A-LevelPhysicsWaves9702/42 · May/June 2020 · Q67 Marks

Telephone signals may be transmitted either by means of an optic fibre or by means of a wire pair. State three advantages of the use of an optic fibre rather than a wire pair. 1. 2. 3.

(a)Telephone signals may be transmitted either by means of an optic fibre or by means of a wire pair. State three advantages of the use of an optic fibre rather than a wire pair. 1. 2. 3.[3]
(b)(i)It is proposed to transmit a signal over a distance of 4.5 x 10³ km by means of an optic fibre. The input signal has a power of 9.8mW. The minimum signal that can be detected at the output has a power of 6.3 × 10⁻¹⁷ W. For this signal power, the signal-to-noise ratio is 21 dB. Calculate: the power of the background noise[2]
(b)(ii)the maximum attenuation per unit length of the optic fibre that allows for uninterrupted transmission of the signal.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[1] greater bandwidth
[1] less noise
[1] less attenuation or fewer repeaters
[1] less crosslinking or greater security
[1] ratio / dB = 10 lg(P₁ / P₂)
[1] 21 = 10 lg [(6.3 × 10⁻¹⁷) / P]
[1] P = 5.0 × 10⁻¹⁹ W
[1] attenuation per unit length = (1 / 4.5 × 10³) × 10 lg [(9.8 × 10⁻³) / (6.3 × 10⁻¹⁷)]
[1] = 0.032 dB km⁻¹

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