A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Principles of Modulation and Bandwidth in Electronics — mark scheme explained
The short answer
Modulation is a fundamental concept in electronics, particularly in communication systems. It involves altering a carrier wave to encode information from an information signal. This process allows the transmission of data over long distances using radio waves or other media. Understanding modulation and bandwidth is crucial for designing efficient communication systems.
The question
An AM radio station broadcasts a carrier wave with a frequency of 1000 kHz. The highest frequency component of the information signal is 5 kHz. Calculate the required bandwidth for this AM broadcast.
[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 calculation questions, show all steps clearly and include units. For identification and comparison questions, provide concise but accurate answers. Always double-check your work for unit consistency and formula accuracy.
What the command words demand
- Calculate
- Perform a numerical calculation to find a specific value.
- Identify
- Recognize and name the correct component or parameter from a given set of data.
- Compare
- Examine and describe the similarities and differences between two or more items or concepts.
- Explain
- Provide a clear and detailed account of how something works or why it is important.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5-7 minutes per question to ensure you have enough time to read the question carefully, perform calculations, and check your answers.
- Identify the given values: f c = 1000 kHz, f M = 5 kHz.0 marks
- Use the formula for AM bandwidth: Bandwidth = 2f M .1 mark
- Substitute the values into the formula: Bandwidth = 2 × 5 kHz = 10 kHz.2 marks
Final answer: The required bandwidth for this AM broadcast is 10 kHz.
Work through every step correctly and you earn all 3 marks.
Another worked example
An FM radio station broadcasts a carrier wave with a frequency of 98.7 MHz and a maximum frequency deviation of 75 kHz. The highest frequency component of the information signal is 15 kHz. Calculate the required bandwidth for this FM broadcast.
- Identify the given values: Δf = 75 kHz (peak frequency deviation), f M = 15 kHz. The carrier frequency (98.7 MHz) does not affect the bandwidth.0 marks
- Use Carson's rule for FM bandwidth: Bandwidth = 2(Δf + f M ).1 mark
- Substitute the values: Bandwidth = 2(Δf + f M ) = 2(75 kHz + 15 kHz) = 2 × 90 kHz = 180 kHz.3 marks
Final answer: BW = 180 kHz
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing AM and FM in terms of what changes (amplitude vs. frequency)
Why it happens: Students may mix up the definitions of AM and FM, leading to incorrect answers when identifying or calculating modulation parameters.
Fix: Review the definitions: In AM, the amplitude of the carrier wave changes; in FM, the frequency of the carrier wave changes.
Using the wrong formula for bandwidth calculation
Why it happens: Students might use the AM bandwidth formula for FM or vice versa, leading to incorrect results.
Fix: Memorize and understand the formulas: Bandwidth = 2f M for AM and Bandwidth = 2(Δf + f M ) for FM (Carson's rule; Δf = peak frequency deviation).
Failing to convert units consistently
Why it happens: Students may mix up units (kHz, MHz) when substituting values into formulas, leading to incorrect calculations.
Fix: Always ensure that all frequencies are in the same unit before performing calculations.
Misinterpreting graphical representations of modulated signals
Why it happens: Students might struggle to identify the carrier frequency and information frequency from a graph, leading to incorrect answers.
Fix: Practice interpreting graphs by identifying the high-frequency carrier wave and the lower-frequency information signal.
Not considering the practical limitations of bandwidth
Why it happens: Students may not realize that the available bandwidth must be greater than or equal to the required bandwidth for a communication system to work.
Fix: Always compare the required bandwidth with the available bandwidth and ensure the former is less than or equal to the latter.
Wrongly adding the carrier frequency into the FM bandwidth.
Why it happens: Students mistakenly think the carrier frequency must be added when calculating FM bandwidth.
Fix: Use Carson's rule: Bandwidth = 2(Δf + f M ), where Δf is the peak frequency deviation and f M is the highest information frequency. The carrier frequency is NOT part of the bandwidth calculation.
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 |
|---|---|---|
| Calculate AM Bandwidth | Find the bandwidth of an AM broadcast from its highest signal frequency. | 3 |
| Calculate FM Bandwidth | Apply Carson's rule to find the bandwidth of an FM broadcast signal. | 4 |
| AM Bandwidth Check | Compare AM bandwidth requirement against channel bandwidth and justify whether transmission is possible. | 3 |
| FM Bandwidth Calculation | Use Carson's rule to find required bandwidth and decide if the channel can support it. | 4 |
| Total across these question types | 14 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.