The Fractional Frequency Of The Pll Frequency Synthesizer

With the quick development of the modern radar, electronic communication, counterspy technology, the synthesizers, which is one of the key components in the system, was demanded for much higher performance. Like: low phase noise, high spurious restraining, wide frequency range, high frequency resolution, fast jump time, high frequency stability and programmable ability. All of these requirements were hard to achieve by the analog circuit nowadays. The main purpose of the frequency synthesizer is to produce a series of high resolution, high stability signals.Fraction-N phase locked loop (PLL) and DDS fraction are the state-of-the-art technologies in recent years. They were shown higher resolution and lower phase noise compared with the frequency synthesizer designed in the traditional way. Compared with fraction -N, the DDS has its drawback that the frequency can not be too high, hence, you can not implement high frequency circuit with single DDS, but which you can do it in fraction-N way. So it's very meaningful to study the fraction-N technology used in the real applications.This paper represents the basic theory of PLL design, including the constitution of PLL, basic theory and performance. Then with much more details on the phase noise analysis of the frequency synthesizer, especially focused on its root cause and characteristic. More analysis on the effects of the various devices produced on the phase noise, the mechanisms and the characteristics of the fraction spurs of the frequency synthesizer and some methodologies to reduce the fraction spurs. At the last, this paper introduce the design procedures of fraction PLL, the details of design 3rd order loop filter, and give the budgets of the phase noise, spurs of the PLL. This paper also gives the testing results of the phase noise and spurs, with descriptions of the hints in the design and debugging phase. The loop filter is the key component of the frequency synthesizer to produce the low phase noise, low spurs stable, the detailed design procedures of the loop filter is also introduced, with the verification of the feasibility.