Overview

Mixers are 3-port active or passive devices. They are designed to yield both a sum and a difference frequency at a single output port when two distinct input frequencies are inserted into the other two ports. This process is called frequency conversion (or heterodyning), and is found in most communications gear. It is used so that we may increase or decrease a signal s frequency. The two signals inserted into the two input ports will normally be a continuous wave, produced within the radio by a local oscillator, and the incoming (for a receiver) or outgoing (for a transmitter) signal. If we want to produce an output frequency that is lower than the input signal frequency, then it is called down-conversion; if we want to produce an output signal that is at a higher frequency than the input signal, it is referred to as up-conversion. Indeed, most AM, SSB, and digital transmitters require mixers to convert up to a higher frequency for transmission into space, while superheterodyne receivers require a mixer to convert a received signal to a much lower frequency. This lower received frequency is then called the intermediate frequency (IF). Receivers must use this lower frequency signal, as it is much easier to efficiently amplify and filter with the IF stages tuned and optimized for a single, low band of frequencies, and the receiver s gain and selectivity are thus increased. The frequency conversion process within the nonlinear mixer produces the intermediate frequency by...