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There are many examples of IQ mixers on the internet; all use 90 degree hybrids to split the LO to the mixers with the outputs being combined in-phase. I'm curious, could both mixers be fed an in-phase LO and a 90 degree hybrid used to combine the signals from the two mixers instead, in a sort of reversed IQ mixer?
Is such a thing even possible? And if not, why?
Yes, this will work.
In fact, that is nothing but emulating an ADC with twice the sampling rate by using two synchronous ADC of the original rate, delaying one branch of the signal by half a sample clock.
Sure, this will perform the same function as a 90 deg hybrid -> mixers -> combiner. The diagram you show will perform the rejection in the hybrid, while the other arrangement would perform the rejection in the mixers and combiner. You may be able to achieve better conversion loss / bandwidth / isolation in one vs the other, since components may be easier or harder to design at the RF/IF frequency. Sadly I don't know any rules of thumb on their behavior. Anyone who does please contribute. A third conversion arrangement feeds each mixer with LOs 90 degs out of phase. Others with multiple 90 deg hybrids exist as well. (Maybe I'll do the math when I get some free time...)
A reverse image search shows the source of the image with a detailed discussion of this topology, although doesn't do much comparison.
