Design Of Differential Acoustic Circulator

Circulators are non-reciprocal devices with several ports,which are widely used in the field of wireless communication.Traditional ferrite circulators can’t be used in the miniaturization equipment due to their large size and incompatibility with integrated circuit(IC)technology.The acoustic circulator based on spatiotemporal modulation angular momentum bias is a new type of magnetic-free circulator,which has become a research hotspot due to its advantages such as small size,low cost and integration.However,the intermodulation products brought about by modulation weaken non reciprocity,affecting the circulator’s isolation and insertion loss.Aiming at the above problems,this paper designed a differential bulk acoustic wave(BAW)circulator,constructed a theoretical model,obtained a differential circuit structure,and improved the performance of the circulator by eliminating intermodulation products.On this basis,a differential circulator based on surface acoustic wave resonators(SAWR)was fabricated,and it was verified that the differential structure can effectively improve the isolation of the circulator and reduce its insertion loss.The specific research work is as follows:The equivalent circuit model of the differential acoustic circulator is constructed,the differential principle is analyzed,and the circuit structure of the acoustic circulator is clarified.Taking the differential BAW circulator modulated by varactors as an example,the BVD model is used instead of a bulk acoustic wave resonator(BAWR),Under the assumption of weak and linear modulation,the varactor can be converted into a variable capacitor,thereby obtaining a small-signal linear model of the circulator;using Kirchhoff’s laws to analyze the current and voltage relationships in the circuit model,the frequency components present in the circuit and the scattering parameters of the differential circulator are derived.The results show that only the fundamental and first order intermodulation components exist in the differential BAW circulator circuit,and the intermodulation products can be eliminated by using a current-mode differential structure.This conclusion lays the foundation for subsequent circuit design.The circuit of differential BAW circulator is designed and simulated.A differential BAW circulator simulation circuit based on varactor diode modulation and switched capacitor modulation is established,and the scattering parameters of the circulator are simulated using the harmonic balance method.The simulation results show that the differential BAW circulator with switched capacitor modulation has better performance.When the series capacitance is 0.5 p F,its insertion loss is about 1.8 d B,isolation is about29.2 d B,and return loss is about 16.2 d B,compared to a single ended structure,the differential circulator has significantly improved insertion loss and isolation.The surface acoustic wave(SAW)circulator is fabricated and tested.By using parallel inductors,the effective electromechanical coupling coefficient of the SAWR is increased,solving the problem of poor performance,which is caused by small difference between series and parallel resonant frequencies of the SAWR;A switch modulated SAW circulator PCB circuit was designed,and the physical objects of a single-ended SAW circulator and a differential SAW circulator were fabricated and tested for performance.The measured insertion loss of the single-ended circulator is about 7.8 d B,and the isolation is about 10.1d B.The measured insertion loss of the differential circulator is about 4.8 d B,and the isolation is about 13.7 d B.It is verified that that the differential structure can effectively improve the performance of the circulator,which is consistent with theoretical analysis and simulation results.