The “I” in IQ stands for “in-phase” and the “Q” stands for “quadrature.” IQ mixers allow the two frequency sidebands to be handled separately using quadrature modulation where phase becomes a variable in data transmission. Quadrature modulation ultimately doubles the information content in a double-sideband transmission since each sideband can contain different information. The block diagram of an IQ mixer is shown in Figure 10:

Figure 10: Block diagram of an IQ mixer.

An IQ mixer comprises two mixers, each with the LO phase shifted by 90° from the other with a hybrid coupler. One mixer handles the in-phase LO component, and the other handles the quadrature component. The I and Q output signals are baseband signals combined into an RF signal for transmission. At the receiver, the process is reversed with the RF signal separated back into 1 and Q baseband channels. Any of the unbalanced or balanced mixer topologies discussed here can be used to create the IQ mixer configuration.

Figure 11: Block diagram of an image reject / single sideband mixer.

An IR/SSB mixer is an IQ mixer with an additional hybrid coupler that is fed by the I and Q ports. The purpose of this additional coupler is to cancel one of the sidebands’ ports with a terminated load. The remaining coupler port is used as the output (IR / receive) or input (SSB / transmit) of the mixer. Any of the unbalanced or balanced mixer topologies discussed above can be used to create an IR / SSB mixer configuration.

Read more about IQ mixers and their uses in single sideband (SSB) and image rejection applications in our related blog post I&Q Mixers, Image Reject Down-Conversion & Single Sideband (SSB) Up-Conversion.