Mirrors and spurs in Spectrum Analyzers

While analyzing the quality of a signal generator the SA shows a number of components next to the base frequency at 6.18MHz.. 
There are multiple causes for these components. The first obvious are harmonics generated either by the generator or internally in the SA.
A second cause is the generation of unwanted mixer products from the various LO's in the SA.
A third cause are mirrors where the quality of the IF filters is insufficient to suppress the opposite mixer output.
A real life example is this measurement




Which of the signals are real?
A common way to reduce spurs and mirrors is to wobble the intermediate frequencies of the SA and use exponential averaging to smear the energy of the unwanted signal over a wider range.
As you can see enabling this form of spur reduction does have some impact. The signals a 46MHz and 10MHz are almost gone




The IMD2 and IMD3 measurements at 12.33MHz and 18.48MHz remain at -42dB and -53dB but how to be sure these are from signal generator and not generated in the SA?
The simplest way to check is to enable some attenuation. Adding -10dB again changes the picture. The noise floor moves up 10dB. 
Most harmonics did go down as is reflected in the IMD2 and IMD3 measurement so the SA did generate most of the harmonics.




A further increase of the attenuation does not change the IMD2 and IMD3 so we can be fairly sure we are now seeing the real content of the signal from the signal generator.
The peak at 42.83MHz should be at 43.26 (=7*6.18MHz) to be a harmonic (its actually the small peak to the right). In fact it is not from the signal generator but from the PC keyboard laying in top of the coax