IQ, Image Reject, and Single-Sideband Mixers

Ed Note: Since this original post much of this content has been updated and combined into our IQ, Image Reject, and Single Sideband Mixer Primer. Check it out!

IQ and Image Reject or Single Sideband mixers use similar circuitry to solve two different fundamental problems in communications and signal processing. IQ mixers address the problem of maximizing information transmission by allowing the user to modulate both the in-phase and quadrature components of a carrier simultaneously, multiplexing two signals onto the carrier*. Image Reject (IR) mixers allow a user to select a signal in a crowded signal environment while suppressing the adjacent image signal, relaxing receiver filtering requirements. Single Sideband (SSB) mixers allow a user to upconvert a signal onto a carrier while suppressing the same image frequency signal, relaxing transmitter filter requirements.

By using quadrature hybrid couplers to manipulate the phase of a signal during conversion, IQ, Image Reject (IR) and Single-Sideband (SSB) mixers enable advanced signal processing capabilities. An IQ mixer allows a system to send twice the information content in a double sideband transmission without using any more bandwidth by using ‘quadrature’ modulation. An IR mixer allows the selection of only one of either the LO + IF frequency or the LO – IF frequency while rejecting the other ‘image’ frequency. This allows for unambiguous signal detection in a crowded signal environment without a tunable RF filter. The SSB mixer is the same structure as the IR mixer and allows the upconversion of an IF signal to only one of the LO + IF or LO – IF frequencies without creating an image of the signal at the other frequency. These mixers are particularly useful when using low IF frequencies, making preselection filtering difficult.

IQ Mixer Operation and Structure

The IQ mixer is the basic building block for higher order IR and SSB mixers. The structure is shown below:

Block Diagram Structure of an IQ mixer

An IQ mixer is actually two mixers with an LO that is shifted by a quadrature hybrid coupler by 90° to each other. In a normal downconversion the phase of the LO is irrelevant because the data is single sided. The IQ mixer requires transmission of both sidebands for accurate reconstruction of the original signal.

An IQ mixer operates on the basis of quadrature modulation, two signals are upconverted into the same block of spectrum in the frequency domain using one LO with either a 0 or 90° phase shift. When the same operation is performed at the receiver in a downconversion, the addition of an extra 90° phase shift will create a complete 180° phase shift, leading to the inversion of one of the original signal components. This will cause the cancellation of one component of the signal, while the other component is reinforced. When an IQ mixer is used as a downconverter it allows both of the original data signals to be retrieved with an appropriately phased LO signal.

IR/SSB Mixer Operation and Structure