AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Only the purpose of each stage is required.

    Full specification wording

    Principles of communication systems (A-level only): Only the purpose of each stage is required.

    A communication system is a chain of stages, each performing one job so that information reaches a distant receiver in a usable form.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 2 of 2
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A communication system is a chain of stages, each performing one job so that information reaches a distant receiver in a usable form. You need to explain why each stage exists, not derive its internal circuit. For example, in a radio link a microphone converts sound into an electrical signal; a modulator imposes that signal onto a carrier wave; an amplifier increases power before transmission; the transmitting aerial radiates the wave; the receiving aerial intercepts it; a tuner selects the wanted carrier; demodulation recovers the original signal; and a loudspeaker converts it back to sound. At every stage ask what would fail if that stage were removed, and describe the transformation of the signal rather than component detail.

    What earns the marks

    • State the overall function of a communication system: to transfer information from a source to a receiver with the signal in a usable form.
    • Identify each stage in a given block diagram and describe the change it makes to the signal, such as conversion, modulation, amplification, transmission, selection or demodulation.
    • Explain the purpose of modulation, including matching the signal to the transmission medium and allowing efficient radiation or sharing of a channel.
    • Explain the purpose of demodulation as recovering the original information signal from the modulated carrier.
    • Explain the purpose of amplification as increasing signal power to overcome attenuation and noise before or after transmission.
    • Explain the purpose of the aerial or transducer stages as converting between electrical signals and radiated waves or sound.

    Common mistakes and how to put them right

    • Describing how a stage is built internally instead of stating its purpose.

      Put it right by linking each stage to the signal transformation it performs.

    • Treating modulation and demodulation as the same process.

      Put it right by stating that modulation imposes information onto a carrier and demodulation extracts it.

    • Assuming amplification removes noise.

      Put it right by stating that amplification increases both signal and noise power, so it cannot improve the signal-to-noise ratio by itself.

    Exam tips

    • Use the wording of the question: if it asks for the purpose of a stage, begin with a verb such as converts, imposes, selects or recovers.
    • Work through the block diagram in order and give one clear sentence per stage rather than a long paragraph.
    • Where a stage has two purposes, such as modulation allowing efficient radiation and channel sharing, state both briefly.

    Try the lesson

    The start of the Principles of communication systems (A-level only): Only the purpose of each stage is required. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In a communication system, what is the purpose of the microphone stage?

    Listen

    A communication system is a chain of stages, each performing one job so that information reaches a distant receiver in a usable form. You need to explain why each stage exists, not derive its internal circuit. For example, in a radio link a microphone converts sound into an electrical signal; a modulator imposes that signal onto a carrier wave; an amplifier increases power before transmission; the transmitting aerial radiates the wave; the receiving aerial intercepts it; a tuner selects the wanted carrier; demodulation recovers the original signal; and a loudspeaker converts it back to sound. At every stage ask what would fail if that stage were removed, and describe the transformation of the signal rather than component detail. A strong answer earns marks for these points. State the overall function of a communication system: to transfer information from a source to a receiver with the signal in a usable form. Identify each stage in a given block diagram and describe the change it makes to the signal, such as conversion, modulation, amplification, transmission, selection or demodulation. Explain the purpose of modulation, including matching the signal to the transmission medium and allowing efficient radiation or sharing of a channel. Explain the purpose of demodulation as recovering the original information signal from the modulated carrier. A common slip: Describing how a stage is built internally instead of stating its purpose. Instead, linking each stage to the signal transformation it performs. Exam tip: Use the wording of the question: if it asks for the purpose of a stage, begin with a verb such as converts, imposes, selects or recovers.

    See it

    A left-to-right flowchart of eight stages in a radio communication link. The microphone leads to the modulator, which leads to the amplifier, which leads to the transmitting aerial. The transmitting aerial leads to the receiving aerial, which leads to the tuner, which leads to the demodulator, which leads to the loudspeaker. Each stage is shown as a single box joined by an arrow to the next.
    It shows the order of stages in a radio communication link, from microphone to loudspeaker.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Receiving aerial
    2. 2. Loudspeaker
    3. 3. Tuner
    4. 4. Microphone
    5. 5. Modulator
    6. 6. Demodulator
    7. 7. Amplifier
    8. 8. Transmitting aerial

    Key words

    Say what each word means before you turn the card.

    Modulation

    Try answering aloud or in your head before turning the card.

    Demodulation

    Try answering aloud or in your head before turning the card.

    Amplification

    Try answering aloud or in your head before turning the card.

    Transducer stages

    Try answering aloud or in your head before turning the card.

    See every definition
    Modulation
    Imposes the information signal onto a carrier wave, matching the signal to the transmission medium and allowing efficient radiation or sharing of a channel.
    Demodulation
    Recovers the original information signal from the modulated carrier at the receiver.
    Amplification
    Increases signal power to overcome attenuation and noise before or after transmission.
    Transducer stages
    Convert between electrical signals and radiated waves or sound, as at the input and output of the system.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A left-to-right flowchart of eight stages in a radio communication link. The microphone leads to the modulator, which leads to the amplifier, which leads to the transmitting aerial. The transmitting aerial leads to the receiving aerial, which leads to the tuner, which leads to the demodulator, which leads to the loudspeaker. Each stage is shown as a single box joined by an arrow to the next.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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