A-LevelPhysicsWaves9702/42 · Oct/Nov 2018 · Q511 Marks

In radio communication, the radio wave is usually modulated. State what is meant by amplitude modulation (AM).

(a)In radio communication, the radio wave is usually modulated. State what is meant by amplitude modulation (AM).[2]
(b)(i)A sinusoidal radio carrier wave has a frequency of 900 kHz and an unmodulated amplitude measured to be 4.0 V. The carrier wave is amplitude modulated by a signal of frequency 5.0 kHz. For the amplitude modulated wave, determine the wavelength,[1]
(b)(ii)A sinusoidal radio carrier wave has a frequency of 900 kHz and an unmodulated amplitude measured to be 4.0 V. The carrier wave is amplitude modulated by a signal of frequency 5.0 kHz. For the amplitude modulated wave, describe the amplitude variation,[2]
(b)(iii)A sinusoidal radio carrier wave has a frequency of 900 kHz and an unmodulated amplitude measured to be 4.0 V. The carrier wave is amplitude modulated by a signal of frequency 5.0 kHz. For the amplitude modulated wave, state the bandwidth.[1]
(c)(i)Communication is sometimes made using satellites in geostationary orbits that have a period of rotation about the Earth of 24 hours. State two other features, apart from the period, of a geostationary orbit.[2]
(c)(ii) 1.Suggest why frequencies of the order of gigahertz are used for satellite communication,[1]
(c)(ii) 2.Suggest why the uplink frequency to the satellite is different from the downlink frequency.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[B1] amplitude of carrier (wave) varies
[B1] variation in synchrony with displacement of information signal
[A1] wavelength = (3.0 × 10⁸)/(900 × 10³) = 3.3 x 10² m
[B1] amplitude varies (continuously) between a maximum and a minimum
[B1] variations repeat 5000 times each second or variations repeat every 0.2 ms or variations above and below 4.0 V
[A1] 10000 Hz
[B2] Any two from: • (orbit is) above the Equator • (orbit is) from west to east/same direction as Earth's rotation • orbit is circular/orbit has a particular radius
[B1] minimal reflection/absorption/attenuation by atmosphere or maximum penetration of/transmission through atmosphere
[B1] uplink signal is greatly attenuated/must be greatly amplified
[B1] prevents downlink signal swamping the uplink signal

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