Based On Sigma-delta Modulation Techniques, Wideband Frequency Synthesizer

The development of modern technology brings more and more difficult requirement for signal source.The output of signal source must have wide frequency range,high frequency resolution,low phase noise,high spurious restraining and can be controlled by program.Frequency synthesizer is the key technology to produce a great deal of frequency signal.It had been an important component of communication,radar, instruments,high-speed computer and navigation equipment.Fractional-N synthesis allows a PLL to achieve arbitrarily fine frequency resolution.Because the technique modulates the instantaneous divide ratio,fractional-N synthesis suffers from fractional spurs.∑-Δmodulation frequency synthesis is an advanced technology that applies in PLL,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.This paper focuses on the analysis of∑-Δmodulation theories such as mathematical models,noise shaping characteristics,etc.Combined with traditional fractional-N synthesizers structures,a project of∑-Δmodulator is proposed.The author participated in the work of designing the whole system and the index assigned.The hardware which was designed and implemented by the author consists of two PLL loops, fractional-N frequency divider with∑-Δmodulator and micro strip low-pass filters.The frequency resolution of the 4GHz～7GHz PLL could achieve 10^-7with the reference frequency of 10MHz,which decides the frequency resolution of the whole system.The results indicate∑-Δmodulation could decrease spurs of fractional-N frequency synthesizer,and the scheme in this paper could be a low cost solution too.