[Radio Wrangler]

Without up conversion and wanting general frequency coverage down to almost DC, you wind up with the LO crossing the IF frequency and giving you a monster spur somewhere in your frequency coverage. Trying to isolate LO to IF to the required extent is impossible.

With the up conversion scheme, you still get the spur, but it is located precisely at 0Hz. Everyone using spectrum analysers knows it's there, but not all know why. You could looka at it as a true response to DC, in this case a response to DC ofset/imbalance in the mixer, which is one component in the leakage path, but not the only one.

David