Research Of Low Distortion Microwave FET Mixer

As one of the most important way of modern communications, wireless communication is widely used among people’s life and work. The transceiver is the most important part of wireless communication system. The high-frequency radio signals need to be converted by the internal mixer to effectively process signal. The performance of mixer often determines the overall performance of the transceiver, so the mixer is very important in the field of wireless communication system.There are many kinds of mixer, which often use the nonlinear components such as diodes or field effect tube as the core of mixing section. Diode mixer is characterized by simple structure and low cost, but its high conversion loss, low degree of linearity are making it just be used in the ordinary microwave system. FET mixer of active structure can provide conversion gain and high linearity, but tend to be with more complicated structure, large noise and a lot of power loss. Passive FET mixer not only have the characteristics of low convertion loss, power loss and low noise, but also have the feature of high linearity. It perform better relative to the first two kinds of mixer.Based on the theory of low distortion mixer, the high electron mobility transistor ATF54143 is chosen as the core mixing module. Then the single tube FET mixer is preliminarily designed. Based on the simulation analysis of the single tube mixer, and the combination of the switch function and equivalent circuit of the transistor, a balanced mixer circuit topology is designed. Then, the high-pass and low-pass filter are added to the RF and IF port. Then, Based on the principle of Marchand balun, a high frequency LO balun of three-metal-layer defected ground structure is designed. In the frequency range of 800 MHz to 3GHz, the return loss is more than 12 dB, the magnitude imbalance is less than 3.4°, and the phase imbalance is less than 0.3dB. After the combination of the respective units, use ADS software to simulating the overall performance of the mixer. At last, the mixer is produced and tested.The low distortion microwave FET mixer works in the frequency range of 1GHz to3 GHz. The frequency of IF signal is 120 MHz, the test results show that the conversion loss is 4dB at least. LO-RF isolation can reach 38 dB, the best value of LO-IF isolation is 81 dB. In the aspect of low distortion, IIP3 is up to 23 dBm.