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Off-Topic Discussion => General Discussion => Topic started by: Ashrak on February 13, 2007, 03:36:47 pm

Title: okhay, wireless technology class etc need help with math
Post by: Ashrak on February 13, 2007, 03:36:47 pm

1. 70 mW + 0 dBm = ? mW (71 mW or 70mW)
2. 3 dBm + 30 dBm = ? mW (probly either 2000mW or 1001.9mW)

need the answers to these two questions, the problem is i dont know which form to add, do i convert to mW and then add em together or do i convert them to dBm and add them together, the awfully **** part is, the answers are different on which way you do it.


any clues? i dont have any left :(

Title: Re: okhay, wireless technology class etc need help with math
Post by: fsi.scsi on February 13, 2007, 05:16:19 pm

First of all, this probably ain't the right forum for your question... maybe a physics forum? :)

I'll try and help you out. 

First, see this link: http://en.wikipedia.org/wiki/DBm (http://en.wikipedia.org/wiki/DBm)

0 dBm = 1 mW as per the equation dB = 10 log(P/Po).  I'd use that equation as a conversion factor, but if you are especially lazy, see the table. 
Oh, and don't take the table data as a straight direct relationship.  Intensity and power are logarithmically related.

EDIT: Maybe it's just me, but just plain adding power levels seems like pointless busy work.

Title: Re: okhay, wireless technology class etc need help with math
Post by: Ashrak on February 14, 2007, 01:23:52 am

Quote from: fsi.scsi on February 13, 2007, 05:16:19 pm

First of all, this probably ain't the right forum for your question... maybe a physics forum? :)

I'll try and help you out. 

First, see this link: http://en.wikipedia.org/wiki/DBm (http://en.wikipedia.org/wiki/DBm)

0 dBm = 1 mW as per the equation dB = 10 log(P/Po).  I'd use that equation as a conversion factor, but if you are especially lazy, see the table. 
Oh, and don't take the table data as a straight direct relationship.  Intensity and power are logarithmically related.

EDIT: Maybe it's just me, but just plain adding power levels seems like pointless busy work.

that really dosent help me :(

i just want to know which units i need to add is it dBm or do i convert to mW and then add?!

Title: Re: okhay, wireless technology class etc need help with math
Post by: Col. Fishguts on February 14, 2007, 03:00:38 am

IIRC, you can go both ways. Just convert everything to the same units and then sum up.

Title: Re: okhay, wireless technology class etc need help with math
Post by: Wanderer on February 14, 2007, 03:06:50 am

Summing dBm (logarithmic) values?

I would convert all to mW and then calculate with those. Then again i haven't touched those for ages.

Title: Re: okhay, wireless technology class etc need help with math
Post by: Col. Fishguts on February 14, 2007, 08:49:57 am

Quote from: Wanderer on February 14, 2007, 03:06:50 am

Summing dBm (logarithmic) values?

Yes, that's where the fun in working with dBs lies. When you have everything in the same dB scale, you can simply sum them up along the transmission line (for example) you're going.

Title: Re: okhay, wireless technology class etc need help with math
Post by: fsi.scsi on February 14, 2007, 10:35:12 am

Ashrak:

70 mW + 0 dBm = 71 mW because 0 dBm = 1 mW. :)

3 dBm + 30 dBm = 1.9953 mW + 1000.000 mW = 1001.9953 mW

I used the logarithmic conversion factor.  And my answers are backed up by this site: http://www.aubraux.com/design/dbm-to-milli-watts-table.php (http://www.aubraux.com/design/dbm-to-milli-watts-table.php)