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

Question 6

(a) In radio communication, the radio wave is usually modulated.

State what is meant by amplitude modulation (AM ). [2]

(b) 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,

(i) determine the wavelength, [1]

(ii) describe the amplitude variation, [2]

(iii) state the bandwidth. [1]

(c) Communication is sometimes made using satellites in geostationary orbits that have a period of rotation about the Earth of 24 hours.

(i) State two other features, apart from the period, of a geostationary orbit. [2]

(ii) Suggest why

1. frequencies of the order of gigahertz are used for satellite communication, [1]

2. the uplink frequency to the satellite is different from the downlink frequency. [2]

[Total: 11]

Reference: Past Exam Paper – November 2018 Paper 42 Q5

Solution:

(a) Amplitude modulation is the variation of the amplitude of the carrier wave in synchrony with the displacement of the information signal.

(b)

(i)

{The (EM) wave travels at the speed of light.

The modulated wave has the same frequency of the carrier wave as it is its amplitude that is being modulated.

v = fλ}

wavelength λ = (3.0 × 10⁸) / (900 × 10³)

wavelength λ = 3.3 × 10² m

(ii)

The amplitude varies (continuously) between a maximum (above 4.0 V) and a minimum (below 4.0 V). The variations repeat 5000 times (since the frequency is 5.0 kHz) each second.

(iii)

{Bandwidth = 2fs = 2×5000 Hz}

Bandwidth = 10000 Hz

(c)

(i) 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

(ii)

1.

Frequencies of the order of gigahertz are used for satellite communication so that there is minimal reflection/absorption/attenuation by atmosphere

or

maximum penetration of/transmission through atmosphere

2.

The uplink signal is highly attenuated as it reaches the satellite. The uplink frequency to the satellite is different from the downlink frequency so as to prevent the downlink signal from swamping the uplink signal.