# Noise in Analog Communication System MCQ

1. White noise is that signal whose frequency spectrum

1. Extends over finite range
2. Has flat spectral density
3. Has special density varying as 1/f
4. Has limited number of frequency components

Explanation

White noise is a type of noise that is produced by combining sounds of all different frequencies together. Because white noise contains all frequencies, it is frequently used to mask other sounds. White noise is that signal whose frequency spectrum is uniform i.e. it has flat spectral density.

2. The RF amplifier A is having 3 dB NF & 5 dB of gain, amplifier B is having 3.5 dB NF & 15 dB. For overall noise figure of the cascaded system, the correct statement is:

1. A as first stage & B as second stage offers minimum over all NF
2. B as first stage & A as second stage minimum over all NF
3. Both the combination will offer same overall NF
4. Over all NF will be less than 3 dB

Answer.1. A as first stage & B as second stage offers minimum over all NF

Explanation

The overall Noise Figure is given by:

$F = {F_1}{\rm{\;}} + {\rm{\;}}\frac{{{F_2} – 1}}{{{G_1}}}{\rm{\;}} + {\rm{\;}}\frac{{{F_3} – 1}}{{{G_1}{G_2}}}$

Given:

For RF Amplifier A:

FA = 3db = 100.3 = 2

GA = 5db = 100.5 = 3.162

For RF Amplifier B:

FB = 3.5db = 100.35 = 2.24

GB = 15db = 101.5 = 31.6

When RF amplifier A is used as 1st stage:

$\begin{array}{l} F = {F_A}{\rm{\;}} + {\rm{\;}}\frac{{{F_B} – 1}}{{{G_A}}}\\ \\ F = 2{\rm{\;}} + {\rm{\;}}\frac{{1.24}}{{3.16}} = 2.04 \end{array}$

When RF amplifier B is used as 1st stage:

$\begin{array}{l} F = {F_B}{\rm{\;}} + {\rm{\;}}\frac{{{F_A} – 1}}{{{G_B}}}\\ \\ F = 2.24{\rm{\;}} + {\rm{\;}}\frac{{2 – 1}}{{31.6}} = 2.24 \end{array}$

We can conclude that when RF amplifier A is at the first stage and amplifier B is at the second stage, then minimum Noise Figure occurs.

3. A system has a receiver noise resistance of 50 Ohm. It is connected to an antenna with an input resistance of 50 Ohm. Calculate the noise figure of the system.

1. 1
2. 2
3. 50
4. 101

Explanation

Given Req = Rs = 50 Ω

The noise figure is given as:

$NF = 1 + \frac{{{R_{eq}}}}{{{R_s}}}$

Req = Equivalent input resistance of antenna

Rs = Noise resistance of the system

Noise figure will be:

1 + 50/50 = 1 + 1

Noise figure = 2.

4. For the 2N338 transistor, the manufacturer specifies P max = 100 mW at 25°C free air temperature and the maximum junction temperature, Tj max = 125°. Its thermal resistance is

1. 10° C/W
2. 100° C/W
3. 1000° C/W
4. 10,000° C/W

Explanation

Given:

Ta = 25°C

Rth(j-a) = ?

Ploss = 100 mW

Junction temperature is calculated by using the thermal resistance as follows.

ΔT [°C] = Rth(j-a) [°C/W] × Ploss [W]

ΔT: Junction temperature rise

ΔT = Tj – Ta

Rth(j-a) : Thermal resistance

Ploss: Power dissipated is a semiconductor device

T: Junction temperature

Ta : Ambient temperature

Tj  – Ta = Rth(j-a) × Ploss

Rth(j-a) = (Tj  – Ta) / Ploss

= [(125 – 25) / 100]

= 1000° C/W

5. The change in the value of an analog signal during the conversion from analog to digital produces _______.

1. Quantization error
2. Resolution error
3. Nyquist error
4. Sampling error

Explanation

• The difference between an input value and its quantized value (such as round-off error) is referred to as quantization error.
• It is the difference between the analog signal and the closest available digital value at each sampling instant from the A/D converter.
• It also introduces noise, called quantization noise, to the sample signal.

6. Gain and NF of a single-stage amplifier are 10 dB and 3 dB respectively. When two such amplifiers cascaded then gain and NF of the cascaded amplifier will be?

1. Gain : 20 dB, NF : 3.2 dB
2. Gain : 20 dB, NF : 6.0 dB
3. Gain : 13 dB, NF : 3.6 dB
4. Gain : 13 dB, NF : 6.7 dB

Answer.1. Gain : 20 dB, NF : 3.2 dB

Explanation

Overall Gain of cascaded amplifiers is given by:

G(db) = G1(db) + G2(db) + ….. —(2)

Calculation:

Given:

G1 = G2 = 10 db

G1 = G2 = 10log(x) = 10 db

log(x) = 1

x = 101 = 10

Therefore, G1 = G2 = 10

Similarly F1 = F2 = 3 db = 2

The overall noise figure is given by,

${\rm{F}} = {{\rm{F}}_1} + \frac{{{{\rm{F}}_2} – 1}}{{{{\rm{G}}_1}}} + \frac{{{{\rm{F}}_3} – 1}}{{{{\rm{G}}_1}{{\rm{G}}_2}}} \ldots$

Where  F1, F2, F3……  are noise figures and G1, G2, G3….. are system gains respectively.

$F = 2{\rm{\;}} + {\rm{\;}}\frac{{(2 – 1)}}{{10}}$

F = 2.1

F(db) = 10log(2.1) = 3.2 db

From equation (2) gain can be calculated:

G(db) = 10 db + 10 db = 20 db

7. ________ is used in protocols for detection of transmission errors.

1. Modifiers
3. Cyclic redundancy code
4. Special code

Explanation

Cyclic redundancy check:

• CRC is a different approach to detect if the received frame contains valid data. This technique involves a binary division of the data bits being sent.
• The divisor is generated using polynomials. The sender performs a division operation on the bits being sent and calculates the remainder.
• Before sending the actual bits, the sender adds the remainder at the end of the actual bits. Actual data bits plus the remainder is called a codeword.
• The sender transmits data bits as codewords. At the other end, the receiver performs division operations on codewords using the same CRC divisor.
• If the remainder contains all zeros the data bits are accepted, otherwise it is considered as there some data corruption occurred in transit.

8. Distortion factor is also known as:

1. noise factor
2. fundamental distortion
3. harmonic distortion
4. total harmonic distortion

Explanation

A degree of distortion of a waveform is typically expressed as the ratio of the effective value of harmonic components to the effective value of the fundamental component.

Total Harmonic distortion

It is defined as the ratio of the sum of the powers of all harmonics to the fundamental frequency.

• It is a measurement of the harmonic distortion present in a signal.
• It is also known as “Distortion factor’
• It is one way to gauge the power supply quality.

9. Which of the following Noise is called Johnson-Nyquist noise?

1. Shot Noise
2. Random Noise
3. Thermal Noise
4. None of the above

Explanation

• Thermal Noise 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.

10. With respect to multimedia, the term SNR stands for:

1. Signal with Noise Rate
2. Signal to Noise Rate
3. Signal and Noise Rate
4. Signal to Noise Ratio