Design Of L Band Digital PLL Frequency Synthesizer

Frequency synthesizer is an important part for radar and other telecommunicatingsystems. It is used to provide local oscillating signal and clock reference for thesesystems and its performance directly influence the function of these systems. Therefore,to design a high performance frequency synthesizer is one of the most important aspectsin the development of telecommunication. The technology of DPLL(Digital PhaseLocked Loop) is important in the area of frequency synthesizing basing on its highstability, high integrity and convenience of frequency converting.This paper designed an L band frequency synthesizer basing on the DPLLtechnology. Digital phase/frequency detector (PFD) is used in this synthesizer, whichovercame the shortcoming of narrow linear detect rang and fixed detect gain in analogdetector. Therefore the loop will lock more easily and work more stable. Besides, thePFD and loop divider is integrated into one chip, thus greatly reduced the volume andcomplexity of the synthesizer. The completed frequency synthesizer performs well, andachieved the expectant goal. The main work will be described in the followingparagraphs.1. Analyzed the characteristics of DPLL, and further researched the PFD, the loopfilter and the voltage control oscillator. Made a design plan for the frequencysynthesizer.2. Chose ADF4112 from Analog Device company to build the DPLL circuit. Theloop filter in the DPLL was chosen as third order active filter, and calculated out thesuitable parameters. Then designed the amplifying and filtering circuits referring to therequirement of radar systems. The final output signal of the DPLL is in the range of1400MHz～1600MHz, with a 1MHz frequency resolution and more than 10dBm power.3. Considering practicability and cost, chose MCU to be the controller which willbe in charge of the communication with ADF4112, including writing the frequencycontrolling bits and reading the locking state. Designed the keyboard inputting and displaying circuit using 8279, so we can easily change the working mode and thefrequency value. And what's more, the output frequency value can be displayed on theLED.4. Completed the writing of control programme. Realized two kinds of workingmode, that is single frequency mode and scan frequency mode. Meanwhile the state ofthe synthesizer can be inspected real time.5. Tested the completed synthesizer, and analyzed the results. Concluded thedefects and put forward improving methods.