AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Carrier wave and information signal.
Full specification wording
Amplitude (AM) and frequency modulation (FM) techniques (A-level only): Carrier wave and information signal.
A carrier wave is a high-frequency sinusoidal oscillation that is transmitted through space and can be modified to carry information.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A carrier wave is a high-frequency sinusoidal oscillation that is transmitted through space and can be modified to carry information. The information signal is the lower-frequency oscillation representing the message, such as audio from a microphone. In AM, the amplitude of the carrier is varied in step with the information signal while its frequency stays constant; in FM, the carrier frequency is varied while its amplitude stays constant. For example, a 1 MHz carrier modulated by a 1 kHz audio tone produces a wave whose envelope or spacing changes at the audio rate. The carrier itself conveys no message until modulation encodes the information onto it.
What earns the marks
- States that a carrier wave is a high-frequency sinusoidal oscillation used to transmit information through space.
- States that the information signal is a lower-frequency oscillation representing the message to be transmitted.
- Explains that modulation varies a property of the carrier, such as amplitude or frequency, in step with the information signal.
- Distinguishes AM, where carrier amplitude varies and frequency is constant, from FM, where carrier frequency varies and amplitude is constant.
- Uses a numerical or graphical example, such as a 1 MHz carrier with a 1 kHz audio signal, to show the different time scales of carrier and information.
- Recognises that the unmodulated carrier alone carries no information until the information signal is imposed on it.
Common mistakes and how to put them right
Confusing the carrier with the information signal.
Put it right: The carrier is the high-frequency wave that is modulated, while the information signal is the lower-frequency message.
Believing that FM changes the amplitude of the carrier.
Put it right: In FM the amplitude remains constant and the frequency varies.
Thinking the carrier frequency equals the information frequency.
Put it right: The carrier frequency is much higher so that the signal can be radiated efficiently.
Exam tips
- Sketch carrier and information waveforms on the same time axis, labelling the carrier period and the slower information period.
- When comparing AM and FM, state explicitly which property varies and which stays constant for each technique.
- Use the terms carrier wave and information signal precisely in written answers rather than vague words such as wave or signal.
Try the lesson
The start of the Amplitude (AM) and frequency modulation (FM) techniques (A-level only): Carrier wave and information signal. lesson, made from checked teaching notes for this specification point.
Have a go first
In an AM radio transmission, which wave is described as the carrier wave?
Listen
A carrier wave is a high-frequency sinusoidal oscillation that is transmitted through space and can be modified to carry information. The information signal is the lower-frequency oscillation representing the message, such as audio from a microphone. In AM, the amplitude of the carrier is varied in step with the information signal while its frequency stays constant; in FM, the carrier frequency is varied while its amplitude stays constant. For example, a 1 MHz carrier modulated by a 1 kHz audio tone produces a wave whose envelope or spacing changes at the audio rate. The carrier itself conveys no message until modulation encodes the information onto it. A strong answer earns marks for these points. States that a carrier wave is a high-frequency sinusoidal oscillation used to transmit information through space. States that the information signal is a lower-frequency oscillation representing the message to be transmitted. Explains that modulation varies a property of the carrier, such as amplitude or frequency, in step with the information signal. Distinguishes AM, where carrier amplitude varies and frequency is constant, from FM, where carrier frequency varies and amplitude is constant. Exam tip: Sketch carrier and information waveforms on the same time axis, labelling the carrier period and the slower information period.
Key words
Say what each word means before you turn the card.
Carrier wave
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Information signal
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Amplitude modulation (AM)
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Frequency modulation (FM)
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See every definition
- Carrier wave
- A high-frequency sinusoidal oscillation that is transmitted through space and can be modified to carry information.
- Information signal
- The lower-frequency oscillation representing the message, such as audio from a microphone.
- Amplitude modulation (AM)
- A technique in which the amplitude of the carrier is varied in step with the information signal while its frequency stays constant.
- Frequency modulation (FM)
- A technique in which the carrier frequency is varied while its amplitude stays constant.
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