Design And Implementation Of Initial Demodulator For QAM Demodulator Chip

Quadrature Amplitude Modulation(QAM) is used to transfer signals by digital TV system based on DVB_C standard. In the receiver of the digital TV signal, high frequency signal is turned to 36MHz middle frequency signal by Tuner chip. Then AD converts the analog middle frequency signal into digital signal. The initial demodulator which this paper presented converts digital signal which is outputted by AD into base band digital signal.This paper introduces the architecture of DVB_C system and basic theory of QAM modulation and demodulation at first, analyzing and discussing two ways to implement the initial demodulator: simple sequencer and generating native carrier to demodulate. This paper mainly designs direct digital frequency synthesizer (DDFS) and low pass filter. By comparing different ways of designing DDFS and considering the requirement of the initial demodulator, a method which uses combinational logic to implement look-up table has been used. A half band low pass filter has been used to implement the low pass filter and it also implements the fuction of sampling. So the I, Q signal could use the half band filter by time sharing. And the resource of hardware is reduced.The initial demodulator module is described by verilog , simulated by ModelSim, Synthesized by Design Compiler. FPGA verification and chip test indicates that this module could work well at the clock frequency of 28.8MHz and demodulate the digital signal to baseband. The spurious free dynamic range (SFDR) of the DDFS which is used in the initial demodulator is 68dB and the stop band attenuation of half band low pass filter gets to 50dB.