A Mixer is a device that translates a signal in frequency by combining the incident signal with a Local Oscillator (LO) signal to create an output signal at either the sum (fin + fLO) or the difference (fin - fLO or fLO - fin) of the input signals.
Mixers may be either passive or active. Passive mixers are based on un-biased semiconductors, usually diodes. In this case all of the energy required to create the output signal comes from the LO signal. Most passive mixers have a conversion loss on the order of 7 dB, a noise figure on the order of 7 dB, and an intercept point dependant on the amount of LO drive provided; LO level is commonly between 0 to +17 dBm for this kind of mixer. The main advantage of a passive mixer is the potential for high linearity. The main disadvantage is the requirement for a relatively high LO drive level.
Active mixers are based on biased semiconductors, such as transistors. In an active mixer, some of the energy used to create the output signal come from DC bias. Active mixers commonly have conversion gain (0 to 14 dB), and vary widely in noise figure (sub dB to >15 dB). The intercept point is more a function of the active device than of the LO drive level, and in fact the LO power required is often quite small - commonly -10 to 0 dBm. The main advantages of active mixers are the potential for gain and the potential for a low noise figure. The main drawback is typically reduced linearity when compared to a passive mixer.
The simplest mixer is the unbalanced mixer, which in the most basic form consists of a single semiconductor junction. The non-linearities in the junction create intermodulation distortion products between the incident signal and the LO, including strong signals at fin + fLO and either fin - fLO or fLO - fin. This structure provides no isolation between the input signal or the LO signal and the output, so these signals also appear strongly in the output spectrum. Output filtering is almost always a requirement for unbalanced mixers.
A single-balancd mixer adds a second junction anti-parallel with the first. In this configuration the LO signal can be separated from the incident and output signals, reducing somewhat the filtering requirements. A double balanced mixer uses a quad of junctions, and can provide quite good isolation between the three mixer ports. A double-balance mixer can also be constructed from transistors arranged into a structure called a Gilbert cell. A further step is to build an image reject mixer, where the un-wanted mixing product is also surpressed.