Two-Quadrant/Four-Quadrant Analogue Multiplier
The IF101 is an analogue multiplier that can be switched between 2-quadrant and 4-quadrant operation.
It cna be used as a voltage controlled amplifier (VCVA) or as a ring modulator (RM).
http://elby-designs.com/contents/en-us/p1092_IF101_-_2Q_4Q.html
Available as an assembled Module and as a DIY project.
This Module is currently available.
$60