Application Of DDS In Cable Modem Upstream Modulator

In this thesis, Some methods of frequency synthesis are mentioned firstly. Then the principle of direct digital frequency synthesis(DDS) is introduced. The thesis analyses the cause of spurious noise, and the effect to DDS output spectra. In this thesis, methods are also presented to reduce spur level of the output of DDS and to achieve high performance of DDS. DOCSIS1.1 is introduced, including PHY layer and MAC layer. There is two-way transmission, both upstream and downstream. This thesis only studies the upstream of PHY layer. A system design of CM upstream modulator based on the technology of DDS is given. The system accords with DOCSIS1.1 specification. The modulator provides two modulation formats (QPSK and 16QAM) in DOCSIS1.1. The system is based on FPGA. The project and module design are introduced and the module of DDS is described in detail. Designing the modules of phase accumulator and phase to amplitude converter in DDS is important. In DDS design, traditional method by looking up table to synthesize waveform is studied. Considering the effect of spurious noise and improving the quality of DDS output,the approach to compress the capacity of ROM and optimize the module design of phase accumulator. Using carry look-ahead adder(CLA) and pipeline design optimizes the phase accumulator module. At last, this paper has presented simulation results and analysis of the results.