(a)[2]
Calculate the minimum input signal power to the receiver.
(b)[2]
Use your answer in (a) to calculate the attenuation in the fibre.
(c)[1]
Determine the attenuation per unit length of the fibre.
Physics 9702 · AS & A Level · Characteristics of alternating currents
Characteristics of alternating currents — practice question
An optic fibre carries digital telephone signals. The input power to the optic fibre is $9.8\,\text{mW}$. As shown in Fig. 12.1, the effective noise level in the receiver circuit is $0.36\,\mu\text{W}$. The signal-to-noise ratio at the receiver must stay at or above $28\,\text{dB}$. If no repeater amplifiers are used for this transmission, the greatest possible length of the optic fibre is $85\,\text{km}$.
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This 5-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Set up $28=10\log_{10}\left(\dfrac{P}{0.36\times10^{-6}}\right)$.” …
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