A-LevelPhysicsElectricity9702/41 · May/June 2017 · Q310 Marks

The digital transmission of speech may be illustrated using the block diagram of Fig. 3.1. [Figure 3.1]

(a)(i)State what is meant by a digital signal.[1]
(a)(ii)State the names of the components labelled X and Y on Fig. 3.1. X: Y:[2]
(a)(iii)Describe the function of the ADC.[2]
(b)(i)in dB, the ratio 'input power to optic fibre noise power at output of optic fibre'[2]
(b)(ii)the noise power at the output of the optic fibre.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[B1] signal consists of (a series of) 1s and 0s or offs and ons or highs and lows
[B1] component X: parallel-to-serial converter
[B1] component Y: DAC/digital-to-analogue converter
[M1] sample the (analogue) signal
[A1] at regular intervals and converts the analogue number to a digital number
[C1] attenuation in fibre = 84 × 0.19 (= 16 dB)
[C1] ratio = 16 + 28
[A1] = 44 dB
[C1] ratio / dB = 10 lg (P₂ / P₁)
[C1] 44 = 10 lg ({9.7 × 10⁻³} / P)
[C1] or -44 = 10 lg (P / {9.7 × 10⁻³})
[A1] power = 3.9 × 10⁻⁷ W

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