Power margin of a fiber-optic system

[MattH150197]

Homework Statement

Im going over some revision for fibre optics and I've come across an example question to calculate the power margin of the system with the following info is given:
transmitter power = -10dBm
Connector loss = 1.5dB
Attenuation = 3dB/Km
Total length= 2KM
Receiver threshold = -30dBm

Homework Equations

Power budget = Pt - Pr
Power Margin = PB - Total losses

The Attempt at a Solution

PB = -10 - - 30 = 20
PM = 20 - ((3*2) + 1.5)) = 13.5dB
However it says the answer is 9.5dB, i didnt write the formulas wrong did i when i was making notes or can you see where I've gone wrong anywhere?

 

[marcusl]

If you have written the problem correctly and completely, then I agree with your result and suspect that the "answer" is wrong.

 

[rude man]

PM = 20 - ((3*2) + 1.5)) = 13.5dB

Should be 12.5 dB. Unfortunately still not 9.5dB.

 

[marcusl]

Oops right, I didn’t subtract that properly in my head!

 

[rude man]

I'd like to know how the given answer was obtained if you ever find out.

 

[MattH150197]

1. Oops! Sorry yeah I meant 12.5, I really have no idea where 9.5dB has come from and it was really frustrating why I was getting different. So thanks for clearing it up guys!