This would be for a fairly decent RX SDR I have (QS1R, a true DDC radio) to connect to my Ft-897D.
Why do these kits tap off the IF?  why not the main RX stage, same side as the RX before the antenna switch?
Why would you tap off the IF? and then, *which* IF would be "better"?

The reason I ask is the QS1R offers 50MHz of spectrum - I dont believe that much bandwidth will be available on an IF tap?

EDIT:  OK I just figured out that what I described is called "2nd receiver" where you tap just before the 1st IF mixer.  But they claim this caveat:
Why must you tune both independently?  Does rigcat not solve that?

Depends on the radio. For example, the IC-7300 is a Direct Sampling Receiver, so by necessity is tapped just after the input tuned circuits, while the K3 is a superhet and samples ahead of the 1^st IF filters.

ok right that all makes sense and lines up with the "2nd receiver" method.  but why do they have that caveat on
why wouldnt omnicat handle this for you?  what is it about the IF method here that makes is special in that manner?

prior to SDR radios, superhets were the standard.

Frequencies were mixed to an IF freq for filtering, processing etc.

When everything is converted to one IF freq, it's easiest to process.

this is why IF filters work so well, and why IF DSP works better than AF DSP

.

It's just easier this way. Your panadapter will always be at the center of your radio's frequency, regardless of which band (or frequency) the radio operates on. Also you don't have to link your radio and the SDR receiver to sync. the operating (VFO) frequency.

Some radios, like a Yaesu FTDX-3000, have a special 'RX ANT' jack in the back.It's perfect for connecting the radio to an external SDR receiver. You can download some software to link the radio with the SRD so they both follow the same VFO frequency. The 'RX ANT' output is switched through an internal relay to protect the SRD when the radio is transmitting. You can build a similar interposing relay yourself to make other radios work in the same manner, as long as the radio can be linked to the SDR through a USB or a data port. Obviously, older radio may not have this option.