What is a double-balanced mixer/modulator, and how do I connect it to the PN sequence generator?

A double-balanced mixer is a passive device that multiplies signals applied to two of its port and outputs the result on its third port. It is a conceptually simple device, consisting of only two tiny transformers and four diodes connected in a cathode-to-anode ring configuration. The fascinating thing about double-balanced mixers is that you can choose any two of the ports to be inputs, and the third automatically assumes the role of output. However, double-balanced mixers are optimized to work best when connected in a specific way. As a mixer, the two transformer-coupled ports are typically designed to be driven by the RF and LO (local oscillator), with the IF (intermediate frequency) output taken from the diode-connected port. (By "diode-connected" we mean the port that connects to the diodes without transformer isolation.) As a digital modulator, one of the transformer-coupled ports is driven by the RF carrier, the output is taken from the other transformer-coupled port, and the ...  more