Just wanted to give my 2c in regard to IMD/IP3 measurements as I did and continue doing lots of them at my work.
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30-40dBm IP3/OIP3 measured levels are not that high. One just need a good "Big Box" Spectrum Analyzer (HP/Agilent/Keysight) with good LowPIM attenuator in front of it. No Bandstop/notch filters are really needed! Signal Combiner can produce some IMDs but usually they are pretty low for those levels of desired measured IP3. Bandpass filters for F1 and F2 are also not needed along with attenuators between those filters and Combiner. Actually if filters are used I'd recommend increasing the attenuators to 10dB each. Instead of Bandpass filters normally we would use an Isolator (circulators with termination on top). But.... that's mostly for VHF+ frequencies. Not sure if proper isolators do even exist for LF/MW/HF frequencies. So, filters may be the best F1/F2 solution before Combiner.
Now, my main work IP3 bench is capable of measuring IP3/OIP3 levels of up to 90-100dBm with ~2W of F1 and F2 each... That's pretty much measuring IP3 of passive components where everything matter including connector cleanliness and proper torque (sometimes overtorquing helps!). At those levels you cannot use lump components for filters and combiners and cavity stuff is the only thing that worked for me. For example I'm using celltower cavity diplexers/duplexers for F1 and F2 mixing and a big double cavity rejection filter in front of the Spectrum Analyzer (tuned to reject CW at F1 and F2 with rejection of close to 100dB). Big negative of such setups is inflexibility. You need to swap bench components to change the frequency or have multiply setups for each frequency of interest. And of course LF-HF and cavity filters probably cannot coexist due to the size :) But any of that is not needed when you measure amplifiers.
In regard to TinySA Ultra. It's a great thing but would be my #1 suspect for creating the intermods on it's own. So, with TinySA one indeed may need rejection filters in front of it... If anything I'd probably use an SDR with good front end for the task like that instead of TinySA.
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Thanks,
Simon
