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I'm trying to learn how to properly measure output power with my spectrum analyzer. I have a transmitter datasheet that claims 15dBM typical output power with BPSK. Assuming the matching network is correctly matched to 50 Ohms, and I can configure it for constant transmit mode. Then if I just plug this transmitter into my SA should I expect to see a peak of 15dBM (well minus any cable loss) or should I expect to see half that because the source impedance is 50 Ohms so it acts like a voltage divider.

Or have I missed the mark entirely and I need to integrate the power over the entire channel width or something like that.

Data sheet is not very helpful it just says what I said above.

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up vote 9 down vote accepted

The spectrum analyzer is a 50 Ohm load. If the transmitter / matching network is specified to deliver +15dBm to a 50 Ohm load, you will measure +15dBm.

If you don't burn it out first! Please make sure that the analyzer can handle a +15dbM input. Some max out at +10dBm, for example. You may need to use an attenuator between the transmitter and the analyzer.

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The actual peak value you measure will depend on the modulation index. Look into Bessel functions for that -- at some combinations of modulation frequency and deviation the carrier will actually drop to zero and all the power will be in the sidebands. But for narrow-band modulation you will probably measure something very close to +15dBm. – Paul Feb 17 at 1:48
This is correct, as long as the spectrum analyser resolution bandwidth is set wider than the modulated signal. A good way to check is to confirm that the displayed power does not change when you change the RBW. Note that for some wide band signals (WiFi?) it is very unlikely your spectrum analyser will have enough bandwidth, and it gets much more difficult. – tomnexus Feb 22 at 2:42

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