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.
Best Answer
This suggests that you deem diodes to be active devices, so presumably a passive device means no diodes. In that case, a structure that's passive will not yield down-conversion for you. A mixer requires a non-linearity, like a diode, in order to function.
The important part of a mixer is the non-linearity which does the down-conversion, not the phase-shifting or adding networks like rat-races that give the mixer other useful properties like balancing some of the signals to specific ports.
This can be seen from the fact that you can make a mixer from a single diode, shunted across a pair of terminals. Apply two signals, one of which should be of adequate amplitude, and sum and difference frequencies appear. It's not a particularly convenient mixer, needing filters to separate the signals. Adding transformers, baluns, transmission lines, simplifies the signal application and extraction, and adds ports with inter-port isolation, but it's the diode or diodes that do the work.