Page 336 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
Summary
■■ Modulation varies either the amplitude or frequency of a carrier wave to carry information in the signal.
■■ Bandwidth is the range of frequencies present in a broadcast signal.
■■ FM broadcasts have higher bandwidth and sound quality than AM.
■■ Analogue and digital signals transmit information.
■■ Analogue-to-digital conversion (ADC) turns analogue
signals into digital signals and digital-to-analogue conversion (DAC) turns them back again.
■■ Regeneration removes noise and interference from digital signals.
■■ For good reproduction of a signal, the sampling rate and number of bits should be high.
■■ Wire-pairs, coaxial cables, radio waves, microwaves and optic fibres transmit signals.
■■ Attenuation of a signal is measured in dB, where ■ number of dB = 10 lg P1 .
■■ Satellites for communication are often in geostationary or polar orbits.
P2
End-of-chapter questions
1 a Draw a sketch graph of an AM wave and use your graph to explain how the AM wave has been formed
from the carrier wave and a signal. Refer to the frequencies involved. [4]
b Draw a sketch graph of an FM wave and use your graph to explain how the FM wave has been formed
from a carrier wave and a signal. Refer to the frequencies involved. [4]
2 The output of a microphone is an analogue signal with a bandwidth of 3.4 kHz.
a Explain what is meant by
i analogue [2]
ii bandwidth. [1]
b Compare the bandwidth of the microphone with the typical range of frequencies that can be heard by
the human ear. Comment on the difference between the two values. [2]
3 An LED provides input power of 1.26 mW to an optic fibre of length 60 m. The output at the other end of the fibre is 1.12 mW.
a Calculate the attenuation in the optic fibre. [2]
b Calculate the attenuation per unit length in the optic fibre. [2]
4 The signal attenuation per unit length of an optic fibre is 0.30 dB km−1. An input signal to the optic fibre is 100 dB above the noise level. The level of the noise remains constant along the optic fibre at 6.0 pW.
Calculate:
a the power of the input signal to the optic fibre [3]
b the maximum length of the optic fibre used, if the signal at the end of the fibre is to remain at least
30 dB above the noise level. [2]
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