For a multimode fiber following parameters are recorded.

LED with drive circuit has a rise time of 15 ns.

LED spectral width = 40 nm

Material dispersion-related rise time degradation = 21 ns over 6 km link.

Receiver bandwidth = 235 MHz

Modal dispersion rise time = 3.9 sec

Calculate system rise time.

Solution:

$\mathrm{t}_{\mathrm{tx}}=15 \mathrm{nsec}$

$\mathrm{t}_{\text {mat }}=21 \mathrm{nsec}$

$t_{\text {mod }}=3.9 \mathrm{nsec}$

$\mathrm{t}_{\mathrm{rx}}=\frac{350}{\mathrm{~B}_{\mathrm{rx}}}$

$\begin{aligned} & \mathrm{t}_{r x}=\frac{350}{25} \\\\ & \mathrm{t}_{r x}=14 \mathrm{nsec}\\ \end{aligned}$

Since,

$\mathrm{t}_{\mathrm{sys}}=\left(\sum_{\mathrm{j}=1}^{\mathrm{N}} \mathrm{t}_{\mathrm{ri}}^2\right)^{1 / 2}\\ $

$\begin{aligned} & \mathrm{t}_{\mathrm{sys}}=\left[15^2+21^2+3.9^2+14^2\right]^{1 / 2} \\\\ & \mathrm{t}_{\text {sys }}=29.61 \mathrm{nsec}\\ \end{aligned}$