My understanding is that RF mixers generally use some sort of active device (sometimes diodes) to produce output rich in harmonics, and rat-race couplers can be used to get the sum or difference between two frequencies.
Would a solely passive microstrip rat-race coupler fit my application of 5.8 GHz FMCW radar downconvertion? I'm only interested in the difference between two RF signals that are very close in frequency.
Additionally, I've seen examples(1) of radar systems that use a rat-race structure in conjunction with diodes as part of the mixer stage, what might be the reason for that?
(1) Here is a screenshot from this youtube video. Two diodes can be seen around the rat-race coupler.