Radiocommunication requires that we shift a baseband information signal to a frequency or frequencies suitable for electromagnetic propagation to the desired destination. At the destination, we reverse this process, shifting the received radiofrequency signal back to baseband to allow the recovery of the information it contains. This frequency-shifting function is traditionally known as mixing; the stages that perform it, as mixers. Any device that exhibits amplitude-nonlinear behavior can serve as a mixer, for example, as we saw in Section 1.7.2, nonlinear distortion results in the production, from the signals present at the input of a device, of signals at new frequencies. Even a rusty screw or bolt on an antenna element can act as a mixer, producing unwanted IMD products that appear at the receiver input.
Although mixers are equally important in wireless transmission and reception, traditional mixer terminology favors the receiving case because mixing was first applied as such in receiving applications. Thus, the signal to be frequency-shifted is applied to the mixer's RF port, and the frequency-shifting power or voltage (from a local oscillator [LO]) is applied to the mixer's LO port, resulting in two outputs at the mixer's intermediate-frequency (IF) port. If the wanted IF is lower in frequency than the RF signal, the mixer is a downconverter; if the wanted IF is higher than the RF, the mixer is an upconverter. Converter may also be ...