Study On The Sigma Delta Modulator PLL Fractional-N Synthesizer

Sigma Delta modulation frequency synthesis is an advanced technology that applies in PLL(Phase-Locked Loop) frequency synthesizer, which could transform the spurs created by Fractional-N Synthesizers into phase noise that could be filtered by inherent low-pass identity in PLL. Then high frequency distinguished rate and much low spurs could be realized in a single loop PLL frequency synthesizer to carry out many kinds of digital modulation and predigest the structure of many signal generators. Nowadays Sigma Delta modulator PLL Fractional-N Synthesizer is widely used in measurement instrument and communication application.This paper focuses on the analysis of Sigma Delta modulation theories such as mathematics models, noise shaping characteristics, etc. Combined with traditional PLL Fractional-N Synthesizers structures, a project of sigma delta modulator is proposed, and the algorithmic analysis is implemented by FPGA. Design of hardware consists of three PLL loops, micro wave sample mixer, Fractional-N frequency divider. The results indicate that Sigma Delta modulation could improve spurs of Fractional-N frequency synthesizer in a certain extent.