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To illustrate how confusing and muddy it can get, the RS white paper in the following link states that the two tones at f1 and f2 of frequency apart delta_f (=f2-f1) will produce 3rd order IMD products at f1-delta _f and f1+delta_f. What the heck? Everywhere i have seen the 3rd order IMD should be at 2f1-f2 and 2f2-f1(attached):
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The equations look different, but they should give the same result.
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I struggle with the idea that when you have two phase uncorrelated sources enter the mixer, somehow there will be constructive and destructive interference. I can easily understand it in time domain when two tones of different frequencies can have constructive and destructive interference along the nodes, but I cannot visualize the same in the mixer within the frequency domain.
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Sig gen tones f1 and f2 enter the DUT and produce IMD tones at 2f1-f2 (IMDtoneA) and 2f2-f1 (IMDtoneB).
However, f1 and f2 then enter the analyser and the analyser will also produce its own 2f1-f2 (IMDtoneA) and 2f2-f1 (IMDtoneB) distortion tones.
IMDtoneA therefore gets injected into the analyser from the DUT and the analyser also generates its own IMDtoneA through its own internal distortion.
These two versions of IMDtoneA will often not have exactly the same phase. Usually in a spectrum analyser, they are very close to being in phase. This is especially the case if the IMD testing is done with closely spaced test tones f1 and f2.
If the two versions of IMDtoneA are the same amplitude and they are in phase then they will sum in voltage to give twice the voltage. This means the analyser will report the level of IMDtoneA to be about 6dB higher than the level of IMDtoneA in the DUT. This is seen on the blue curve on the right-hand side of the graph below.
It can happen that the two IMDtoneA terms are not in phase, so in theory at least, they can cancel, and the analyser will report the level of IMDtoneA to be lower than the level of IMDtoneA in the DUT. This is the red curve. The phase could be anywhere in-between these two cases.
If you look at the graph below, it shows the measurement uncertainty caused by the relative amplitude and phase of 'DUT IMDtoneA' and 'Analyser IMDtoneA' inside the analyser. The green arrow and text shows that it is desirable to keep the analyser generation of IMDtoneA to be at least 20dB lower that the DUT generation of IMDtoneA. This keeps the measurement uncertainty displayed on the analyser to below +/- 1dB. Most RF engineers will be well aware of this graph and it's a good idea for anyone who uses a spectrum analyser to be aware of it too.
The same graph can be found in app notes by HP/Agilent/Keysight for example.