Research And Design Of 5?10GHz Ultra-low Phase Noise Frequency Synthesizer

Frequency synthesis technology has developed rapidly in recent years.As a core part of the communication system,its application range has become more and more extensive.The performance index of the frequency synthesizer determines the performance index of the system to some extent.At present,most frequency synthesizers have begun to take into account the requirements of comprehensive indicators such as broadband,ultra-low phase noise,small stepping,frequency agility,and miniaturization.This paper proposes a 5?10GHz small step ultra-low phase noise frequency synthesizer based on the dual phase-locked loop structure and harmonic mixing technology.First,by analyzing the phase noise model of the phase-locked loop circuit,it is determined that the mixing structure is adopted to reduce the loop frequency division ratio N to improve the phase noise index.Due to the low phase noise broadband output of 5?10GHz,there are very high requirements on the input local oscillator signal of the mixer,which increases the design difficulties,and finally generates high-quality local oscillator signals through the use of harmonic mixing.Then,the double-loop structure is used to achieve full frequency band coverage of 5-10 GHz.After reasonable frequency planning,1/5 frequency band is output by PLL 1 and 4/5frequency band is output by PLL 2.The high-quality local oscillator signal in the loop 2 is provided by the output of the loop 1,and the phase-locked loop 2 also uses a mixing structure to reduce the frequency division ratio in the loop to improve the phase noise of the output signal.At the same time,because the system index requires small step output.Utilize the high resolution of DDS output frequency,make its output as the reference input signal of loop 1.The output signal of DDS adopts up-conversion and frequency-division processing,which effectively improves its output spurious and phase noise.Finally,based on the above scheme,the key chip selection was completed and related simulations were performed to verify the feasibility of the scheme,and complete the circuit and structural design.The actual product has been debugged,and the final measured results show that the output frequency of the frequency synthesizer can achieve 5?10GHz coverage,the phase noise can reach-116 d Bc/Hz@10k when the output frequency is 10 GHz,the frequency resolution is 1Hz,and the spurious suppression can reach-60 d Bc,harmonic suppression is better than-35dBc.