Bloody awful things. I’ve built a few simple analogue PID loops and simple analogue computers over the years and it’s an art and a half. I think the biggest pain I had was before the availability of reasonably stable log converters was trying to use matched transistor pairs to perform log operations. Took three days to work out where an error was creeping in and it was from the thermal offset voltage of the buffer opamp input. The circuit was drifting due to my desk lamp warming it up. The IC was out of spec on drift too! :(

I did succeed in producing a four quadrant multiplier in the end but required some unholy things to be done.

That and noise and the cost of precision make it almost useless compared to a digital computer now. The rise of direct sampling SDRs confirms this. Crazy dynamic range, excellent noise floor and hardly any analogue up front.

The 4QM is one of the most useful analog devices in music synthesis, where absolute precision is unnecessary. There it's called a ring modulator because it emulates the sound of a bell. It produces both the sum and the difference of the two sinewaves fed into it.

Edit: Just looked up "ring modulator" on Wikipedia. It says the name comes from the ring of diodes in the canonical circuit. I'd always heard the genesis was from the bell-like sound it made. And there are better ways to build analog multipliers than four diodes in a circle.

Similar to diode balanced mixers in RF stuff.

Yep. It's a mixer in RF terminology. Or a heterodyne. Lots of names for this thing.