It slowly occurred to me that I am doing this still too complicated and there is an even simpler measurement approach. (if this is too much details for this group feel free to shut me up)
Using only one clean signal (harmonics really low) followed by a step attenuator it should be possible to observe the harmonics generated inside the tinySA and be able to predict everything what happens and calculate the IIP3 and SFDR
So I took a regular tinySA (no "special" this time) and started with a powerlevel calibrated signal at about -4dBm at about 9MHz.
This is how it looks with 0dB external attenuation
2nd harmonic at -54dBc and 3rd harmonic at -63dBc.
Calculating the IIP3 gives (-4 + (-4 - -63)/2) = +25dBm
Reducing the input with 10dB (using the external step attenuator) implies the 2nd harmonic should move to -64dBc and the 3rd to -83dBc and the IIP3 should stay the same
2nd order moves to -66dBc (seems ok) and 3rd order moves down 30dB so into the noise floor.?Let's pessimistically assume it is at -98dB so -84dBc. All looks good.
Calculating the IIP3 gives (-14 + ( -14 - -84)/2 ) = +21dBm which is a bit less so my assumption about the level of the 3rd harmonic may be too pessimistic.
Further reducing the input to -24dBm gives:
The RBW was reduced to 10kHz otherwise the 2nd harmonic would be invisible. Its now at -100dBm thus -76dBc which is indeed again 10dB better as it should be.
With the RBW at 30kHz this would have been invisible so the SFDR with an RBW of 30kHz seems to be 76dB.
In other words:?The internally generated intermodulation products of on or more tones at -24dBm or lower will no longer be visible at an RBW of 30kHz
So much to learn.
