dbm value increase by factor of ten?

Discussion in 'Homework Help' started by kee23, Sep 23, 2012.

1. kee23 Thread Starter New Member

Feb 1, 2012
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0
Hello,

There was this lab question asking that it says "the dbm value increased by a factor of ten, explain why using math"

I don't know what it asks me to do, but i'm guessing that whatever the power I have came up with say 25mW and the dbm came up with 14dbm, but somehow i have to make it increase factor of ten?

I don't know how....

2. WBahn Moderator

Mar 31, 2012
18,085
4,917
It's a bit of a strange question. If the original power was 1mW, then that would have been 0dBm. What would it mean for the dDm value to increase by a factor of ten in that case? Of if the original power had been 100μW, you would have started out at -10dBm. What would an 'increase' by a factor of ten mean then?

It sounds like a question that must be very specific to the lab that was performed. For instance, you did a lab and took some measurement and got 2dBm. Then you made some changes and took the same measurements and got 20dBm. You are then supposed to show how the changes you made result in that change in the observed change in measured dBm.

I seems like it would make a lot more sense to have the power change by a factor of ten and then explain mathematically why that resulted in the measured value changing by +10dBm.

3. vk6zgo Active Member

Jul 21, 2012
677
85
Unless you tell us what the rest of the question looks like,we are going to be battling to help much.

From the dB formula: dB=10 logP1/P2,every time you increase power by a factor of 10,the new level is +10dB w.r.t. the original power level.

(log10=1,so 10 log10 =10).

0dBm =1mW,so +10dBm =10mW,+20dBm=100mW,& so on.

It seems to me that they are saying the original power level has been
increased by a factor of 10,which in turn means it has been increased by 10dB,so that:-
If the original was,for example,0dBm,it would now be+10dBm.

If your figure of 25mW (+14dBm) is the result you get after the increase.the original level must have been 2.5mW,or (+4 dBm).

If +14 dBm is the original level,an increase of 10dB will give you +24dB,or 250mW.

If the lab question was set up using real instruments,the level meter may readout in dBm,which would account for the slightly strange phrasing of the sentence.

4. WBahn Moderator

Mar 31, 2012
18,085
4,917
Nah, I think it takes more than that to account for the phrasing (but I sure don't know what!).

As written, it sounds like it was developed by someone that was extremely niave or inexperienced. An increase in a factor of ten in the dBm reading has no concrete meaning whatsoever. It's a very different thing going from 1dBm to 10dBm as going from 1.5dBm to 15dBm. Like you, I suspect them are trying to get at either a 10dBm increase or a factor of ten in the actual power, but don't understand the concepts well enough to phrase it correctly.

It would certainly help to have more details about the lab and measurements that were made about which the question is being asked.

5. vk6zgo Active Member

Jul 21, 2012
677
85
Yep! I see what you mean!

An increase in the meter reading by "a factor of 10" from +1dBm to +10 dBm, is an increase of 9dB,whereas a change in the reading of the same "factor" from +1.5dBm to +15dBm is an increase of 10dB.**