Study Of The Spur Elimination Technique On Frequency Synthesizer Based On Sigma-delta Modulation

Fractional-N frequency synthesizers based on the sigma-delta modulation, whichcan gain a fine frequency step and a good noise performance, are widely studied andapplied. The issues of the fractional spurs arised in the fractional-N phase locked loopbased on the sigma-delta modulation and the methods which are used for thesuppression of the fractional spurs have been mainly focused in this thesis. The maintasks are illustrated as following:1. The detailed analysis of the first order sigma-delta modulator, which is availablefor the Phase Locked Loop, is issued in this thesis, and the (MultistAge noise SHaping)MASH, which has a better noise-shaping performance than the first order sigma-deltamodulator, has been deduced. What’s more, the effects, which are deviated from thestages and the change of the bit-width of the input data of MASH, on the outputphase-noise performance of the fractional-N module are analyzed. It is concluded thatthe comprehensive performances of the fractional-N divider’s output would be better ifthe48-bit bit-width and the MASH1-1-1are implemented.2. As the sigma-delta modulator used in the fractional-N PLL is a Finite StateMachine (FSM), its output sequence is of periodic behavior. It is the root cause offractional spurs in fractional-N synthesizer. The detailed theoretic analysis about theperiodic behavior of the sigma-delta modulator has been given in this thesis, and theformula to determine the accurate locations of the spurs has been propsoed. Largeamount of experimental spectra indicate that the calculated results from the formulacoincide with these spur locations of the experimental spectra.3. According to the spur issues of the fractional-N PLL, firstly, two kinds of spursuppression algrithms in these existing reference papers, namely Deterministic methodsand Stochastic methods, are compared and analysized. Specifically, the Deterministicmethods can suppress the fractional spur by prolonging the period of the sigma deltamodulator’s output sequence. Via comparison and analysis, it is concluded that thesedeterministic methods have high hardware cost, and little improvement on the noiseperformance of the output spectrum. This kind of methods has few practical values in the specific applications. On the other hand, the stochastic methods disturb the existingperiod of the output sequence by adding the random dithering signal to solve the spurproblems. These stochastic methods have lower hardware cost and better spurperformance compared with these deterministic methods. However, these methods alsohave different drawbacks. And most of these spurious carrier signals have residual spursafter adding these existing methods. It can be concluded that all these methodsaforementioned above are not efficient to solve the spur issues.4. After the analysis of these existing stochastic methods, a new spur suppressionmethod is proposed and compared with these existing methods in this thesis. Via largeamount of verifications, it is concluded that this new approach has a better performancein suppressing the spurs.5. MASH structure and the spur suppression structure are implemented in the(Field Programmable Gate Array) FPGA. At the same time, the verifications of thesemethods and the spectrum tests in the band4060MHz-7060MHz are accomplished byusing the designed structures and the existing RF platform.Via much experimental verification, the output spectrum purity can be improved byapplying the fractional-N divider designed in this thesis, the spurs in a majority ofsignals in the band can be suppressed by applying the proposed method. The followingperformances can be achieved in this phase locked loop: the frequency resolution islower than1Hz, the in-band and out-of-band spur suppression is better than-80dBc, thenoise performance is better than-95dBc/Hz@10kHz...