I want to obtain the signal power in dBm (RMS) as it would be displayed by a power meter using a spectrum analyzer (in my case EXA N9010A).
But one thing makes me confused: The spectrum is a plot with frequency on x-axis and y-axis is in dBm. In my case, signal bandwidth is 20 MHz. Depending where I put my the marker, I get different answers. Inband, for example -12 dBm. Out-of-band (noise floor) -80 dBm. This would suggest to me that the actual unit is dBm/Hz, not dBm. However, this makes no sense either: I would need to integrate the power across the spectrum to obtain the total RMS power. In my example, to first order: -12 dBm + 10*log10(20e6) = 61 dBm. Clearly wrong!
So which kind of power does the spectrum analyzer actually show and why is it frequency dependent?