

= 2(2 + 12.5) = 29 W

Output noise PSD:

Area under PSD given total power,

output noise power:

2 × 0.05 × 4 × 10³

= 400 W

Output SNR = \(\dfrac{29}{400} = 0.0725\)

Explanation:

In any communication system, during the transmission of the signal, or while receiving the signal, some unwanted signal gets introduced into the communication, making it unpleasant for the receiver, questioning the quality of the communication. Such a disturbance is called Noise.

What is Noise?

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.

Noise in communication system generally divided into two categories-

i) External noise like atmospheric noise, industrial noise solar noise, and cosmic noise.

ii) Internal noise like shot noise, thermal noise, low and high-frequency noise.

Internal and external noises are the main source of noise in the receiver. 

Noise temperature is one way of expressing the level of available noise power introduced by a component or source. The power spectral density of the noise is expressed in terms of the temperature (in kelvins) that would produce that level of Johnson–Nyquist noise.

It is useful in dealing with UHF.

Note:

Thermal Agitation:

• It is also called Johnson-Nyquist noise
• It is distributed across the entire frequency spectrum range so it is also called white noise
• Thermal noise is generally observed in the passive component like R, L & C due to the random movement of charge carrier electrons.
• Random noise power or thermal noise power generated by a resistor is given by

Pn = KTB      -------    (1)

Where,

K = Boltz man's constant

T = temperature

B = Bandwidth over which noise is measured

From equation (1), we can observe that

The thermal noise power generated by a resistor is proportional to bandwidth (B), Boltzmann constant (K). and temperature (T).

But it is not dependent on-resistance of the resistor