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Learn7408Paper 3B · Option: ElectronicsData communication systemsModulation and transmission
Data communication systems · Subtopic 1 of 1

Modulation and Transmission

Information rides on a high-frequency carrier. In AM the carrier's amplitude follows the signal; bandwidth =2fm=2f_m. In FM its frequency varies; bandwidth ≈2(Δf+fm)\approx2(\Delta f+f_m), wider but much less affected by noise. Signals travel through copper wire, optical fibre or free space (radio, microwaves); time-division multiplexing sends many channels down one link by interleaving samples.

AQA 7408 Paper 3B≈ 40 minRevision 1 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Explain why carrier waves and modulation are used.
  • Calculate AM and FM bandwidths.
  • Compare transmission media.
  • Describe time-division multiplexing and link bit rate.
Pause and think

Why can't we just broadcast speech directly as radio waves at 11 kHz?

Section 2 of 14

Core idea

Start hereRiding on a carrier

A voice contains frequencies of a few kilohertz, which no practical aerial can broadcast. So the voice is loaded onto a high-frequency carrier wave, by changing either its amplitude or its frequency.

The big idea

AM bandwidth 2fm2f_m; FM bandwidth ≈2(Δf+fm)\approx2(\Delta f+f_m). Choosing a medium is a trade-off between bandwidth, range, cost and interference.

1. Modulation

Why AM needs 2fm2f_m
  1. (1+msin⁡2πfmt)sin⁡2πfct(1+m\sin2\pi f_mt)\sin2\pi f_ct
    Carrier amplitude follows the signal.
  2. =sin⁡2πfct+m2[cos⁡2π(fc−fm)t−cos⁡2π(fc+fm)t]=\sin2\pi f_ct+\tfrac m2\left[\cos2\pi(f_c-f_m)t-\cos2\pi(f_c+f_m)t\right]
    Product-to-sum identity.
  3. frequencies fc−fm, fc, fc+fm\text{frequencies }f_c-f_m,\ f_c,\ f_c+f_m
    Carrier and two sidebands.
Bandwidth =(fc+fm)−(fc−fm)=2fm=(f_c+f_m)-(f_c-f_m)=2f_m.
Worked exampleFitting stations into a band

AM band 540540–16111611 kHz; fm=4.5f_m=4.5 kHz.

Goal
Number of stations.
Strategy
Width ÷ bandwidth per station.
  1. 1611−540=1071 kHz1611-540=1071\text{ kHz}
    Band width.
  2. 10719.0=119\frac{1071}{9.0}=119
    Each station needs 99 kHz.
Answer119119 stations.
✓ CheckFM's 180180 kHz per station explains why FM uses the much wider VHF band ✓.

2. Media and multiplexing

Transmission media

Each medium has strengths.

Key takeawayTDM Channels take turns in short time slots; the link must carry the sum of their bit rates.
Common misconceptionFM bandwidth Include both the deviation and the signal frequency: 2(Δf+fm)2(\Delta f+f_m).
Pause and think

Why is a radio aerial roughly half a wavelength long?

Summary

  • AM 2fm2f_m; FM 2(Δf+fm)2(\Delta f+f_m).
  • FM resists noise.
  • Fibre: high bandwidth, low loss.
  • TDM interleaves channels.
Key words
Carrier wave
High-frequency wave that is modulated.
Sideband
Frequencies either side of the carrier.
Multiplexing
Sharing one link between channels.
HistoryFirst voice broadcast. Reginald Fessenden made the first AM voice-and-music broadcast on Christmas Eve 1906, playing violin to ships at sea.
Fun factWhy FM sounds better. Lightning and electrical noise change a wave's amplitude, which AM radios turn into crackle. FM radios ignore amplitude.
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