Design Of A Frequency Synthesizer Based On Combination Of DDS And PLL

PLL (Phase Locked Loop) is a closed-loop control system to track the input signal. It has been a very wide range of applications in various fields of the radio technology. With the continuous development of integrated circuit technology, PLL has gradually become a low-cost, easy to use multi-functional component. At present, the main application of PLL has been in modulation and demodulation, frequency synthesis and other fields. PLL has compared phases of the reference signal and output signal phase, produced a phase error voltage to adjust the output signal frequency, and made the frequency of the output signal consistent with the reference signal.In this paper, at first the principle of PLL was introduced, in this basis the author has analyzed the advantages and disadvantages of PLL and DDS, combining with the characteristics of two methods has proposed a new method of frequency synthesis. PLL has been the advantages of flexible control, small size, low cost and high output frequency, frequency bandwidth, the spectrum of good quality, etc., but the frequency resolution has been limited by the frequency of the output signal and can't reach very high. The frequency resolution of DDS has been higher, but the output frequency has been limited by the digital circuit, can not do a very high, and the noise of the output signal is relatively larger. In the loop formed by PLL and DDS, DDS's output signal is a reference signal of the loop phase detector, PLL complete the final output. This can take full advantages of the high-frequency resolution of DDS and good quality of the PLL output frequency. Finally, we design the system simulation and detailed circuits.