[G0HZU_JMR]

One thing to bear in mind is that the Rigol spectrum analyser in that blog demo is being asked to cope with all of the mixer terms being produced at the mixer output port. This will include the two wanted test tones and also the two image tones and quite a few other mixer terms. This can be risky, especially if the analyser is being used close to its own distortion limits.

I see he's used the IF port as the output port even though the system is an up-conversion.

The usual aim is to try and keep the analyser IMD3 terms at least 20dB lower than the predicted IMD3 terms from the device under test. This will keep the uncertainty (caused by the analyser's own IMD) below about 1dB.

If you plan to do lots of mixer testing with an IF of 10.7MHz I'd recommend you make a narrowband diplexer at 10.7MHz. This will correctly terminate the mixer from DC up through VHF and it will also provide a bandpass response at 10.7MHz.

The diplexer consists of two inductors, two capacitors and two 50 ohm resistors so it is fairly quick and easy to make. It would protect the analyser from all the other mixer products arriving at the IF port. A typical 10.7MHz narrowband diplexer would have a response as in the plot below: