Practical Mixers 101

Introduction to Mixers

A microwave frequency mixer is a 3-port electronic circuit that combines two or more signals into one or two composite output signals, and are categorized as switching mixers or nonlinear mixers– although many mixers are configured with both. Using diodes, the mixer is passive and has a conversion loss. Using active devices, such as transistors, a conversion gain is possible. A variety of circuit topologies exist for mixers and can be as simple as one that uses a single diode or more complex for enhanced performance.

Switching mixers include single-balanced and double-balanced mixers, widely used and known for reliability, while nonlinear mixers offer the ability for higher frequencies output. A single-ended mixer is usually based on a single Schottky diode or transistor. A balanced mixer typically incorporates two or more Schottky diodes or a Schottky quad. A balanced mixer offers advantages in third-order intermodulation distortion performance compared to a single-ended mixer, because of the balanced configuration. The popular modern mixer designs are Schottky diodes, GaAs FETs, and CMOS transistors depending upon the application. FET and CMOS mixers are often used in higher volume applications.

Basic Mixer Operation

Conceptually, the three ports on a mixer are 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 to down-convert it or where the high-frequency signal is output in an upconverter. The local oscillator (LO) port is where the power is applied. The LO signal is the strongest signal and turns the diodes on and off in a switching mixer which then reverses the path of the RF to the IF. In other words, the IF port is where the modified RF signal is filtered to become the IF signal.

While the mixer operates within its linear range, increases in IF output power corresponds to increases in RF input power. Conversion compression occurs outside the linear range. The 1-dB compression point is where the conversion-gain is 1dB lower than the conversion gain in the linear region of the mixer. The LO power coupled into the mixer controls performance. Inadequate LO power for a given mixer degrades conversion-gain and noise figure and, therefore, system sensitivity. Conversion gain factor is specified at a particular LO drive level and is defined as the ratio of the numeric single-sideband (SSB) IF output-power to the numeric RF input-power such that a positive value for an IF output-power greater than the RF input-power indicates conversion gain. Conversely, a negative result occurs for conversion loss.

Understand the System Requirements and Link Budget

When considering which mixers to use in a given system, the RF/Microwave signal-processing components performance data and specification will dictate the link budget limitations for gain, noise, frequency, and linearity parameters that can be applied to a mixer. Many system level design software, and well-designed system-level spreadsheet analysis, will include Link budgets and worst case performance requirements for each critical performance parameter of a system. Link budget calculations are an essential step in the design of any RF system. The link budget calculation allow the losses and gains to be planned, so that changes can be made to the system to meet its operational requirements or optimal performance. A simple link budget equation for Received Power can resemble: Received Power (dB) = Transmitted Power (dB) + Gains (dB) − Losses (dB), where the Gains and Losses are the component contributions of all components in the signal chain.