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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?

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The actual measurement of Spectrum Analyzers is "power / filter bandwidth".

Your spectrum analyzer has a filter bandwidth that you can typically configure. For example, if your filter bandwidth is 10 kHz and you measure -12dBm, that's -12 dBm/10 kHz = -12 dBm - 40 dBHz = -52 dBm/Hz.

why is it frequency dependent?

Because you're using a spectrum analyzer!

The SA's job is to tune through your band of interest: stop at a frequency for a period of time, measure what power is coming through a filter, tune to the next frequency, measure, repeat. That way, you get a Power Spectral Density estimate for different frequencies.

I don't which bandwidth you're currently using, but it's probably less than your signal bandwidth, and hence, you don't measure all the energy, but just a part of it.

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  • \$\begingroup\$ Ah! Is filter bandwidth the same as either resolution bandwidth (100kHz) or video bandwidth (10kHz)? Otherwise I wonder how to obtain it ... In case of ResBW: Ptot = -12 dBm - 50 + 73 dBHz = -2 dBm would make more sense ... If I've a 20 MHz signal centered at 2.1 GHz (LTE) then I guess I should be able to see/measure the total power (assuming no other harmonics/bands) \$\endgroup\$
    – divB
    Commented Jul 14, 2016 at 0:29
  • \$\begingroup\$ ... or is it "Analysis bandwidth"? That would be 25 MHz in my case. So -12dBm-74dBHz+73dBHz = -12 dBm? \$\endgroup\$
    – divB
    Commented Jul 14, 2016 at 0:51
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    \$\begingroup\$ 'Filter Bandwidth' as Marcus describes is more commonly referred to as the 'resolution bandwidth'. The video bandwidth setting controls how noise the signal will appear on screen. \$\endgroup\$
    – Andrew W.
    Commented Jul 14, 2016 at 1:23
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    \$\begingroup\$ The 'noise bandwidth' of the resolution filter is what you want. Usually, this can be approximated by the resolution bandwidth or filter bandwidth of the filter with negligible error. If the SA has the quality and stability to actually distinguish meaningfully between the two slightly different values, then you will find a figure for noise bandwidth available somewhere, on screen on the analyser behind a 'filter properties' tab or similar, or tabulated in its manual. If it has a dBm/rt(Hz) marker, then it will use the noise rather than resolution bandwidth internally for scaling it. \$\endgroup\$
    – Neil_UK
    Commented Jul 14, 2016 at 5:54

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