The Uhf Band High-performance Receiver Front-end System

The development of modern communication systems has proposed higher and higher requirement on sensitivity and selectivity of the receiver. Better selectivity can reject the interference signal and improve the frequency utilization more effectively. And lower noise figure can achieve higher sensitivity and greater dynamic range of system. In the narrowband case, however, the sensitivity and selectivity of the receiver are contradictory. The high temperature superconductor(HTSC) receiver provides a good solution to this problem.The UHF band HTSC receiver which has both narrowband and low insertion loss characteristic is studied in this dissertation. The operating frequency band of the receiver is from 415MHz to 420MHz. The noise figure is required to be lower than 0.9dB. Conventional methods can not achieve this goal.The receiver front-end in the paper consists of the HTSC pre-filter, the cryogenic low noise amplifier (LNA) and the refrigerating system. The system operates in the lower UHF band. It leads to two difficulties. First, the physical size of the filter will be too large to be manufactured on the 3 inch HTSC substrate in the laboratory. Second, the stability and the reflection coefficient of the pHEMT transistor used in the LNA do not perform well in the lower UHF band. To solve these problems, the capacitive loading technology is used to reduce the size of the filter. The source series feed back is used to enhance the stability and weaken the contradiction between noise figure and the reflection coefficient. Also, the resistive loading technology is used to achieve unconditional stability within a broad frequency band.The noise figure of the receiver front-end is less than 0.8dB. The input reflection coefficient is lower than -15dB. The gain is about 18dB. The minimum detectable signal is about -106dBm. The dynamic range is about 107dB. The out-of-band rejection is more than 70dB at±10MHz offset from center frequency. These results present the excellent performance and great potential of the HTSC receiver.