Converting to dBm from pwelch function

How do I convert Pxx that I get from using the pwelch to dBm. I know we can convert to dB by 10*log10(Pxx) but I was unsure how we would do it to get dBm.

Answer 1

[Wayne King]

Since 1 milliwatt is 0 dBm, you can do the following:

 10*log10(1e3*Pxx)

from the above, you see that a power of 1 milliwatt will equal 0 dBm.

or equivalently,

10*log10(Pxx)+30

[Divya Venkatesha]

Hi Wayne,

Thanks for your reply. I tried that and it doesnt seem to stack up. I have an input power level of -80 and if I add 30 to the 10log10(Pxx) I get 71dB which gives me a gain of 151dB. But I have tried using voltages and the gain should be 75dB which is also verified by simulations. So I am really not sure what units Pxx has. Any feed back will be appreciated.

Many Thanks Divya

[Wayne King]

What units is your input time series in?

[Divya Venkatesha]

My input is in dBm.