Help Understanding Mixing Circuits?

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http://forum.allaboutcircuits.com – This has been driving me crazy. I'm learning about communications circuits, more specifically mixers and modulators. It's been a bumpy road, since I'm having a lot of trouble grasping some of the core concepts, such as sideband energy. But now I would just like to gain some understanding about nonlinear components' role in mixer circuits. All the sources I've seen so far state simply that 'nonlinear components create sum and difference frequencies whenever two signals are applied to them.' But that's not much help, because: 1) Why is it only nonlinear components? And 2) How come a componen (Embedded)