因为卫星广播电视的电磁波信号在空间的传播路程很大,而天线接收到的电磁波信号功率与电磁波传输路程的平方成反比,所以在天线上接收到的卫星电视信号非常微弱。就接收系统的灵敏度而言,普通电视接收机根本无法接收卫星电视信号。要接收卫星电视信号,除了接收系统需要有十分讲究的天线外,卫星接收机性能的优劣也是十分重要的。接收机包括室外单元(即微波低噪声下变频器或称SHF高频头)和室内单元(即中频接收机或称调制解调器)。在卫星电视接收站(TVRO)系统中,几乎无例外地采用超外差接收体制。室外单元的功能是将接收天线捕捉到的卫星电视电波进行低噪声、高增益放大,然后用外差原理变换成中频。在TVRO站中,微波低噪声下变频器是一个极其重要的部件,在天线尺寸选定后,它的性能的优劣,特别是其低噪声特性的好坏,决定着整个接收系统的增益——噪声温度比G/T值。从而也就决定 Because the electromagnetic wave signal of satellite radio and TV broadcasts the space very much, and the electromagnetic wave signal power received by the aerial is inversely proportional to the square of the electromagnetic wave transmission distance, the satellite television signal received on the aerial is very weak. In terms of the sensitivity of the receiving system, ordinary television receivers simply can not receive satellite television signals. To receive satellite television signals, in addition to receiving the system needs to be very particular about the antenna, the pros and cons of satellite receiver performance is also very important. Receivers include outdoor units (ie microwave low noise down converters or SHF tuners) and indoor units (ie IF receivers or modems). In the satellite television receiving station (TVRO) system, almost without exception, super heterodyne reception system. Outdoor unit’s function is to capture the satellite antenna captured by the antenna low-noise, high-gain amplifier, and then heterodyne principle into IF. In the TVRO station, the microwave low noise downconverter is an extremely important part. After selecting the size of the antenna, its performance, especially its low noise characteristic, determines the gain of the entire receiving system - - Noise temperature ratio G / T value. Which also decided