[G0HZU_JMR]

I've not used the Siglent so I don't know what to expect from it. The same applies to your sig gens and combiner.

If we go back maybe 45 years, then I think there were various design rules in place for the design of a lab spectrum analyser. If you were to take the 1500MHz HP8568B apart, you would see that the front end is entirely passive up until after the second mixer. The first amplifier in the system is at the second IF.

I think this was done to try and maintain predictable distortion performance. In other words, the two-tone distortion performance at the front end should be dominated by the first mixer for fairly well spaced test tones. The second mixer will have similar TOI as the front end mixer but it is protected by the conversion loss of the first mixer and any additional loss in the IF1 post mixer attenuator and the first IF filter. This makes sure that it obeys the >-20dB IMD distortion in the second mixer compared to the distortion in the first mixer.

This makes these analysers easy to understand with fairly predictable distortion performance. Modern analysers like the Rigol and Siglent will have solid state switches and solid state attenuators ahead of the first mixer. There will also be an amplifier at the first IF. This means that all of these devices can (in theory at least) contribute a tiny amount to the overall distortion seen on the display.

It's fairly easy to predict the HP8568B TOI and noise floor performance. I first looked at the design of this analyser over 30 years ago. I've just had a quick go at designing something similar to the HP8568B front end using excel. I've used a RBW of 10kHz and assumed that the default 10dB attenuation is at the front end.

You can see that the spreadsheet predicts an input IP3 (TOI) of 23dBm, a noise floor of -100dBm and a spurious free dynamic range of 82dB. See also a chart from the user manual that shows that the HP8568B has a typical SFDR of 82dB at 10kHz RBW. This agees very well with the spreadsheet.

It's 30 years since I studied the HP8568B design in any detail, but I was tasked to do this at work so I could learn how the frequency planning was done and how the front-end design was optimised for low distortion. The line entries in the spreadsheet are done from memory but they should at least be in the right ballpark for the HP8568B up to the 2nd IF at least.