A Frequency Synthesizer Based On FN-PLL

Frequency synthesizer is one of the most important parts used in modern communication and electronic systems. With the development of modem communication techniques, frequency synthesizer is required to have higher performance.With the development of the digital Phase-Lock Loop and the Large Scale Integrated Circuit, Phase-Lock Loop frequency synthesizer becomes a new hot spotin researching on frequency source. Using the technology of Fractional-N and Sigma-Delta modulation in Phase-Lock Loop frequency synthesizer can get fine frequency resolution without the loss of PD frequency, So the Fractional-N Phase-Lock Loop(FN-PLL) frequency synthesizer settle the problem between the high frequency resolving and fast frequency switching, and because of Sigma-Delta modulation technology, it also have low phase noise. This kind of FN-PLL frequency synthesizer has some advantages not only the low phase noise and fine frequency resolution, but also low expense and short developing period.We firstly introduce the theory of the direct digital Phase-Lock Loop frequency synthesizer. The Phase-Lock Loop is based on negative feed-back control theory, when one of its control variable changed, the system can adjust by itself and get stable on a new state. Then, we import the FN-PLL frequency synthesizer, analysis the way it avoid the conflict the between the high frequency resolving and fast frequency switching, and the phase noise it bring in. At the last we adopt the Sigma-Delta modulation to eliminate the phase noise.We also design the controller of the FN-PLL frequency synthesizer. The control data is input by PC, then through PCI bus we transfer the control data to FPGA, in FPGA we process the control data by the demand of the synthesizer and send the processed data into synthesizer to control the work of it.Lastly, we read many IC databook and select the IC , then we design the FN-PLL frequency synthesizer and its controlling board on software and hardware. Because it is a negative feedback system, when we design it, we pay important attention to the stability of it, and keep enough phase margin.