What Is a Frequency Mixer?

A mixer is a circuit whose output signal frequency is equal to the sum, difference, or other combination of the two input signal frequencies. The mixer usually consists of a non-linear element and a frequency selection loop.

Frequency conversion

Mixing refers to the process of transforming a signal from one frequency to another. Its essence is the process of linearly shifting the frequency spectrum. In the superheterodyne receiver, the purpose of mixing is to ensure that the receiver obtains high sensitivity, sufficient amplification and proper passband, and can also work stably. The mixing circuit includes three components: a local oscillator, a non-linear device, and a band-pass filter. ^[1]

Work

frequency

Driven by reducing the cost of RF products, how to use low-cost, highly integrated CMOS technology to achieve high-performance RF integrated circuits has become the focus of RF integrated circuit design research. People have continuously proposed RF circuit structure and design technology based on CMOS technology, and gradually introduced mature CMOS RF products, and achieved great results. As a key module of the receiver, the research of CMOS design technology of the mixer is also one of the very important topics. ^[2]

Therefore, the design of the mixer usually needs to take into account performance indicators such as conversion gain, linearity, noise figure, isolation between ports, and power consumption. 1) In order to make up for the loss of the IF filter and reduce the contribution of the subsequent circuit noise of the mixer to the system noise, the mixer needs to have a certain conversion gain. However, if the gain is too large, it will affect the output of the mixer. 2) The linearity of the mixer is the most important performance among various performances, which directly determines the dynamic range of the receiver. When the power of the RF input signal is too large and exceeds the 1 dB compression point of the mixer, the power of the intermediate frequency output signal will be greatly attenuated than expected, and deviate from the original linear trajectory; due to the cubic non-linearity of the mixer Term, the third-order intermodulation caused by the RF signal of the adjacent channel will cause serious interference to the IF output signal, usually IIP3 (inpu t th ird- orde r intercept point) or O IP3 (ou tpu t third- o rde r intercept point) to characterize the ability of the mixer to suppress third-order intermodulation. 3) In order to reduce the system noise and the design pressure of the LNA, the mixer should have a lower noise figure. 4) The local oscillator signal LO swing in the mixer is usually large, which can easily cause signal feedthrough and cause interference, especially the feedthrough to the RF input end, affecting other receivers or causing self-mixing (for zero-IF receivers) The performance impact is very large). Therefore, the mixer needs to have good isolation. 5) Power consumption is an issue that all modules must consider. Reducing the power consumption of the mixer can effectively reduce system power consumption. ^[2]