Research And Implementation Of High-precision Signal Source Based On FPGA

During the magnitude transfer of the resistor-capacitor,due to the phase difference of90°between the resistor and the capacitor,the capacitor value cannot be traced to the resistor reference value by the conventional transformer bridge method.Although the double right angle bridge can solve the problem of the phase of the right angle power supply interfering with its amplitude adjustment,in order to realize the accurate definition of the four-terminal-pair?4TP?impedance,it is necessary to build an extremely complicated circuit and increase the experimental difficulty.The National Institute of Metrology of China has proposed an impedance measurement method based on the Wagner branch,which enables simple and easy-to-operate impedance measurement with the prerequisite of preparing a precise signal source.The signal source has two requirements:1)being capable of generating two sinusoidal signals,which can accurately change the phase of 90°with no change of the signal amplitude before and after the phase change;2)having very high stability in the amplitude of these two signals with the amplitude resolution of more than 16 bits,and the accuracy of the frequency and amplitude reaches the level of10^-4in the meantime.At present,the signal source in the market cannot meet the above requirements at the same time.Therefore,this paper has developed a high-precision signal source that meets the above two requirements.The specific details of this paper are as follows:1)After the basic principle of DDS has been analyzed,the spur problems introduced by the phase truncation error,amplitude quantization error and nonlinear error of digital-to-analog converter,and the relationship between the RMS effective value of DDS signal and the RMS value of fundamental wave are discussed in detail with a view of obtaining the relevant parameters of the DDS signal source,so as to provide a solid theoretical basis for the design of high precision dual DDS signal sources.2)The correctness of the digital circuit is verified with the application of the Quartus II software in the designing of the digital circuit portion of the DDS signal source,and the Modelsim software and the SignalTap II function of the Quartus II software in its simulation.In order to develop a high-precision dual-channel DDS signal source bases on master-slave dual-processor coordination,the signal source uses two FPGA processors to coordinate work to generate two sinusoidal signals and communicates with the PC through the serial port to change the phase and frequency in real time.In the dual-channel DDS signal,the FPGA control circuit board and the DAC module circuit board are connected by ports to perform digital-to-analog isolation in coaxial lines for signal transmission,so that the output signals of the respective channels are completely electrically isolated and do not interfere with each other.Therefore,the output signal of dual DDS signal source can accurately change the phase,independent of the amplitude and phase of the output signal.3)Finally,the design of the high-precision dual DDS signal source is tested with the application of the Tektronix DSO5034 oscilloscope,Agilent 34451A and an AC voltmeter Fluke 5790,and the test results are analyzed to evaluate the signal source stability and frequency amplitude resolution.The test results show that the dual DDS signal source designed in this paper can achieve phase accurate change,independent on amplitude and phase.The signal source meets the design requirements as follows:the amplitude resolution of the two signals of DDS signal source reaches 18 bits,the accuracy of the frequency reaches the level of10^-6,the accuracy of the amplitude reaches the level of10^-6,the stability of the amplitude reaches the level of10^-6.