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6.1 概述中频调制发射机和两次变频差转机中均需将中频已调信号变换为指配频道的射频信号,此过程通常称为上变频。电视上变频的基本特点是,被变换的是调幅波,要求实现线性变频,即载频移置后调幅包络的波形和频谱结构均无显著变化。要实现变频功能,必须包括借以产生新增频率分量的非线性混频器件和本地振荡源,以及必要的输入、输出滤波装置。
6.1 Overview The intermediate frequency modulated transmitter and the two frequency converted differential transformers need to convert the intermediate frequency modulated signal to the assigned channel RF signal. This process is commonly referred to as upconversion. The basic characteristic of frequency conversion on TV is that the amplitude modulated wave is transformed and the linear frequency conversion is required, that is, there is no significant change in the waveform and spectrum structure of the amplitude modulation envelope after the carrier frequency shift. In order to realize the frequency conversion function, it is necessary to include the non-linear mixing device and the local oscillation source by which the newly added frequency components are generated, and the necessary input and output filtering devices.