(Aktu Btech) Analog and Digital Communication Important Unit-2 Angle Modulation

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Q1. Show the generation of narrow band FM using phase modulator.

Ans.

• 1. PM and FM are closely related in sense that the net effect of both is variation in total phase angle. 
• 2. In PM, phase angle varies linearly with f(t) whereas in FM phase angle varies linearly with the integral of f(t). 
• 3. We can get FM by using PM provided that at first the modulating signal is integrated and then applied to phase modulator. 

• 4. Similarly we can generate a PM wave using frequency modulator provided that f(t) is first differentiated and then applied to frequency modulator.

Q2. Discuss the transmission bandwidth of FM signals and explain Carson’s rule.

Ans. A. Bandwidth of FM signals : 

1. The bandwidth of FM signal is given by,  

Here, n = number of significant sidebands produced.

Thus, the bandwidth of FM system is given as twice the frequency deviation.  

B. Carson’s rule : 

1. An empirical formula for the bandwidth of a single-tone wideband FM is given by Carson’s rule. 

2. According to this rule, the FM bandwidth is given by 

where, m_f = FM modulation index

Special case: 

Q3. Write a short note on performance comparison of frequency modulation and amplitude modulation.

Ans. 

Q4. Explain the detection of phase modulation.

Ans. These are two methods of detection in phase modulated system: 

• 1. Fixed reference detection. 
• 2. Differential detection.

A. Fixed reference detection: 

• 1. The source phase is given a fixed reference in this detection approach. To achieve optimal efficiency, this carrier information should be transmitted with the least amount of power.
• 2. There are several ways to calculate the reference phase from the carrier frequency.
• 3. A secondary tone (in a very small band outside the data band) that is harmonically connected to the carrier frequency and contains information about the latter’s phase may be delivered.
• 4. Because of its distinct frequency, a delay distortion may modify the phase of this tone by a different amount to the carrier. This must be made up for.
• 5. A phase reference may also be sent in bursts at regular intervals during transmission.
• 6. Data is frequently divided into words or records and sent and examined independently. Each of them may be preceded by a carrier frequency burst.

B. Differential detection: 

• 1. It makes no attempt at the receiver to produce a fixed reference phase.
• 2. The data are coded using phase changes.
• 3. To do the detection, the received signal is delayed one symbol interval and compared to the signal that is currently being received.
• 4. The phase change between the symbol intervals is indicated by this comparison.
• 5. The detected phase change is then translated into bits, dibits, and so on.

Q5. What are the advantages of frequency modulation ? The carrier frequency of an FM broadcast transmitter is 100 MHz and maximum frequency deviation is 75 kHz. If the highest audio frequency modulating the carrier is 15 kHz, what is the approximate bandwidth of the signal ?

Ans. A. Advantages of frequency modulation: 

• 1. Frequency modulation is less susceptible to noise. 
• 2. Signal to noise ratio doubles on doubling the bandwidth. 
• 3. Bandwidth required is more, so the system performance would increase. 

B. Numerical: 

Q6. Discuss external noise in detail. 

Ans. The noise that is originated outside the receiver is known as external noise. 

The noise includes:

A. Atmospheric noise: 

• 1. Atmospheric noise is made up of spurious radio signals with components spread across a wide frequency range.
• 2. It propagates over the earth in the same manner that ordinary radio waves of the same frequencies do, so that static from all thunderstorms, local and distant, can be received at any location on the ground.
• 3. If the storm is local, the static will be more severe but less frequent.
• 4. Since field strength is inversely related to frequency, this noise will interfere with radio reception more than television reception.
• 5. At frequencies over 30 MHz, atmospheric noise becomes less severe.

B. Extraterrestrial noise:

• 1. Extraterrestrial noise, often known as space noise, can be detected at frequencies ranging from roughly 8 MHz to slightly above 1.43 GHz. 
• 2. This noise can be classified under two categories:
• a. Solar noise: 
  • 1. The sun radiates throughout a wide frequency spectrum, including the frequencies we use for communication.
  • 2. The sun is a constantly changing star that passes through cycles of peak activity, resulting in electrical disturbances such as corona flares and sunspots.
• b. Cosmic noise: 
  • 1. Distant stars generate RP noise similarly to our sun since they are also suns and have extremely high temperatures.
  • 2. Due to their distance from the earth, a number of disturbances build up and interfere with the communications system.
• c. Industrial noise:
  • 1. The intensity of noise produced by humans between 1 and 600 GHz easily surpasses that of all other sources.
  • 2. This topic covers a variety of sources, including leakage from high-voltage lines, electrical machinery, electric motors, and switching equipment.

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