--- title: "Analog communication block diagram; FDM of three signals" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:analog-block-fdm-three-signals" source: https://www.bctengineer.com/q/semester_6/communication_systems/analog-block-fdm-three-signals site: BCT Engineering --- # Analog communication block diagram; FDM of three signals Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2064 Shrawan (CS I), 4+4 marks - Video: [Frequency Division Multiplexing (FDM) Explained](https://www.youtube.com/watch?v=UwWmDwbsDBs) — 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/analog-block-fdm-three-signals - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question Discuss a block diagram of an analog communication system. Explain FDM system with neat diagrams, assuming three different message signals are to be transmitted simultaneously. ## Short answer An analog communication system comprises source, modulator, channel, demodulator, and destination. In FDM, three 4 kHz bandwidth-limited signals modulate separate carriers (e.g., 100, 105, 110 kHz) using SSB and are summed; bandpass filters and demodulators recover them at the receiver. ## Answer #### Block diagram of an analog communication system An analog communication system carries a continuously varying message from a source to a user over a channel. The message is first converted to an electrical signal and then modulated onto a carrier. ```text [Information source] | [Input transducer] | [Transmitter (modulator)] | [Channel] <---- noise, interference | [Receiver (demodulator)] | [Output transducer] --> user ``` - **Information source and input transducer:** produce the message, e.g. speech converted to voltage by a microphone. - **Transmitter:** amplifies, filters and **modulates** (AM/FM/PM) the message onto a carrier suited to the channel, then power-amplifies it. - **Channel:** a wire, coaxial cable, optical fibre or free space. It attenuates and distorts the signal and adds noise. - **Receiver:** selects the wanted signal, amplifies it, demodulates it and filters it to recover the message. - **Output transducer:** a loudspeaker or display that turns the electrical signal back into its original form. #### FDM system for three message signals Assume three messages $m_1(t), m_2(t), m_3(t)$, each band-limited to $W = 4$ kHz, and SSB modulation with carriers $f_{c1} = 100$ kHz, $f_{c2} = 105$ kHz and $f_{c3} = 110$ kHz (upper sideband kept, 1 kHz guard band). ```text TRANSMITTER m1 ->[LPF]->[SSB mod fc1]->[BPF1]--+ m2 ->[LPF]->[SSB mod fc2]->[BPF2]--+-->(+)--> channel m3 ->[LPF]->[SSB mod fc3]->[BPF3]--+ RECEIVER +->[BPF1]->[demod fc1]->[LPF]-> m1 channel -+->[BPF2]->[demod fc2]->[LPF]-> m2 +->[BPF3]->[demod fc3]->[LPF]-> m3 ``` Spectrum of the multiplexed signal: ```text | m1 |g| m2 |g| m3 | 100 104 105 109 110 114 kHz |<--------- 14 kHz ---------->| ``` Working: 1. LPFs band-limit each message to $W$. 2. Each message modulates its own carrier. BPFs keep one sideband, so each signal sits in its own band. 3. The adder combines them. The composite signal (bandwidth $3W$ + guard bands = 14 kHz here) is sent over one channel. 4. At the receiver, BPFs centred on each band separate the three signals. Coherent demodulators with $f_{c1}, f_{c2}, f_{c3}$ and LPFs recover $m_1, m_2, m_3$. Guard bands and good filters prevent crosstalk between the channels. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is the correct order of an analog communication system? A. Source, input transducer, transmitter, channel, receiver, output transducer B. Source, channel, transmitter, receiver C. Transducer, receiver, channel, transmitter D. Source, transmitter, transducer, channel Answer: A. The message is transduced, modulated, sent over a noisy channel, demodulated and converted back. (Key point: Block diagram order) 2. What is the transmitter chain for each message in the FDM system? A. LPF, SSB modulator with its own carrier, BPF, then summed B. Sampler, commutator, adder C. BPF, mixer with same carrier for all, adder D. LPF, quantiser, encoder Answer: A. Each message is band-limited, SSB-modulated on its own carrier, filtered and added. (Key point: Transmitter FDM chain) 3. With W = 4 kHz, carriers 100, 105, 110 kHz (USB), what range does the composite occupy? A. 100-114 kHz (14 kHz) B. 100-110 kHz C. 96-114 kHz D. 100-112 kHz Answer: A. Third message spans 110-114 kHz, so total 100-114 = 14 kHz, i.e. 3W plus guard bands. (Key point: Composite bandwidth) 4. How does the receiver recover each message? A. BPF centred on each band, coherent demodulator with the same carrier, LPF B. Single LPF C. Time-slot gating D. Envelope detection of the sum Answer: A. BPFs separate bands, coherent demodulation shifts to baseband and LPFs remove products. 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