The Application Research On Direct Digital Synthesis Technology Based On Single Chip Microcomputer

The invention of Direct Digital Synthesis (DDS) is the revolution of signal frequency synthesis technology. DDS has shown its advantages increasingly along with the development of microelectronics technique. Now it has been widely used in the fields of radar, communication, apparatus, etc.In this thesis, a new direct digital synthesizer controlled by 89C55 single chip-microcomputer (abbr:SCM) has been successfully designed and assembled, based on detailed analysis of the distribution of spurious spectrum of DDS signal and the characteristic of different DDS chips. With modularized construction, the apparatus can be conveniently used to study various frequency synthesis circuits by changing the card with different DDS chips (AD9851, AD9854, AD9951). Several innovative measures have been made to suppress the clutters and spurs in output signals, which include the complete photoelectric coupling circuit between DDS module and SCM system, the high frequency amplifying circuit operating in floating ground, and a set of 7th order elliptical low pass filters with eight channels. The correction of those circuits and its component values were simulated first by PSPICE, and then testified in system debugging. For overcoming the difficulties of multi-bytes data conversion and processing in SCM, a new program using C51 has been designed.Furthermore, a virtual instrument interface for this apparatus has been developed on the platform of Labview. So the frequency of synthetical signal can be adjusted not only on the front-board of SCM system, but also on the virtual instrument interface. The frequency tuning resolution can reached 0.001 Hz while the output signal ranged from 1MHz to 20MHz. Using this apparatus, a lot of experiments on the circuit modules with different DDS chips have been made to find out the key factors that disturbed the synthetical signals. It has come to some important conclusions that might be valuable to investigators and engineers who are engaged in DDS technique.DDS is one of the most advanced frequency synthesizing techniques. However, there are still some disadvantages in using DDS, for example, the narrow bandwidth, which more or less limits its application. In the last part of the thesis, some suggestions are proposed to further improve the result of frequency synthesis.