Theory and Math

Mixers, directional couplers, bartlett's bisection theorem, constant current sources, PLL and an excellent introduction to HP's ADS.

A really slick design guide for all sorts of filters. Includes DSP filters.

This is a basic introduction to the concepts of charge, voltage, and current. Discusses Coulomb's Law, Ohm's Law, and power. Includes graphics and animations.

Quite a bit of theory, practical advice, math and diagrams, from basic electronics theory to very advanced physics.