[Radio Wrangler]

Just a few odd comments.

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.

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.

David