--- title: "Modulation; analog communication system block diagram" subject: "Communication Systems" chapter: 1 key: "semester_6/communication_systems:modulation-analog-block-diagram" source: https://www.bctengineer.com/q/semester_6/communication_systems/modulation-analog-block-diagram site: BCT Engineering --- # Modulation; analog communication system block diagram Communication Systems · Chapter 1: Introduction - Asked in: 2079 Chaitra (CS I), 4+6 marks - Video: [Introduction to Analog and Digital Communication | The Basic Block Diagram of Communication System](https://www.youtube.com/watch?v=qhjj6WG7Rgc) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/modulation-analog-block-diagram - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1 ## Question Define modulation. Draw the functional block diagram of analog communication system and briefly explain each block. ## Short answer Modulation varies carrier amplitude, frequency, or phase in proportion to the message. An analog system has a source, input transducer, transmitter (modulation, power amplifier), channel (adding noise), receiver (RF/IF amplification, demodulation), output transducer, and destination. ## Answer #### Modulation **Modulation** is the process by which some characteristic of a high-frequency carrier $c(t) = A_c\cos(2\pi f_c t + \phi)$, namely its **amplitude**, **frequency** or **phase**, is varied in accordance with the instantaneous amplitude of the message signal $m(t)$. The results are amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM) respectively. For example, in AM, $s(t) = A_c[1 + k_a m(t)]\cos 2\pi f_c t$. Modulation shifts the message spectrum from baseband to a band around $f_c$, which allows practical antenna sizes, multiplexing of many signals, and better noise performance. #### Functional block diagram of an analog communication system ```text +--------+ +----------+ +-------------+ | Source |-->| Input |-->| Transmitter | | | |transducer| | mod + amp | +--------+ +----------+ +------+------+ | Noise +-----v-----+ ------->| Channel | +-----+-----+ | +--------+ +----------+ +------v------+ | Desti- |<--| Output |<--| Receiver | | nation | |transducer| | amp + demod | +--------+ +----------+ +-------------+ ``` #### Blocks 1. **Information source:** generates the message, e.g. speech, music, picture. Its output is usually non-electrical. 2. **Input transducer:** converts the message into an electrical baseband signal $m(t)$, e.g. microphone, camera, temperature sensor. 3. **Transmitter:** processes $m(t)$ for efficient transmission. It band-limits and amplifies the message, **modulates** it on a carrier generated by an oscillator, then amplifies the modulated signal (power amplifier) and couples it to the antenna or cable. 4. **Channel:** the medium between transmitter and receiver: wire pair, coaxial cable, optical fibre or free space. It attenuates the signal and introduces distortion; **noise** and **interference** are added mainly here. 5. **Receiver:** selects the wanted signal from the antenna, amplifies it (RF amplifier), converts it to an intermediate frequency (mixer + local oscillator in a superheterodyne receiver), filters and amplifies it, and **demodulates** it to recover $m(t)$, followed by audio/video amplification. 6. **Output transducer:** converts the recovered electrical signal back into the original form, e.g. loudspeaker, display. 7. **Destination:** the listener, viewer or machine that uses the message. **Example:** FM radio: microphone, FM transmitter at 88–108 MHz, free space, FM receiver with discriminator, loudspeaker. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Varying which carrier parameter gives AM, FM and PM respectively? A. Frequency, amplitude, phase B. Phase, frequency, amplitude C. Amplitude, phase, frequency D. Amplitude, frequency, phase Answer: D. AM varies amplitude, FM frequency and PM phase of the carrier according to the message. (Key point: Modulation parameters) 2. In a superheterodyne receiver, what do the mixer and local oscillator do? A. Demodulate the message directly B. Convert the received signal to an intermediate frequency C. Generate the carrier for transmission D. Convert the message to digital form Answer: B. The mixer with the LO shifts the signal to a fixed IF for filtering and amplification before demodulation. (Key point: Superheterodyne receiver) 3. Which describes the transmitter's job? A. Band-limit and amplify the message, modulate it on a carrier, power amplify, and couple to the antenna B. Only convert sound to voltage C. Only select the wanted station D. Only reproduce sound Answer: A. The transmitter prepares the message for the channel by modulation and amplification. (Key point: Transmitter processing) ## Close variants set in other exams - [Analog communication block diagram; modulation and its reasons](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-comm-modulation-reasons) — Communication Systems, chapter 1, asked 3× ## Related in Communication Systems - [Block diagram of analog communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-comm-block-diagram) — Communication Systems, chapter 1, asked 2× - [Communication system block diagram; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acomm-block-diagram-needs-modulation) — Communication Systems, chapter 1 - [Digital vs analog communication; DCS block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-vs-analog-dcs-block) — Communication Systems, chapter 1 - [Reasons for modulation; digital communication block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-reasons-digital-adv-duplex) — Communication Systems, chapter 1 - [Analog communication block diagram; FDM of three signals](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-block-fdm-three-signals) — Communication Systems, chapter 5 - [Duplex digital communication system vs analog](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-comm-duplex-vs-analog) — Communication Systems, chapter 1 ## Related in other subjects - [Communication system and AM block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-am) — Basic Electronics Engineering, chapter 5 - [Block diagram of digital communication system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ablock-diagram-digital-comm) — Data Communication, chapter 1, asked 8× - [Communication system and block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acomm-system-block-diagram) — Basic Electronics Engineering, chapter 5, asked 7× - [Functional block diagram of excitation control](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-control-block-diagram) — Power Plant Equipment, chapter 3