Calculate Signal Power from dBm vs Wavelength Graph

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Homework Help Overview

The discussion revolves around interpreting a graph of "dBm vs Wavelength" to calculate total power over a specified wavelength range. Participants are exploring the conversion of dBm values to power in watts and the implications of integrating these values.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning the validity of integrating the graph as described, particularly regarding the dimensions of the resulting quantity. There is a discussion about whether the y-axis represents power per unit wavelength or another quantity, and the implications of this on the integration process.

Discussion Status

The discussion is ongoing, with participants providing insights and questioning assumptions about the graph's axes and the units involved. Some guidance has been offered regarding the need for clarity on the y-axis measurement and its implications for calculating total power.

Contextual Notes

There is a lack of clarity regarding the specific units on the y-axis of the graph, which is critical for determining the correct approach to integration. Participants are also considering the context in which the power will be used, suggesting that unit cancellation may play a role in their calculations.

n0_3sc
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I have a graph of "dBm vs Wavelength".
I want the total Power in a certain area of the graph.

Do I convert the graph to:
"Power (Watts) vs Wavelength" then just Integrate within the required wavelength region?
ie.
[tex]\int_{\lambda_1}^{\lambda_2} Power d\lambda = Total Power[/tex]
 
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The way you described, you'll get some quantity with dimensions "power x length", which is not "power". If you had a graph of "Power per unit wavelength vs Wavelength", then you could integrate and obtain total power in the region of your choice. For example, see Planck's law in Wikipedia or somewhere else.
 
n0_3sc said:
I have a graph of "dBm vs Wavelength".
You have not sufficiently described the quantity on the y-axis. Besides, are you sure it's not in units of dBm/Hz or somesuch? It is meaningless to speak of power at a given wavelength or frequency.
 
The y-axis is a power measurement. It is in decibels with respect to a milliwatt (dBm).
Yes it is meaningless to talk of power per nm...

I realized now that it just needs to be some form of energy unit because it will be used as a ratio with other quantity's of the same units thus the units end up cancelling.
 

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