A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Block Diagram of Real-Time Communication System — mark scheme explained
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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For questions asking about the purpose of stages, allocate marks for each correct identification and explanation. For more complex questions, break down the marks for different aspects of the answer (e.g., definition, example, application).
What the command words demand
- Identify
- Recognize and name the purpose of each stage in a communication system.
- Explain
- Provide a detailed account of why a particular stage is necessary or how it functions.
- Describe
- Give a clear and concise overview of the role of a specific stage.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 2-3 minutes per mark to ensure you have enough time to provide detailed and accurate answers.
- 1. Information Source : Captures and converts physical information into an electrical signal.1 mark
- 2. Transducer : Converts one form of energy (e.g., sound waves) into another (e.g., electrical signals).1 mark
- 3. Signal Processing : Modifies the electrical signal to make it suitable for transmission, including amplification, filtering, and modulation.1 mark
- 4. Modulator : Encodes the information onto a carrier wave by changing its amplitude, frequency, or phase.1 mark
- 5. Transmitter : Sends the modulated signal into the transmission medium (e.g., air, cable).1 mark
- 6. Transmission Medium : The physical path through which the signal travels from the transmitter to the receiver.1 mark
- 7. Receiver : Captures the transmitted signal and prepares it for further processing.1 mark
- 8. Demodulator : Extracts the original information from the modulated carrier wave by reversing the modulation process.1 mark
- 9. Signal Processing (at Receiver) : Improves the quality of the demodulated signal and makes it suitable for output, including amplification, filtering, and error correction.1 mark
- 10. Transducer (at Receiver) : Converts the electrical signal back into its physical form (e.g., sound waves, images).1 mark
- 11. Information Sink : Presents the final output in a usable and understandable form to the user.1 mark
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).
Work through every step correctly and you earn all 11 marks.
Another worked example
Explain the role of a modulator in a communication system and why it is necessary.
- 1. Role of Modulator : A modulator encodes the information onto a carrier wave by changing its amplitude, frequency, or phase.2 marks
- 2. Necessity : Modulation is essential because it allows the information to be transmitted over long distances using high-frequency waves that can travel through various media (e.g., air, cables). Without modulation, the original low-frequency signal would not be able to propagate effectively over long distances due to its limited range and susceptibility to interference.2 marks
Final answer: The role of a modulator is to encode the information onto a carrier wave by changing its amplitude, frequency, or phase. This is necessary because it allows the information to be transmitted over long distances using high-frequency waves that can travel through various media effectively.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing the roles of the transmitter and receiver
Why it happens: Students may mix up the functions of these two stages because they both deal with signals. The transmitter sends the signal, while the receiver captures it.
Fix: 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
Why it happens: Students might overlook why modulation is necessary, focusing only on its process.
Fix: 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
Why it happens: Transducers are used at both the information source and sink, which can lead to confusion about their specific functions.
Fix: 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
Why it happens: Students may not distinguish between the different tasks performed by signal processing at these two stages.
Fix: 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
Why it happens: Students might underestimate the role of the transmission medium in the communication process.
Fix: 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
Why it happens: Students might focus only on modulation without considering its reverse process.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| System Block Diagram | Identify and explain the purpose of each stage in a communication system. | 11 |
| Explain Modulator Role | Describe what a modulator does and explain why modulation is needed for transmission. | 4 |
| Signal Processing Functions | Explain what signal processing does at both the transmitter and the receiver. | 4 |
| Explain Transducer Purpose | Explain why transducers are needed at both the information source and sink stages. | 4 |
| Explain Component Purpose | Identify the transmission medium and explain its role in a communication system. | 4 |
| Total across these question types | 27 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.