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I think my problem is more to do with working out the origins and derivations of the linear equations from the first principles in the graphs.
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Another way to look at this is to forget about the TOI point and just inject two clean distortion free tones into the analyser. Set the tone level really low to start with. Choose a convenient RBW, say 10kHz and set the front-end attenuator to 10dB and the reference level to -5dBm. On your Siglent analyser, I think that the noise floor (DANL) would be at about -90dBm with these settings if you have the preamp turned off.
In theory, you should be able to keep increasing the input tone levels up to a point where the IMD tones would have the same power as the displayed -90dBm DANL. In practice, it will be difficult to judge where this happens, you could briefly select a narrower RBW to make it easier to see the -90dBm IMD tone level as it begins to compete with the DANL and become visible.
When you reach this point, the difference between the input test tone levels and the -90dBm DANL is the spurious free dynamic range with a 10kHz RBW.
You would obviously have to set the analyser to a span where the phase noise of the analyser doesn't mask the DANL. Maybe try a 5MHz span with 1MHz tone spacing to try and minimise the impact of the LO phase noise. Set the RBW to 10kHz. I'm guessing that your analyser phase noise will be -120dBc/Hz at 1MHz offset. 10kHz RBW is 40dBHz so you should be able to see about an 80dB dynamic range if there was no IMD. However, I think the Siglent will probably have a SFDR of about 73dB with 10kHz RBW because of the IMD it will generate.
Alternatively, if you know the input TOI of the analyser and the DANL from the datasheet, you can predict the SFDR with a reasonable amount of accuracy without having to do any hardware tests. Just use the equations you already have. The equations allow you to work backwards from the published TOI and noise floor data to work out the SFDR at a given RBW setting.
This generally works well for classic spectrum analysers that use a diode pair or diode ring mixer at the front end. I don't know if the Rigol and Siglent analysers will follow the same classic equations. There are solid state devices in the front end of these analysers and the linearity of these devices probably won't be as predictable. Therefore, the tests outlined above might not give the expected results.