What does a mixing diode mean? What are the application scenarios?

What does a mixing diode mean? What are the application scenarios?

A mixing diode is a Schottky barrier diode that is made using the principle of a metal semiconductor junction formed by the contact between a metal and an N-type semiconductor. When metal comes into contact with semiconductor, a Schottky barrier, also known as a surface barrier, is formed at their interface to hinder the passage of electrons. In order for carriers in semiconductors to easily cross potential barriers and enter metals, it is necessary to use N-type semiconductors with much higher electron work functions (the energy required for electrons to escape from the semiconductor or metal surface) than metals. When a forward bias is applied to a diode, the potential barrier decreases and most charge carriers (electrons) enter the metal from the semiconductor.

Compared with general diodes, frequency mixing diodes have the characteristics of high frequency, low noise, and small reverse current due to the use of majority carriers and the absence of minority carrier storage effects. They are mainly used in mixers. When using diode mixing, Schottky and point contact diodes are commonly used in the frequency range of 500-10000Hz.

Application of Mixing Diode

A mixer generally consists of four parts: an input signal loop, a local oscillator, nonlinear devices, and a filtering network. The nonlinear device here only implements frequency conversion, and the local oscillator signal is generated by the local oscillator. If nonlinear devices generate both local oscillator signals and achieve frequency conversion.

The so-called mixing refers to adding two different signals (such as a useful signal and a local oscillation signal) to a nonlinear device, and taking their difference or sum frequencies. Mixers can be classified into diode mixers, transistor mixers, field-effect transistor mixers, and varactor mixers based on the different nonlinear devices used. Mixers can be divided into single diode mixers, balanced mixers, ring mixers, differential pair mixers, and parametric mixers according to their different operating characteristics.

When designing a mixer, the following points should be noted:

(1) The larger the frequency mixing amplification factor, the better. The mixing amplification factor refers to the ratio of the amplitude of the intermediate frequency output voltage of a mixer to the amplitude of the variable frequency input signal voltage, also known as the mixing voltage gain. Increasing the frequency mixing amplification factor is a powerful measure to improve the sensitivity of the receiver.

(2) Require the intermediate frequency output circuit of the mixer to have good selectivity to suppress unwanted interference frequencies.

(3) In order to reduce the frequency distortion and nonlinear distortion of the mixer, as well as various mixing phenomena caused by the local oscillator frequency, it is required that the mixer operates in an area where the nonlinear characteristics are not too severe, so that it can complete frequency conversion while generating less interference in various forms.

(4) The smaller the noise coefficient of the mixer, the better. When designing the mixer, it is necessary to select the circuit and components as well as the operating point current of the components reasonably according to the requirements given for the allocation of the total noise coefficient of the equipment.

(5) Consider the operational stability of the mixer, such as the instability of the output caused by the frequency instability of the local oscillator.

(6) Pay attention to the connection conditions between the input and output terminals of the mixer, and fully consider how to match when selecting circuits and designing circuits.

The so-called diode mixer refers to a mixer in which the nonlinear device adopts a mixing diode. L1C1 is the input signal loop tuned to the input signal frequency fS; L2C2 is the output intermediate frequency circuit, tuned to the intermediate frequency fi. The output intermediate frequency circuit is directly connected in series with the mixing diode D, the signal input voltage μ S, the local oscillator voltage μ O, and the bias power supply EO, and the output intermediate frequency voltage μ i is all reversed to the mixing diode.

This diode mixer has the characteristics of simple circuit, low noise, and high operating frequency, but the mixing voltage gain is relatively low. When this circuit is used in large signal mixing, the mixing diode operates in a switching state, which can obtain a large dynamic range, so it is widely used in microwave circuits. If a mixer composed of a single device is used, its nonlinearity and dynamic range are not ideal. Nowadays, most TV tuners use balanced mixers and differential pair mixers composed of two or more components to improve the performance of the mixer.

In the field of microwave, if two mixing diodes are used as nonlinear devices, a short wave mixer with mixing diodes can be formed.

This mixer has the following characteristics:

(1) It belongs to the balanced circuit form and can overcome the shortcomings of single diode mixers in terms of nonlinearity and dynamic range;

(2) It has the advantages of stability and reliability, low frequency conversion gain, and no regeneration effect;

(3) Low mixing noise;

(4) The dynamic range of the mixer depends on the magnitude of the local oscillator voltage.

I believe that by reading the above content, everyone has a preliminary understanding of the mixer diode. At the same time, I hope that everyone can make good summaries during the learning process, so as to continuously improve their professional level.