---
title: "Working principle of FDM"
subject: "Telecommunications"
chapter: 3
key: "semester_8/telecommunications:multiplexing-fdm-principle"
source: https://www.bctengineer.com/q/semester_8/telecommunications/multiplexing-fdm-principle
site: BCT Engineering
---

# Working principle of FDM

Telecommunications · Chapter 3: Signal Multiplexing

- Asked in: 2076 Chaitra, 7 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_8/telecommunications/multiplexing-fdm-principle
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3

## Question

Define multiplexing and explain the working principle of FDM system with a neat block diagram.

## Short answer

Multiplexing sends several independent signals over one shared medium. In FDM, total bandwidth is divided into non-overlapping bands; each signal modulates a different carrier, band-pass filtered, summed and transmitted. Receiver uses BPFs, demodulators and LPFs to recover each channel.

## Answer

**Multiplexing** is the technique of sending several independent signals simultaneously over one common transmission medium, so that the medium's capacity is shared and the cost per channel is reduced.

#### Principle of FDM
In **Frequency Division Multiplexing**, the total bandwidth of the medium is divided into non-overlapping frequency bands. Each message signal is shifted by modulation to its own band; all the shifted signals are added and sent at the same time. At the receiver, filters separate the bands and demodulators bring each one back to baseband.

#### Block diagram
```text
 TRANSMITTER
 ch1 -[LPF]-[Mod f1]-[BPF]-+
 ch2 -[LPF]-[Mod f2]-[BPF]-+--(SUM)--> line
 chN -[LPF]-[Mod fN]-[BPF]-+

 RECEIVER
       +-[BPF f1]-[Demod f1]-[LPF]-> ch1
 line -+-[BPF f2]-[Demod f2]-[LPF]-> ch2
       +-[BPF fN]-[Demod fN]-[LPF]-> chN
```

#### Working
1. **Band limiting:** each input (e.g. voice) passes through an LPF to limit it to 0.3–3.4 kHz.
2. **Modulation:** each signal modulates a different sub-carrier $f_1, f_2, \ldots, f_N$. Telephone FDM uses **SSB-SC** so each voice channel needs only 4 kHz (3.1 kHz speech + guard band).
3. **Band-pass filtering:** removes the unwanted sideband and carrier.
4. **Combining:** a summing network adds all channels into a composite baseband signal, which may then modulate a final radio carrier.
5. **Receiver:** BPFs centred on each band pick out each channel; product demodulators with locally generated carriers (same frequency and phase) recover the baseband, and LPFs remove the remaining high-frequency components.
```text
 amplitude
   |  [ch1]  [ch2]  [ch3]  [ch4]
   |  |   |g |   |g |   |g |   |
   +--+---+--+---+--+---+--+---+--> f
      f1     f2     f3     f4
 g = guard band
```

**Guard bands** between channels allow for practical filter roll-off and reduce crosstalk.

#### Example – FDM telephone hierarchy (CCITT)
| Level | Made from | Channels | Band |
|---|---|---|---|
| Basic group | 12 voice channels | 12 | 60–108 kHz |
| Supergroup | 5 groups | 60 | 312–552 kHz |
| Mastergroup | 5 supergroups | 300 | 812–2044 kHz |
| Supermastergroup | 3 mastergroups | 900 | 8516–12388 kHz |

Other uses: AM/FM radio and TV broadcasting, cable TV, first-generation (AMPS) mobile.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the principle of FDM?
   A. Signals take turns using the whole bandwidth
   B. Each signal is shifted by modulation into its own non-overlapping frequency band and all are sent simultaneously
   C. Each signal uses its own wavelength of light
   D. Signals are digitised and interleaved
   Answer: B. The medium bandwidth is divided into bands, one per signal, transmitted at the same time. (Key point: FDM principle)

2. Why does a telephone FDM channel need 4 kHz?
   A. 3.1 kHz speech plus guard band, using SSB-SC
   B. Double sideband needs 8 kHz
   C. Because the sampling rate is 4 kHz
   D. Because the carrier occupies 4 kHz
   Answer: A. SSB-SC removes one sideband and carrier, so 0.3-3.4 kHz speech plus guard fits in 4 kHz. (Key point: Voice channel spacing)

3. What is needed in the receiver to recover each channel?
   A. A commutator synchronised to the transmitter
   B. BPF per channel, then a product demodulator with a local carrier of the same frequency and phase, then LPF
   C. Only a low-pass filter
   D. A prism and grating
   Answer: B. BPF selects the band, coherent demodulation returns baseband, LPF removes high-frequency components. (Key point: Receiver recovery)

4. Which FDM hierarchy is correct?
   A. Group 12 channels, supergroup 60 channels (5 groups), mastergroup 300 channels (5 supergroups)
   B. Group 24, supergroup 120
   C. Group 12, supergroup 36
   D. Group 60, supergroup 12
   Answer: A. Basic group 12 channels (60-108 kHz), supergroup 5 groups = 60, mastergroup 5 supergroups = 300, supermastergroup 3 mastergroups = 900. (Key point: CCITT hierarchy)

## Related in Telecommunications

- [FDM and TDM with block diagrams](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-block) — Telecommunications, chapter 3
- [FDM hierarchy and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afdm-hierarchy-t-carrier) — Telecommunications, chapter 3, asked 2×
- [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4×
- [Analog vs digital multiplexing; 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aanalog-digital-mux-24-channel) — Telecommunications, chapter 3
- [Digital carrier system and multiplexing techniques](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-carrier-multiplexing) — Telecommunications, chapter 3
- [CCS and SS7 working principle](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs-ss7-working-principle) — Telecommunications, chapter 5, asked 2×

## Related in other subjects

- [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
- [CRO block diagram and working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acro-block-diagram) — Basic Electronics Engineering, chapter 7
- [Working principle of dc generator with diagram](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-principle-diagram) — Electrical Machines, chapter 3
- [FDM principle; SCPC and DAMA FDMA](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-scpc-dama) — Communication Systems, chapter 5