Research Of Direct Digital Synthesizer Based On FPGA

Direct Digital Synthesis(DDS) is a technique which generates analog signals by digital means, it has many advantages, including high frequency resolution, fast frequency switching time, maintaining contiuous phase while changing frequency. But dedicated DDS chips have fixed features and performance in output control and frequency switching time, and it needs the assistance of mircrocontroller. With FPGA, one can deisgn DDS flexibly, accomplish multi-waveform output and various modulation modes, and integrate features of multiple chips into one device.This paper studies, designs and implements a DDS based on FPGA, which can output various waveforms and implement digital modulation. The main work of this thesis is as follows: 1, it analyses the characteristics of DDS in both time and frequency domain. This paper studies the phase-truncation error, quantization error and the effects of both on spurious signals. It discusses several techniques and algorithms to improve system performance, including Nicholas Phase Accumulator, jitter injection technique, memory compression technique and Taylor series correction algorithm. 2, it designs the hardware system. The hardware structure and IC chips are determined. And it designs the functional circuits for this system, which includes voltage convertion, clock input, DA and signal amplification, and power up configuration modules. 3, it proposes the performance expectations of the system, completes software design with Verilog HDL and functional simulation with ModelSim. It adopts memory compression and jitter injection technique, develops functionalities for outputting various waveforms in design of the synthesizer, and it studies the implementation of digital modulation with DDS. 4, it implements the system and analyses the results. This paper realizes its frequency output on PCB and analyses the frequency domain performance, frequency accuracy and stability with oscilloscope. And the results are studied and explained.The practical tests verify that, the frequency synthesizer developed by this paper can adjust both frequency and phase dynamically, output various waveforms( sine, square, sawtooth and triangular waveform), implement digital modulation( 2ASK, 2FSK,2PSK) and other functionalities. The output signal can have a dynamic frequency range from 0 to 20 MHz, frequency resolution better than 1Hz, frequency switching time less than 80 ns, SFDR( spurious-free dynamic range) of sine wave better than 30 dB, and frequency stability on 10-3 order of magnitude. As the size and power consumption are small, the synthesizer developed by this paper reaches the level of currently selling signal generators on the domestic market.