---
title: "Frequency division multiplexing explained"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:fdm-explain"
source: https://www.bctengineer.com/q/semester_5/data_communication/fdm-explain
site: BCT Engineering
---

# Frequency division multiplexing explained

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2079 Baisakh, 4 marks; 2076 Asoj, 8 marks; 2072 Chaitra, 3 marks (asked 3 times)
- Video: [Frequency Division Multiplexing (FDM) Explained](https://www.youtube.com/watch?v=UwWmDwbsDBs) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/fdm-explain
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Explain the operation of FDM (frequency division multiplexing) with a diagram.

## Short answer

FDM divides the link's bandwidth into separate frequency bands, each modulated onto its own carrier and sent simultaneously. Guard bands prevent adjacent channel interference; the receivers use band-pass filters and demodulators to recover original signals.

## Answer

**Frequency division multiplexing (FDM)** is an analog multiplexing technique in which the link bandwidth is divided into non-overlapping frequency bands, and each signal is modulated onto its own carrier frequency and sent at the same time over the shared link.

#### Operation
1. Each source signal (e.g. a voice signal of 4 kHz) modulates a different carrier frequency *f1, f2, f3* (AM, FM or SSB). This shifts each signal to its own band.
2. The modulated signals are added (combined) in the multiplexer and transmitted together on the link.
3. At the receiver, the demultiplexer uses **band-pass filters** to separate the bands. Each is then demodulated with its carrier to recover the original signal.
4. Narrow **guard bands** are left between channels to prevent adjacent channel interference.

```text
 m1(t) -> [Mod f1] \                     / [BPF f1]->[Demod]-> m1
 m2(t) -> [Mod f2] -->[ + ]==== link ===< [BPF f2]->[Demod]-> m2
 m3(t) -> [Mod f3] /                     \ [BPF f3]->[Demod]-> m3

 Frequency spectrum on the link:
 | ch1 |g| ch2 |g| ch3 |
 f1        f2        f3        (g = guard band)
```

#### Key points
- Total link bandwidth ≥ sum of channel bandwidths + guard bands. For n channels of bandwidth *B*: *Blink* ≥ n*B* + (n−1)*Bguard*.
- All channels are transmitted simultaneously, each continuously.
- Example: three voice channels of 4 kHz with 1 kHz guards need 3×4 + 2×1 = 14 kHz.
- Used in AM/FM radio, TV broadcasting, cable TV, first-generation cellular (AMPS) and telephone trunks.
- Drawbacks: crosstalk, noise accumulates in the analog signal, bandwidth wasted by guard bands, and intermodulation distortion.

## Check yourself

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

1. How does FDM send many signals simultaneously?
   A. Each is modulated onto a different carrier frequency and the results are added
   B. Each takes a time slot
   C. Each uses a code
   D. They are sent one by one
   Answer: A. Each signal is shifted to its own band. (Key point: FDM operation)

2. What separates the channels at the receiver?
   A. Band-pass filters
   B. Low-pass filter only
   C. Time gates
   D. Codes
   Answer: A. BPFs select each band before demodulation. (Key point: Demultiplexing)

3. Why are guard bands left between channels?
   A. To prevent adjacent channel interference
   B. To add data
   C. To lower power
   D. To sync
   Answer: A. They separate channels. (Key point: Guard bands)

4. Three 4 kHz voice channels with 1 kHz guard bands need how much bandwidth?
   A. 12 kHz
   B. 13 kHz
   C. 14 kHz
   D. 15 kHz
   Answer: C. 3 x 4 + 2 x 1 = 14 kHz. (Key point: Bandwidth calc)

## Related in Data Communication

- [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3×
- [Multiplexing techniques; analog one explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Alist-mux-techniques) — Data Communication, chapter 6

## Related in other subjects

- [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2×
- [Frequency counter operation with block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Afrequency-counter) — Digital Logic, chapter 11, asked 14×
- [Multiplexing display concept and operation](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Amultiplexing-display) — Digital Logic, chapter 11, asked 5×
- [Division operation and hardware](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adivision-hardware) — Computer Organization and Architecture, chapter 5