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The diode ring mixer is the nice one, much more easy to understand than the others. The others have all sorts of foibles and difficulties that you have to get your head around. It's one of a class called 'double balanced' mixers. An unbalanced mixer leaks plenty of the RF and LO inputs out with the IF products. A single balanced mixer acts to null one of them, and you get some compensation of non-linearity. A double balanced mixer acts to null both leakage paths, and gives a bit better compensation of non-linearity.
It is routine to drive diode ring mixers very hard with lots of LO signal, and then you can view, for a first understanding, the diodes as being turned hard on and hard off by the domineering LO. So the mixer becomes an inverting switch to the RF to IF path, controlled by the LO. Mathematically, the RF signal gets multiplied by +1 or -1 and you have a switching mixer.
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So the goal is suppress both the strong LO and RF as much as possible, leaving mostly IF in the output. So linearity is not a bad thing right? You want more linearity in a non-linear mixing device. I guess the input and output baluns are simple broadband impedance matching transformers optimised for minimum core losses. It looks so much like a full wave rectifier circuit. (Lately I am interested in the design of bandboard balun or unun because i do not know if it is possible to have a 50 ohms input matched to a high impedance LC resonance (10k-100k). I only see 64:1 balun and never any higher ratio. I guess any higher ratio will introduce unacceptable insertion losses.
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Intermods, don't stop at 2nd order harmonics. The excrement hits the fan when you consider the 3rd harmonic of one signal mixing with the second of the other and the two spurs created flank the original signals at the same spacing which puts them in the right place for maximum trouble. 4th and 3rd put spurs one more spacing out on either side of those.... and so on. It all gets very messy very quickly and the only escape is to try to make things as linear as possible and to run the signal levels low enough for the spurs to be lost in the noise.
The diode ring mixer is rare in domestic stuff because you get about 6 dB loss in them, while transistor and valve mixers can give a bit of gain. So a diode ring usually means you need an extra stage of gain along the line, but they do tend to out-perform active mixers in aareas harder to fix than mere lack of gain. It all comes down to pennies. A couple of American FM tuners had diode ring mixers.
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I like your figurative language. D. There seem to be differences in "nastiness" between even- and odd-order intermodulation products. Did folklore said odd numbers are dodgy, evens are good?
The reason I thought about ring diode mixer is because i saw his SSB homebrew rig a few days ago and I was wondering what advantages it offers:
https://www.youtube.com/watch?v=w0baCsMlZEA&t=67s