This paper designs a high-precision signal source based on DDS(Direct Digital Frequency Synthesis) technology for the needs of quartz wafer parametric testing system in pi-network. UsingOven Controlled Crystal Oscillator (OCXO) as the time base, the signal source uses AD9959 integrated chipof the AD Company in American, and is controlled by ARM micro-controller. The signal source can generate four-channel sinusoidal signals whichranges from] to 200MHz and itsfrequency, phase and amplitude can be independently adjusted. This technologyprovides technical support for quartz wafer parametric testing system and improves the accuracy of measurement of quartz wafer. This paper analyses the equivalent circuits of quartz wafer unit, the pi-network, the select of chips, the principle of DDS technology and the hardware and software design of DDS signal source. Among them, AD9959 is a high-performance DDS chip with four channels, and its internal clock frequency can reach 500MHz, furthermore, the chip is integrated with four independent DDS channels and its composed of 32-bit frequency control registers; The STM32F407 chip is used which comes with three 12-bit ADCs, two 12-bit DACs and two USB ports and can simplifY the external circuit and facilitate the realization of the control of the entire system, so it provides convenient conditions for the testing system. The experimental results show that the frequency error of the signal source is less than +/- 0.1 ppm, and the accuracy of the series resonant frequency of the quartz wafer testing system can reach +/- 2ppm when using the signal source. Compared with the testing instruments widely used in China, which are based on oscillator and impedance testing methods, this testing system has the advantages of wide frequency testing range, fast frequency switching speed, high frequency resolution and accuracy. The signal source meets the requirements of the quartz wafer testing system very well.
