--- title: "Communication system and AM block diagram" subject: "Basic Electronics Engineering" chapter: 5 key: "semester_2/basic_electronics_engineering:define-comm-am" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/define-comm-am site: BCT Engineering --- # Communication system and AM block diagram Basic Electronics Engineering · Chapter 5: Communication System - Asked in: 2070 Bhadra, 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_2/basic_electronics_engineering/define-comm-am - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-5 ## Question Define communication system. Explain amplitude modulation communication system with the help of necessary block diagrams. ## Short answer A communication system transfers info between source and destination via channel. Amplitude modulation varies carrier amplitude proportional to message; modulation index m≤1, bandwidth 2fm. Transmitter chain: mic, audio amp, modulator with carrier oscillator, power amp, antenna. Superheterodyne receiver converts to fixed IF then demodulates audio. ## Answer A **communication system** transfers information from a source to a destination through a channel using electrical signals. #### Amplitude modulation (AM) In AM, the amplitude of a high-frequency carrier wave is varied in proportion to the instantaneous amplitude of the low-frequency message (modulating) signal. The carrier frequency and phase stay constant. Carrier: vc = Vc sin ωc t. Message: vm = Vm sin ωm t. AM wave: v = Vc (1 + m sin ωm t) sin ωc t, where the modulation index m = Vm/Vc (0 < m ≤ 1). Bandwidth = 2 fm. The spectrum has the carrier fc and the sidebands fc − fm and fc + fm. #### Block diagram of an AM transmitter ```text +-----+ +---------+ +-----------+ +---------+ +---------+ | Mic |-->|Audio amp|-->| Modulator |-->|Power amp|-->| Antenna | +-----+ +---------+ +-----------+ +---------+ +---------+ ^ | +-----------+ +--------+ |Carrier osc|-->| RF amp | +-----------+ +--------+ ``` 1. The microphone converts sound to an electrical message signal, which is amplified by the audio amplifier. 2. The oscillator generates the high-frequency carrier, which is boosted by the RF amplifier. 3. The modulator combines the two to give the AM wave. 4. The power amplifier raises its power, and the antenna radiates it. #### AM receiver (superheterodyne) ```text Ant->[RF amp]->[Mixer]->[IF amp]->[Detector]->[Audio amp]->[Speaker] ^ [Local osc] ``` The RF stage selects the station, the mixer converts its frequency to a fixed IF (455 kHz), the IF amplifier gives gain and selectivity, the detector recovers the audio, and the audio amplifier drives the loudspeaker. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In AM, what is varied? A. Carrier amplitude, in proportion to the message amplitude B. Carrier frequency C. Carrier phase D. Message frequency Answer: A. Carrier frequency and phase stay constant. (Key point: AM definition) 2. What is the modulation index and its limit? A. m = Vc/Vm, with m ≥ 1 B. m = Vm × Vc C. m = Vm/Vc, with m ≤ 1 D. m = fm/fc, any value Answer: C. m > 1 causes overmodulation and distortion. (Key point: Modulation index) 3. What is the bandwidth of an AM wave? A. 2 fm B. fm C. fc D. 2 fc Answer: A. Sidebands at fc ± fm give a bandwidth of 2 fm. (Key point: Bandwidth) 4. What does the mixer do in the superheterodyne receiver? A. Detects the audio B. Amplifies audio C. Converts the station frequency to a fixed IF (455 kHz) D. Generates the carrier for transmission Answer: C. The IF amplifier then gives gain and selectivity at a fixed frequency. 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