(Aktu Btech) Analog and Digital Communication Important Unit-1 Amplitude Modulation

With the help of Aktu’s Quantum Notes, discover the keys to B.Tech success. These brief yet thorough notes concentrate on crucial and commonly questioned Analogue and Digital Communication concerns. Become exam-ready today! Unit-1 Amplitude Modulation

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Important Questions For Analog and Digital Communication:
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Q1. With the help of block diagram explain the working of communication system.

Ans.

• 1. Communication is the process of transmitting information from one point to another through a series of processes.
• 2. A communication system consists of three basic components: a transmitter, channels, and a receiver.
• 3. There are two types of communication: broadcasting and point-to-point communication.
• 4. Fig. shows a simple block diagram of an electronic communication system.  

• 5. Different blocks of source are discussed below:
  • i. Input transducer: It turns the data to be transferred into an electrical message signal. Speech, text, video, and other forms of input are all possible. An example of an input transducer is a microphone.
  • ii. Transmitter: It is a device that makes input electrical information suitable for efficient transmission over a given channel. 
  • iii. Channel: Channels are the means by which information is transmitted from source to destination. It is worth noting that the term channel is frequently used to refer to the frequency band assigned to a specific service or transmission, such as a television channel. Noise can interfere with a signal at any point in the communication system, but it has the greatest impact when the signal is at its weakest. It signifies that the noise in the channel or at the receiver’s input is the most noticeable.
  • iv. Receiver: The receiver does the reverse process as the transmitter. To obtain original information, demodulation of the wave coming from the receiver via channel is performed. The receiver’s output can be routed to a loudspeaker or video display. 
  • v. Output transducer: It converts electrical input to a form of message required by user. For example, speech, image, video, text etc. The loud speaker is an example of output transducer.

Q2. What are the fundamental limitations in communication system ?

Ans. The fundamental limitations in communication system are:

• 1. Noise limitation: Baseband transmissions that are not modulated are largely influenced by noise in the channel due to their low frequency.
• 2. Bandwidth limitation: Due to low bandwidth it is not possible to have multiplexed signals from baseband signals. 
• 3. Equipment limitation: The original form of baseband signal is not received at destination due to strong transient noise of active equipment in communication system. 

Q3. What is modulation ? Why the modulation is needed ?

Ans. A. Modulation: 

• 1. Modulation is defined as the process of varying a signal’s carrier characteristic in response to the instantaneous value of the baseband or modulating signal.
• 2. The modulated signal is the signal produced by the modulation process.

B. Need of modulation before transmitting: 

1. Multiplexing: 

• i. Multiplexing is the process of sending several messages over a single channel. Because their baseband (spectrum) are similar, the signals will interfere if they are delivered without modulation.
• ii. However, utilizing multiplexing techniques, such as frequency division multiplexing and temporal division multiplexing, the signals can be sent without interfering.

2. Practicability of antenna: 

• i. When free space is used as the communication medium, the message signals are transmitted via an antenna at the transmitter.
• ii. For affective transmission the height of the antenna should be of the order of the wavelength to be transmitted. Therefore, if the wavelength of the signal is 5 kHz then height of the antenna should be

which is impracticable. 

• iii. Modulation, which provides frequency shifting, can be used to minimise the height of the antenna. For modulation, higher frequency signals with short wavelengths are used.
• iv. Therefore, if an audio frequency is translated to radio frequency of 1 MHz, then height of the antenna is

which is practically achieved. 

3. Narrowbanding:

• i. Even if we assume that the antenna’s height is practical, the audio signals sent range from 50 kHz to 10 kHz, with a wavelength ratio of 200.
• ii. So a single antenna cannot receive all the signal. Now if the audio signals are translated to radio range frequency of 1 MHz, then ratio becomes

which is approximately unity. So a single antenna can receive all the signals.

Q4. Explain ring modulator for the DSB-SC generation.

Ans.

• 1. The Chopper type balanced modulator (Ring modulator) is made up of four diodes coupled in a ring configuration; thus, the circuit is also known as a ring modulator.
• 2. The baseband signal m(t) can be chopped by a switching or chopper T circuit. One such chopper circuit is shown in Fig. 

• 3. The diodes are used as a switching device. The carrier signal is such that its amplitude

is the maximum frequency component present in the baseband signal m(t).

• 4. For a positive cycle of the carrier signal, all the four diodes are forward biased. 
• 5. The four conducting diodes provide a short circuit to the baseband signal m(t) and the output is zero for the period of positive half cycles. 
• 6. Conversely, for negative carrier cycles, all four diodes are reverse biassed and provide a channel for the signal m(t) to reach the output.
• 7. As a result, for a positive half cycle of the carrier, no output appears, however for a negative half cycle, m(t) appears at the output, resulting in a chopped version of m(t) at the chopper circuit’s output.
• 8. The parallel R-L-C network tuned to o works as a bandpass filter, and the DSB-SC signal is obtained at its output.
• 9. The circuit is known as a double-balanced modulator because it is balanced with regard to both the baseband signal and the carrier.

Q5. Draw the block diagram of an SSB-SC transmitter employing sideband suppression filter and explain. 

Ans. A. Filter Method of SSB (Single Sideband Generation): 

• 1. The filter method of SSB creation is the most straightforward. A filter removes the undesirable sideband after the balanced modulator.
• 2. Depending on the carrier frequency and other requirements, the filter may be LC, crystal, ceramic, or mechanical.
• 3. Fig. depicts a block diagram of the Filter technique of SSB (Single Sideband) production.
• 4. The balanced modulator and the sideband-suppression filter are the transmitter’s main circuitry.
• 5. Although two suppression filters are depicted in the block diagram, only one is employed at a time depending on which sideband is to be suppressed (LSB or USB).

B. The main characteristics of suppression filter are listed below: 

• 1. Suppression filter must have a flat bandpass and extremely high attenuation outside the bandpass. 
• 2. The quality factor must be sufficiently high. 
• 3. It should be highly frequency selective. 
• 4. The bandwidth of the filter must not be greater than the maximum frequency component present in the baseband signal.

Q6. Define multiplexing. Explain with a block diagram the process of FDM.

Ans. A. Multiplexing: It is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.  

B. FDM: 

• 1. When transmitting several signals over the same channel, the signals must be separated so that they do not interfere with one another and may thus be separated at the receiving end.
• 2. This is achieved by splitting the signals based on frequency or me. The process of splitting signals based on frequency is known as frequency division multiplexing (FDM).
• 3. Fig. depicts a block diagram of an FDM system. Although the incoming message signals are considered to be low-pass, their spectra may not always exhibit non-zero values all the way down to zero frequency.
• 4. We have demonstrated a low pass filter after each signal input, which is aimed to remove high frequency components that do not contribute significantly to signal representation but can disrupt other message signals that use the common channel.
• 5. These low pass filters may be eliminated if the input signals are initially suitably band restricted.
• 6. The filtered signals are fed into modulators, which adjust the signal’s frequency ranges so that they occupy mutually exclusive frequency intervals.
• 7. Carrier frequencies are received from a carrier supply to execute these frequency transformations.
• 8. Single sideband modulation is the most extensively utilised modulation method in frequency-division multiplexing.

• 9. After the modulators, band-pass filters are utilized to limit the band of each modulated wave to its defined range.
• 10. The outputs of the band pass filters are then merged in parallel to provide the input to the common channel.
• 11. At the receiving terminal, a bank of band-pass filters with parallel inputs is employed to separate the message signals based on frequency occupancy. Individual demodulators then recover the original message signals.

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