Fairview Microwave RF Mixer
Signal Chain Discussion Series: Mixers, Upconverters, and Downconverters
Frequency translation is an essential function needed in many signal chains. Superheterodyne circuits require frequency mixing into an intermediate frequency (IF) band and from the IF to an RF band. This is done as there are practical limitations to modulation, synthesis, conversion, and mixing electronics that make directly generating RF signals impractical or otherwise undesirable. In some cases, such as with radio astronomy there may be several frequency translation stages between the baseband and the RF. Even though advances in direct digital synthesis and direct digital conversion of RF signals has led to the development of microwave monolithic integrated circuits (MMIC) and radio frequency integrated circuits (RFICs) that can directly convert/synthesis RF signals to several gigahertz, applications with operating frequencies beyond a few gigahertz still often require frequency translation. Moreover, the performance limitations of direct digital synthesizers and direct digital converters prevent the use of these solutions for some applications where using baseband modulators/demodulators and frequency translation stages is still needed to reach bandwidth, signal-to-noise ratio (SNR), dynamic range, noise, or interference requirements.
The backbone of frequency translation circuits is the RF mixer. A mixer is a three-port nonlinear device that “mixes” the frequencies from two input signals and outputs a signal that is the sum or difference of the input frequencies.
We offer several types of RF mixers including 2.92mm mixers, SMA mixers, field replaceable 2.92mm mixers, and field replaceable SMA mixers. Our SMA mixers include double balanced mixers as well as triple balanced mixers.
A local oscillator (LO) signal is sent to one port of the mixer, in some cases the LO signal is tunable to create a tunable frequency translation stage, and either the RF or IF port is the input/output port depending on if upconversion or downconversion is being performed. During upconversion both the input frequencies (IF and LO) will be seen on the RF port in double sideband upconversion mixers/upconverters. Single sideband upconversion mixers or upconverters will be designed to cancel the sum or difference frequency within the mixer circuitry and it will not be seen at the output.
The frequency translation capabilities of mixers is only possible due to the nonlinear nature of what is called a mixer circuit. Many nonlinear circuits can be used as mixers, but not all perform well for any given application. The most common mixer devices are made using gallium arsenide (GaAs) transistors, Schottky diodes, silicon field effect transistors (FETs), and complementary metal oxide semiconductor (CMOS) transistors.
High performance mixers use symmetry in the design of the mixer to create a balance effect that provides enhanced isolation, consolation of many of the intermodulation products, improved common mode signal rejection, and better conversion efficiency than non-balanced designs. This is beneficial for many circuits, as excessive intermodulation products may appear within the operating bands of the signal chain, as can inadequate isolation if the operating bandwidth of the system is wide enough or has several bands throughout the spectrum.
Some mixers require substantial LO power, which may present limitations for the design as high power LOs tend to generate additional noise, which may impact the receiver SNR/dynamic range negatively. Depending on a mixer’s conversion loss, additional amplification may be needed to compensate for poor conversion loss performance, or additional link budget could be attained by using a better performance mixer with lower conversion loss. Mixers, being intrinsically nonlinear, may also require filtering to prevent undesired mixing products to be sent to other signal chain components. Depending on the design and signal powers involved, reflections may occur between a mixer and other signal chain components that could develop into standing waves or otherwise by mixed and produce undesirable frequency outputs. Hence, careful design and analysis of the signal chain components around a mixer are necessary to ensure desired operation.
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