AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Principles of modulation; bandwidth.
Full specification wording
Amplitude (AM) and frequency modulation (FM) techniques (A-level only): Principles of modulation; bandwidth.
Modulation means varying a property of a carrier wave so that it carries information.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Modulation means varying a property of a carrier wave so that it carries information. In amplitude modulation (AM) the amplitude of the carrier changes in step with the audio or data signal, while the carrier frequency stays fixed. In frequency modulation (FM) the carrier amplitude stays constant and its frequency shifts slightly above and below the centre value as the signal varies. A modulated wave is not a single frequency: sidebands appear at the carrier frequency plus and minus the modulating frequency, so the bandwidth needed is wider than the original signal. FM generally needs a wider bandwidth than AM but is less affected by amplitude noise. Bandwidth is the range of frequencies a channel must carry, and it limits how much information can be transmitted.
What earns the marks
- Modulation varies a property of a carrier wave to carry information.
- In AM the carrier amplitude varies while the carrier frequency remains constant.
- In FM the carrier frequency varies while the carrier amplitude remains constant.
- A modulated carrier produces sidebands at the carrier frequency plus and minus the modulating frequency.
- Bandwidth is the range of frequencies contained in the transmitted signal.
- FM typically requires a wider bandwidth than AM for the same modulating signal.
- Bandwidth limits the rate at which information can be transmitted.
Common mistakes and how to put them right
Saying that FM changes the amplitude of the carrier.
Put it right: FM changes the carrier frequency while the amplitude stays constant.
Treating a modulated wave as a single frequency.
Put it right: Sidebands appear around the carrier, so a range of frequencies is present.
Assuming AM and FM always need the same bandwidth.
Put it right: FM generally needs a wider frequency range than AM.
Exam tips
- Label a carrier wave and show how its amplitude or frequency changes when a modulating signal is applied.
- Use the sideband idea to explain why a modulated signal occupies a band of frequencies rather than one frequency.
- Compare AM and FM in a short table covering what varies, bandwidth and noise immunity.
Try the lesson
The start of the Amplitude (AM) and frequency modulation (FM) techniques (A-level only): Principles of modulation; bandwidth. lesson, made from checked teaching notes for this specification point.
Have a go first
In amplitude modulation of a carrier wave, which property of the carrier is varied in step with the information signal?
Listen
Modulation means varying a property of a carrier wave so that it carries information. In amplitude modulation (AM) the amplitude of the carrier changes in step with the audio or data signal, while the carrier frequency stays fixed. In frequency modulation (FM) the carrier amplitude stays constant and its frequency shifts slightly above and below the centre value as the signal varies. A modulated wave is not a single frequency: sidebands appear at the carrier frequency plus and minus the modulating frequency, so the bandwidth needed is wider than the original signal. FM generally needs a wider bandwidth than AM but is less affected by amplitude noise. Bandwidth is the range of frequencies a channel must carry, and it limits how much information can be transmitted. A strong answer earns marks for these points. Modulation varies a property of a carrier wave to carry information. In AM the carrier amplitude varies while the carrier frequency remains constant. In FM the carrier frequency varies while the carrier amplitude remains constant. A modulated carrier produces sidebands at the carrier frequency plus and minus the modulating frequency. A common slip: Saying that FM changes the amplitude of the carrier. Instead, FM changes the carrier frequency while the amplitude stays constant. Exam tip: Label a carrier wave and show how its amplitude or frequency changes when a modulating signal is applied.
Key words
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Carrier wave
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Amplitude modulation
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Frequency modulation
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Bandwidth
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See every definition
- Carrier wave
- A wave whose property is varied so that it carries information.
- Amplitude modulation
- A technique in which the amplitude of the carrier changes in step with the audio or data signal while the carrier frequency stays fixed.
- Frequency modulation
- A technique in which the carrier amplitude stays constant and its frequency shifts slightly above and below the centre value as the signal varies.
- Bandwidth
- The range of frequencies a channel must carry, limiting how much information can be transmitted.
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