Can you subtract db from dbm

IanP Advanced Member level 5. Click to expand I agree with IanP. I think it's clear enough. Sorry I've expressed in wrong manner. These questions sometimes leave even experienced engineers scratching their heads. Decibels are found everywhere, including power levels, voltages, reflection coefficients, noise figures, field strengths and more.

What is a decibel and how should we use it in our calculations? Azulykit Advanced Member level 4. Activity points 8, One often sees a variety of letters after dB indication various reference values. Again, beware of the comments in this thread. Much of it is in error and will just add to the confusion. Re: dbm addition subtract Azulykit said:.

First of all there are a series of mistakes in the various posts above so be careful with all of the comments this one too.

Now dBm — dBm, when converted to linear scale, becomes, you guessed it, a division! Share this: Twitter Facebook. Like this: Like Loading Leave a Reply Cancel reply Enter your comment here Fill in your details below or click an icon to log in:. Email required Address never made public. So, if you add a gain of 3 dB to a power of 10 dBm, you obtain 13 dBm.

But what you really did, is multiplying a factor of 2 with a power of 10 mW obtaining 20 mW, which is 13 dBm! So far so good, adding is much easier to do in your head than multiplying, and this makes decibels so handy.

But there is a problem: since adding decibels together correspond to multiplying the original factors, how can one add combine the power of two signals?

Well, you can't. You can't add dBm to dBm. There is no way of doing this directly in dBm, you have to convert both powers in Watts, add them together and convert them back in dBm. This is a big limitation of decibels and a common pitfall; fortunately this operation is not very common. To convert from power to amplitude and vice versa, the impedance must be entered.

If you're just converting powers or amplitudes, you can leave the impedance blank. Of course, with the calculator, we find that —2.

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