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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Identify the given values: f c = 1000 kHz, f M = 5 kHz.
- S2
Use the formula for AM bandwidth: Bandwidth = 2f M .
- S3
Substitute the values into the formula: Bandwidth = 2 × 5 kHz = 10 kHz.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Identify the given values: f c = 1000 kHz, f M = 5 kHz.
- S2
Use the formula for AM bandwidth: Bandwidth = 2f M .
- S3
Substitute the values into the formula: Bandwidth = 2 × 5 kHz = 10 kHz.
Final answer: The required bandwidth for this AM broadcast is 10 kHz.
Common mistakes
- Confusing AM and FM in terms of what changes (amplitude vs. frequency) — 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 — 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 — Always ensure that all frequencies are in the same unit before performing calculations.
- Misinterpreting graphical representations of modulated signals — Practice interpreting graphs by identifying the high-frequency carrier wave and the lower-frequency information signal.
- Not considering the practical limitations of bandwidth — 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. — 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
- Full worked solution (all marking points)3 marks