A-LevelPhysicsAlternating currents9702/43 · May/June 2020 · Q67 Marks
The transmission of signals using optic fibres has, to a great extent, replaced the use of coaxial cables. Advantages of optic fibres include greater bandwidth and very little crosslinking.
- (a)(i)Suggest an advantage of greater bandwidth.[1]
- (a)(ii)State what is meant by crosslinking.[2]
- (b)In telecommunications, a signal power of 1.0mW is used as a reference power. Signal powers relative to this reference power and expressed in dB are said to be measured in 'dBm'. Show that a signal power of 13dBm is equivalent to 20mW.[2]
- (c)A signal of input power 20mW is transmitted along an optic fibre for an uninterrupted distance of 45 km. The optic fibre has an attenuation per unit length of 0.18dBkm⁻¹. Calculate the output power P from the optic fibre.[2]
Mark scheme
Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.
- [B1] greater information carrying capacity
- [B1] power/energy is radiated
- [B1] signal picked up by adjacent fibre/wire
- [C1] ratio / dB = 10 lg(P₂/P₁)
- [A1] 13 = 10 lg [P/(1.0 × 10⁻³)] AND so P = 20 mW
- [C1] 45 × 0.18 = 10 lg (20/P)
- [A1] P = 3.1 mW
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