Electronic – How to measure a fast-varying RF power in a circuit

Caveat: I have not tried this myself -- by the time you get this to work you will know more about it than I do.

Instead of putting your IF at DC, put it at some fixed frequency, say 1 MHz. Now you can run the signal through a a narrow bandpass filter to select out just the IF signal you want without worrying about DC drift issues. Connect the filtered signal to an rf detector (see also here and here). Now low-pass filter and amplify the detector output and use an ADC to measure the signal. This output will (within a certain range of input power) be proportional to the signal power you are trying to measure.

Obviously there's more to it than that; if there wasn't Agilent, Anritsu, and R&S wouldn't be able to charge 6 figures for a spectrum analyzer. But this ought to give you something to experiment with.

Edit

After consulting this book I realize that a 1 MHz IF is probably not a good idea. It is preferable to use an IF outside the input band of the analyzer, to facilitate image rejection. For example, if you were mixing a 100 MHz signal down to 1 MHz using a 101 MHz LO, you'd also get any 102 MHz signals mixed down into your IF band. In the early-70's spectrum analyzer architecture discussed in the book they actually use 3 or 4 stages of up- and down-mixing to different IF's to achieve different filtering requirements. Their first stage mixes up above the input band to enable the best image rejection.

I've tried a similar setup and I've realized that you're using a very small sweep time (500 us). Your transient, that seems so wide in the picture, last only about 30 us. When using a quasi-peak detector it will be certainly removed.

I can show you my pictures. In my setup I've used a 10 dBm signal, 868 MHz central frequency, and an offset of 200 kHz when looking for the transient. In all the figures RBW is 120 kHz and VBW is 100 kHz.

Here it is the zero-span picture, no PA ramp is used:

My sweep time is 5 s. The pulse is then about 0.8 seconds wide, and the transient is less than a few ms (didn't measure it accurately).

When using a Quasi-Peak detector I get this:

If I repeat both measurement after activating PA ramp in the transmitter, the transient peak disappears:

And using a Quasi-Peak detector:

Conclusions:

Your phase noise is in fact fine, and so are the transients. The fact that the transient doesn't change when enabling PA ramp seems to indicate that there is some kind of implementation error with this feature. Anyway your transient will strongly smooth out under a Quasi-Peak detection.