A-Level · Physics · AQA · Mark scheme decoded

AQA A-Level Physics: Block Diagram of Real-Time Communication System — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Physics specificationlast verified 2 July 2026

The short answer

In the realm of electronics and communication systems, understanding the block diagram of a 'real-time' communication system is crucial. This topic focuses on the purpose of each stage in such a system, which helps in grasping how information is transmitted from one point to another efficiently and accurately.

The question

Identify the purpose of each stage in the following block diagram of a real-time communication system: Information Source → Transducer → Signal Processing → Modulator → Transmitter → Transmission Medium → Receiver → Demodulator → Signal Processing (at Receiver) → Transducer (at Receiver) → Information Sink. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

S1
1. Information Source : Captures and converts physical information into an electrical signal.
S2
2. Transducer : Converts one form of energy (e.g., sound waves) into another (e.g., electrical signals).
S3
3. Signal Processing : Modifies the electrical signal to make it suitable for transmission, including amplification, filtering, and modulation.
S4
4. Modulator : Encodes the information onto a carrier wave by changing its amplitude, frequency, or phase.
S5
5. Transmitter : Sends the modulated signal into the transmission medium (e.g., air, cable).
S6
6. Transmission Medium : The physical path through which the signal travels from the transmitter to the receiver.
S7
7. Receiver : Captures the transmitted signal and prepares it for further processing.
S8
8. Demodulator : Extracts the original information from the modulated carrier wave by reversing the modulation process.
S9
9. Signal Processing (at Receiver) : Improves the quality of the demodulated signal and makes it suitable for output, including amplification, filtering, and error correction.
S10
10. Transducer (at Receiver) : Converts the electrical signal back into its physical form (e.g., sound waves, images).
S11
11. Information Sink : Presents the final output in a usable and understandable form to the user.

Model answer

Worked through, with each step tagged to the mark it earns.

S1
1. Information Source : Captures and converts physical information into an electrical signal.
S2
2. Transducer : Converts one form of energy (e.g., sound waves) into another (e.g., electrical signals).
S3
3. Signal Processing : Modifies the electrical signal to make it suitable for transmission, including amplification, filtering, and modulation.
S4
4. Modulator : Encodes the information onto a carrier wave by changing its amplitude, frequency, or phase.
S5
5. Transmitter : Sends the modulated signal into the transmission medium (e.g., air, cable).
S6
6. Transmission Medium : The physical path through which the signal travels from the transmitter to the receiver.
S7
7. Receiver : Captures the transmitted signal and prepares it for further processing.
S8
8. Demodulator : Extracts the original information from the modulated carrier wave by reversing the modulation process.
S9
9. Signal Processing (at Receiver) : Improves the quality of the demodulated signal and makes it suitable for output, including amplification, filtering, and error correction.
S10
10. Transducer (at Receiver) : Converts the electrical signal back into its physical form (e.g., sound waves, images).
S11
11. Information Sink : Presents the final output in a usable and understandable form to the user.
Final answer: The purpose of each stage is as follows: Information Source (captures and converts), Transducer (converts energy forms), Signal Processing (modifies for transmission), Modulator (encodes information), Transmitter (sends signal), Transmission Medium (physical path), Receiver (captures and prepares), Demodulator (extracts original information), Signal Processing (at Receiver) (improves quality), Transducer (at Receiver) (converts back to physical form), Information Sink (presents output).

Common mistakes

Confusing the roles of the transmitter and receiver — Remember that the transmitter is responsible for sending the modulated signal into the transmission medium, whereas the receiver captures and processes the received signal.
Forgetting the purpose of modulation — Understand that modulation is essential for transmitting information over long distances by encoding it onto a carrier wave. This allows the signal to travel through various media effectively.
Misunderstanding the role of transducers — At the information source, a transducer converts physical information into an electrical signal. At the information sink, it converts the electrical signal back into its physical form.
Confusing signal processing at the transmitter and receiver — At the transmitter, signal processing modifies the electrical signal for transmission (amplification, filtering, modulation). At the receiver, it improves the quality of the demodulated signal (amplification, filtering, error correction).
Not recognizing the importance of the transmission medium — Understand that the transmission medium is crucial for the physical path through which the signal travels. It can be air, cables, optical fibers, or other media, chosen based on factors like distance and interference.
Overlooking the purpose of demodulation — Demodulation is essential for extracting the original information from the modulated carrier wave. It reverses the modulation process to recover the electrical signal representing the information source.

Where the marks go

Full worked solution (all marking points)11 marks