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

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

Mixer diode is a kind of Schottky barrier diode, which is made of the principle of metal-semiconductor junction formed by contacting metal and N-type semiconductor. When a metal and a semiconductor are in contact, a Schottky barrier, that is, a surface barrier, is formed at their interface, which hinders the passage of electrons. In order for the carriers in the semiconductor to easily cross the potential barrier and enter the metal, it must use an N-type semiconductor whose electron work function (the energy required for electrons to escape from the surface of the semiconductor or metal) is much larger than that of the metal. When the diode is forward biased, the potential barrier drops, and most carriers (electrons) enter the metal from the semiconductor.

Compared with ordinary diodes, mixing diodes work with majority carriers and have no minority carrier storage effect, so they have the characteristics of high frequency, low noise and small reverse current, and are mainly used in mixers. When using the diode mixing method, in the frequency range of 500 ~ 10,000Hz, Schottky type and point contact type diodes are mostly used.

Applications of Mixer Diodes

A mixer generally consists of four parts: an input signal loop, a local oscillator, a nonlinear device, and a filter network. The nonlinear device itself here only realizes the frequency conversion, and the local oscillator signal is generated by the local oscillator. If the nonlinear device not only generates the local oscillator signal, but also realizes frequency conversion.

The so-called frequency mixing is to add two different signals (such as a useful signal and a local oscillator signal) to the nonlinear device, and take the difference frequency or sum frequency. Mixers can be divided into diode mixers, transistor mixers, field effect transistor mixers and varactor mixers according to the different nonlinear devices used. Mixers can be divided into single tube mixers, balanced mixers, ring mixers, differential pair mixers and parametric mixers according to different working characteristics.

The following points should be paid attention to when designing the mixer:

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

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

(3) In order to reduce the frequency distortion and nonlinear distortion of the mixer and various mixing phenomena caused by the local oscillator frequency, the mixer is required to work in an area where the nonlinear characteristics are not too serious, so that it can complete the frequency conversion, It can also reduce various forms of interference.

(4) It is required that the noise figure of the mixer should be as small as possible. When designing the mixer, it is necessary to reasonably select the line and device and the operating point current of the device according to the requirements given by the distribution of the total noise figure of the equipment.

(5) The working stability of the mixer should be considered, such as the unstable output of the mixer caused by the unstable frequency of the local oscillator.

(6) Pay attention to the connection conditions of the input and output terminals of the mixer. When selecting circuits and designing circuits, full consideration should be given to how to match them.

The so-called diode mixer is a mixer in which the nonlinear device in the mixer adopts a mixing diode. L1C1 is the input signal circuit, which is tuned to the input signal frequency fS; L2C2 is the output intermediate frequency circuit, which is tuned to the intermediate frequency frequency fi. The output intermediate frequency circuit is directly connected in series with the mixing diode D, the signal input voltage μS, the local oscillator voltage μO and the bias power EO, and the output intermediate frequency voltage μi is all reacted 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 low. When this circuit is used in large-signal mixing, the mixing diode works in a switch state, which can obtain a larger 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. At present, most color TV tuners use a balanced mixer and a differential pair mixer composed of two or more devices to improve the performance of the mixer.

In the microwave field, if the nonlinear device uses two mixing diodes, a mixing diode shortwave mixer can be formed.

This mixer has the following characteristics:

(1) It is in the form of a balanced circuit, which can overcome the shortcomings of single-tube mixers in terms of nonlinearity and dynamic range;

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

(3) The mixing noise is small;

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

I believe that by reading the above content, you have a preliminary understanding of mixing diodes. At the same time, I hope that you will make a good summary during the learning process, so that you can continuously improve your professional level.