Abstract: AFC( Automatic Frequency Control) is realized with technology of DDS ( Direct Digital Synthesis) and high accuracy frequency measurement, quadrature double channel receiver is made up of circuits of IF quadrature sampling directly and coherent sampling clock realized with DDS. By doing so, the effect of all kinds of unstable factors such as frequency shift and the limits placed by analogy circuit is diminished effectively , even eliminated. Meanwhile, the performance of the receiver is improved greatly, for example, the limits on I( improvement factor) and SCV( visibility) placed by traditional receiver is exceeded. Experiments indicate that SCV can be enhanced 12 dB on an average. Additionally, some practical problems in realizing are discussed.

Key words: DDS, AFC, Coherent clock, IF quadrature sampling