A-LevelPhysicsAlternating currents9702/41 · Oct/Nov 2019 · Q57 Marks
A section of a coaxial cable is shown in [Figure 5.1].
- (a)(i)Suggest two functions of the copper braid. 1. 2.[2]
- (a)(ii)Suggest one application of a coaxial cable for the transmission of electrical signals.[1]
- (b)(i)The constant noise power in a transmission cable is 7.6μW. The minimum acceptable signal-to-noise ratio is 32 dB. Calculate the minimum acceptable signal power P_MIN in the cable. P_MIN = ...........................................................W[2]
- (b)(ii)The input power of the signal to the transmission cable is 2.6W. The attenuation per unit length of the cable is 6.3dBkm⁻¹. Use your answer in (i) to determine the maximum uninterrupted length L of cable along which the signal may be transmitted. L = ...........................................................km[2]
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Mark scheme
Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.
- [B1] provides return for the signal
- [B1] shields signal from noise
- [B1] e.g. connection between aerial and TV set
- [C1] gain / dB = 10 lg (P1/P2)
- [A1] 32 = 10 lg {PMIN / (7.6 × 10−6)}
- [(A1)] PMIN = 0.012 W
- [C1] attenuation per unit length = (1 / L) × 10 lg (P₁ / P2)
- [A1] 6.3 = (1/L) × 10 lg (2.6/0.012)
- [(A1)] L = 3.7 km
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