The Design Of Sigma-Delta Modulator In Fractional Phase-Locked Loop For GFSK Modulation

Gaussian frequency shift keying(GFSK)is a continuous phase modulation method,and it has an extensive use in low-speed personal communication(such as Bluetooth)because of its constant envelope,compact spectrum and narrow spectrum ability.In the closed-loop GFSK modulation structure,the baseband data filtered by Gaussian filter is transformed into a range of sequences by the Sigma-Delta modulator(SDM).The sequences control the frequency divider,resulting in a certain frequency offset of the output of the voltage-controlled oscillator(VCO)to realize GFSK modulation.However,the SDM in the current design has the problem of serious interference to the spurious signal in quantization noise power spectral density,resulting in the existence of spurious components in the output spectrum of the fractional phase-locked loop.Therefore,it is urgent to carry out the research of the new SDM.The paper mainly focuses on the design of SDM that is suitable for fractional phase-locked loop.This paper realizes the design of the closed-loop GFSK modulation circuit based on the improved SDM.In this paper,the principle of the closed-loop GFSK modulation is introduced and analyzed firstly.Then,based on the existing problems of MultistAge noise SHaping(MASH)structure,this paper proposes an improved structure(denoted SH-MASH).The SH-MASH structure absorbs and synthesizes the advantages of prime-modulus MASH structure and SP-MASH structure.With the theoretical derivation and simulation with Matlab,it is proved that the SH-MASH structure yields the maximum sequence length for all input values and for all initial conditions.The noise power spectrum of the proposed structure is closed to ideal noise curve.Then,the SH-MASH structure is applied to the fractional phase-locked loop.We perform behavior simulation and verification based on Matlab,and perform phase noise analysis based on Cppsim software.The designed fractional phase-locked loop is then applied to GFSK closed-loop modulation to verify the performance of the SH-MASH structure.By the use of the behavior model of fractional phase-locked loop,the phase noise model of the fractional phase-locked loop,and the behavior model of GFSK modulation,we completes the performance simulation of the SH-MASH structure.The simulation results show that the improved SH-MASH structure can effectively suppress the fractional spurs and realize the closed-loop GFSK modulation.Compared with the other structures,the SDM based on SH-MASH structure is superior to other structures without significantly increasing the hardware cost and it is comparable to the HK-MASH structure which is recognized the best structure in the current industry,but the SH-MASH structure has a simpler structure.Therefore,the improved method proposed in this paper has high academic value.The characteristics and innovations of this paper is proposed an improved MASH structure that yields the maximum sequence for all input and for all initial conditions.At the same time,the quantization noise of the SH-MASH structure can be effectively suppressed,so that the loop filter of the fractional phase-locked loop can eliminate the quantization noise to the maximum extent and obtain good spectral characteristics.