The Design Of The DDS System Of Digitized FM Based On FPGA

The technology of Frequency Synthesis, which consist of Direct Synthese, Phase-locked Synthese and Direct Digital Synthese is widely used in the area of communition,aeronautics and instrument,etc. The system realized this time which is achieved by FPGA is a DDS system that has the function of digitized frequency modulation of analog signals.DDS is a method that can transform the digitized signals into analog signals by means of DAC. With the help of the high-speed ROM which is used as a look-up-table, the sin-waveforms or other waveforms can be obtained by using the high-speed DAC. The common DDS is composed of three part. The Phase Adder,, which accomplish the accumulation of the phase; The Phase Code-Amplitude Code Transformer, which is realized by ROM; The DAC,which transforms the digital signals into analogy signals.By using the DDS system, we can obtain signals with high definition in frequency and with sequential phase. Meanwhile we can acquire digital FM signals and PM signals by means of changing the phase word.The basic idea of digitized frequency modulation of analog signals is as follows: First, transform the analogy signals into digital signals by ADC, Then add this digital signals with a fixed frequency word and acquire the frequency word which is controlled by the analogy signals. Finally, apply this frequency word to the DDS system and realize the digitized frequency modulation.The hardware of the design is ACEX IK series FPGA made by Altera Inc.The whole system is described with VHDL and developed by MAX+PLUS II ,The test demonstrates that the system coincide with the theoretical analyse in lower frequency, but in higher frequency the waveform has obvious distortion because of the lower speed of the devices.