The Basics of RF Mixers

In the radio frequency signal chain, the superheterodyne receiver is a method that uses locally generated oscillating waves to mix with the input signal and transform the frequency of the input signal into a predetermined frequency. The principle of superheterodyne was first proposed by E.H. Armstrong in 1918. This method is developed on the basis of the heterodyne principle in order to meet the needs of long-distance communication for high-frequency and weak signal reception. The performance of this receiving method is better than that of high-frequency (direct) amplified receiving, so it is still widely used in the reception of remote signals, and has been widely applied to measurement technology and other aspects.

The mixer directly processes the RF signal amplified by the LNA after the low noise amplifier (LNA). In order to realize the mixing function, the mixer also needs to receive the local oscillator (LO) signal from the voltage-controlled oscillator, and its circuit completely works in the radio frequency band. Today we will take a brief look at RF mixers.

(1) Definition and type

The RF mixer is a three-port electronic circuit that can combine two or more signals into one or two composite output signals. It can be divided into switching mixers or nonlinear mixers. Many mixers are used Of these two configurations. Mixers that use diodes are passive mixers and will generate conversion loss. Mixers that use active devices such as transistors may have conversion gain. There are multiple circuit topologies for mixers. Simple circuit topologies are as simple as using a single diode, and complex circuit topologies can be used to improve performance.

Double balanced mixer. Basics

Switching mixers include single-balanced and double-balanced mixers, which are widely used and reliable, and nonlinear mixers can provide higher frequency output. Single-ended mixers are usually based on a single Schottky diode or transistor. A balanced mixer usually contains two or more Schottky diodes or one Schottky tetrode. The balanced mixer has a balanced configuration, so it has an advantage in third-order intermodulation distortion performance compared with a single-ended mixer. According to the needs of the application, modern popular mixers are designed with Schottky diodes, GaAs FETs and CMOS transistors. FET and CMOS mixers are commonly used for high volume applications.

(2) RF Mixers Basic operation

Conceptually, the three ports on the mixer are the radio frequency (RF) port, the local oscillator port (LO), and the intermediate frequency port (IF).

The radio frequency (RF) port is used to apply high-frequency signals to achieve down-conversion, or to output high-frequency signals in an up-converter.

The local oscillator (LO) port is used to connect the power supply. The LO signal is the strongest signal, and the RF path can be reversed to the IF path by turning on or off the diode in the switching mixer.

The intermediate frequency (IF) port is used to filter the modified radio frequency signal into an IF signal.

Three balanced mixer

When the mixer is operating in its linear range, the increase in IF output power corresponds to the increase in RF input power. Outside the linear range, conversion compression occurs. The 1-dB compression point is the point where the conversion gain is 1 dB lower than the conversion gain in the linear region of the mixer. The LO power coupled into the mixer can affect performance. Insufficient LO power for a given mixer will reduce the conversion gain and noise figure, resulting in a decrease in system sensitivity. The conversion gain coefficient is specified according to a specific LO drive level. It is defined as the ratio of the digital single sideband (SSB) IF output power to the digital RF input power. When the IF output power is greater than the RF input power, the coefficient is positive, indicating frequency conversion Gain. Conversely, when the coefficient is negative, conversion loss will occur.

Waveguide frequency converter mixer

(3) System requirements and link budget

When considering which mixers to use in a given system, the performance data and specifications of the RF/microwave signal processing components will dictate the link budget limits available for the mixer's gain, noise, frequency, and linearity parameters. Many system-level design software and well-designed system-level spreadsheet analysis will include the link budget and worst-case performance requirements for each key performance parameter in the system. In any RF system design, link budget calculation is an essential step. In the calculation of the link budget, the gain and loss can be planned to change the system to meet its operational requirements or to achieve the best performance. A simple received power link budget formula is similar to the following: received power (dB) = transmission power (dB) + gain (dB)-loss (dB), where gain and loss are the components of all components in the signal chain.