|Schematic symbol for a mixer|
There are three ports on a mixer,the radio frequency (RF) port, the local oscillator port (LO), and the intermediate frequency port (IF).
The RF port is where the high frequency signal is applied that you want to downconvert it, or where the high-frequency signal is output in an upconverter.
The local oscillator (LO) port is where the "power" for the mixer is injected. In this case, the power that is applied is RF, not DC like it would be in an amplifier. The LO signal is the strongest signal, and is used to turn the diodes on and off in a switching mixer (which is nine out of ten mixers). The switching action effectively reverses the path of the RF to the IF.
The IF port is where the RF signal that was modified by the LO signal is passed, and its waveform is filtered to become the IF signal.
Passive mixers use one or more diodes and rely on the non-linear relation between voltage and current to provide the multiplying element. In a passive mixer, the desired output signal is always of lower power than the input signals. Active mixers can increase the strength of the product signal. Active mixers improve isolation between the ports, but may have higher noise and more power consumption; an active mixer can be less tolerant of overload. Mixers may be built of discrete components, may be part of integrated circuits, or can be delivered as hybrid modules.
Mixers may also be classified by their topology. Unbalanced mixers allow some of both input signals to pass through to the output. A single balanced mixer is arranged so that either the local oscillator (LO) or signal input (RF) is suppressed at the output, but not both. A double balanced mixer has symmetrical paths for both inputs, so that neither input signal appears at the output, only the product (IF) signal. Double balanced mixers are more complex and require higher drive levels than unbalanced and single balanced designs. Selection of a mixer type is a trade off for a particular application.