The quadrature amplitude modulation and demodulation QAM(quadrature amplitude modulation and demodulation) frequency utilization ratio and the power utilization ratio is a high efficiency digitized modulation and demodulation solution to strengthen the anti- interference function. In the medium and big capacity digital microwave correspondence system,the high cable TV networks data transmission and the satellite correspondence etc was extensively applied. DDS technical realization depend on digitized spare part of the high speed,high performance. According to the need to carry out various more complicated frequency complicated phase complicated range function expediently. The DDS(Direct Digital Synthesis) is a new digital frequency synthesis technique based on theconcept of phase. This paper presents the work principle of DDS. And by using the big capacity FPGA (Cyclone)which is the latest product declared by the Altera company, here brings up a design project to realize thecore of the DDS system. And we also use the QUARTUSⅡ6.0’simulation to wave form. By making use of some optimizing measure, the performance of the DDSsystem designed comes to our anticipant purpose, finally, the experiment result demonstrate that the proposed method has relatively high cost performance ratio. The AD9761 is a complete dual channel, high speed, 10-bit CMOS DAC. The AD9761 has been developed specifically for use in wide bandwidth communication applications where digital I and Q information is being processed during transmit operations. This paper is depended on FPGA for QAM realization, Use the VHDL language to make process of the QAM modulation，Mainly is the reflection of QAM constellation and the DDS signal source module. Through the reflection of QAM constellation of the input information to control DDS to produce to correspond range, phase already modulated digital signal. That process need the Quartus II software and the download chip Cyclone. the chip export pass the D/A conversion, which use AD9761 chip electric circuit . Then pass a low pass filter are the already modulated of QAM signal we want to get.
Key words: QAM；VHDL；DDS；QAM constellation；Quartus II；AD9761；
digitized modulation and demodulation；Cyclone；D/A conversion
