The Research Of A Fractional Digital Frequency Synthesizer Based On Delta-Sigma Frequency Discriminatior

With the development of communication technology,frequency synthesizer is getting more and more wildly used in circuit technology. The frequency synthesizer with low phase noise,high frequency spectrum purity and high output frequency band has become the main trend of the development of the frequency synthesis.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.The output signal frequency of integer frequency synthesizer is the integral multiple of reference signal frequency,Loop frequency resolution is the reference frequency.In order to precise control the output signal frequency,needs to use low frequency about the reference signal,so the loop bandwidth is correspondingly narrow.Narrow loop bandwidth is undesirable because of long switching times,inadequate suppression of VCO phase noise,and susceptibility to noise.Fractional frequency synthesizer overcomes the contradiction between the frequency resolution steps and phase discriminator frequency settings of the integer frequency synthesizer.It can work at high frequency reference signal and obtain high precision output signal frequency at the same time,reducing the Bandwidth limitation on the filter.But,because of the technique modulates the instantaneous divide ratio,fractional frequency synthesizer suffers from fractional spurs.TheΔ-Σnoise spectrum moving technology is an advanced method which developing with the digital processing technology.It allows the phase noise frequency spectrum move to the high frequency, thus it can be filtered by loop filter easily.This method can suppress the fractional noise effectively.Correspondingly,Δ-Σpure digital frequency discriminator is a potential structure.The methodology of the frequency synthesizer is similar to that of the 1 bit ADC technique for that the phase error detection is done by only determine whether the transition from DSFD(Delta-Sigma Frequency Discriminator) comes before or after the reference transition.The accuracy of detection ensured by increasing the sampling frequency,so the influence of simulate parameters on the detection accuracy minimized.The circuit structure ofΔ-Σpure digital frequency discriminator is simple and completely solves the problem of fractional noise,so use this circuit may get high-performance frequency synthesizer.Because most signals are digital processing,this circuit easies to be integrated and has great flexibility. The frequency synthesizer which based onΔ-Σfrequency discriminator can not only be used in high-tech systems,but also have deep potential to replace most conventional frequency synthesizer.This paper mainly focuses on researching the fractional frequency synthesizer which based onΔ-Σfrequency discriminator.Firstly,the characteristic and development of the frequency synthesizers and the basic theory of the traditional frequency synthesizers are introduced.And analyze the reason of bringing the frequency noise.Next,present the theory of the fractional frequency synthesizer which based onΔ-Σfrequency discriminator and educe the frame picture of it.At last give a FPGA design of the fractional frequency synthesizer which based onΔ-Σfrequency discriminator,correspondingly contrast this new fractional frequency with the traditional one by simulating and realized it by hardware platform,then conclude that the new one is more excellent than the old one.