A-LevelPhysicsWaves9702/42 · May/June 2019 · Q58 Marks
For a signal transmitted along an optic fibre, state what is meant by: attenuation
- (a)(i)For a signal transmitted along an optic fibre, state what is meant by: attenuation[1]
- (a)(ii)noise.[2]
- (b)(i)The initial section of the transmission line for a signal from a telephone exchange is illustrated in Fig. 5.1. At the exchange, the input signal to the transmission line has a power of 2.5 × 10⁻³W. After the signal has travelled a distance of 52 km along the transmission line, the power of the signal is 7.8 × 10⁻¹⁶W. The signal is then amplified. Calculate the attenuation per unit length, in dBkm⁻¹, in the transmission line. attenuation per unit length = .............................................. dBkm⁻¹[3]
- (b)(ii)The gain of the amplifier is 115dB. Calculate the power of the signal at the output of the amplifier. power = ................................................................ W[2]
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Mark scheme
Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.
- 5(a)(i) loss of (signal) power/amplitude/intensity B1
- 5(a)(ii) unwanted/random signal B1 superposed on (transmitted) signal B1
- 5(b)(i) attenuation = 10 lg(P2/P1) C1 attenuation per unit length = (1 / L) × 10 Ig(P2/P1) C1 = (1/52) × 10 lg [(2.5 × 10−3) / (7.8 × 10–16)] = 2.4 dB km-1 A1
- 5(b)(ii) gain / dB = 10 lg(P2/P1) C1 115 = 10 lg [P/ (7.8 × 10-16)] P = 2.5 × 10-4 W A1
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