A-LevelPhysicsAlternating currents9702/42 · May/June 2021 · Q48 Marks
A sinusoidal carrier wave has a constant amplitude and a frequency of 1.2 MHz. The carrier wave is modulated by a signal wave such that a 1.0 V displacement of the signal wave causes a change in frequency of 25 kHz. The signal wave has frequency 8.0 kHz and amplitude 2.0 V. An audio signal is transmitted by means of a modulated radio wave. The variation with frequency of the amplitude of the radio wave is shown in [Figure 4.1]. For this transmission, determine:
- (a)(i)State the name of this type of modulation of the carrier wave.[1]
- (a)(ii)For this modulated carrier wave, determine the variation, if any, in: 1. its amplitude 2. its frequency.[3]
- (b)(i)the wavelength, in km, of the carrier wave[2]
- (b)(ii)the bandwidth[1]
- (b)(iii)the frequency of the audio signal.[1]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.
- [1] frequency (modulation)
- [1] zero
- [1] frequency (of 1.2 MHz) varies by ±50 kHz
- [1] frequency varies (by ±50 kHz) at a rate of 8000 times per second
- [1] wavelength = (3.00 × 10⁸) / (240 × 10³)
- [1] = 1.25 km
- [1] bandwidth = 30 kHz
- [1] frequency = 15 kHz
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