Design And Realization Of 6-bit CMOS Digitally Controlled Attenuator At Millimeter-Wave Band

The phased array is an antenna array technology in which the phase of the antenna unit is controllable.The reception and transmission of electromagnetic beams are realized by the antenna array unit.Due to large bandwidth,anti-interference ability and high detection accuracy,the phased array is widely used in high-precision military and civilian fields.The phased array system is a complex system composed of sub-modules such as T/R modules,feeder networks and digital modules.The performance of each sub-module will affect the function of the entire system.As a key component of the T/R module,the gain control module has the functions of compensating for the gain deviation between different channels and reducing the side lobe level.Its research is of great value.Therefore,in order to meet the demand for high-performance attenuator for millimeter-wave phased array radars,this paper introduces the attenuation accuracy,phase-shift variation,insertion loss and linearity of digital attenuator,which is of great significance to support the development of millimeter-wave integrated circuits.Millimeter-wave phased array systems have requirements on the high-performance attenuator,and its phase-shift variation and attenuation accuracy are important for the beam scanning accuracy of the phased array.First,this paper introduces the performance parameters of active attenuator and passive attenuator,including attenuation range,attenuation step,operating bandwidth,phase-shift variation,attenuation accuracy,linearity and so on.The optimization of the performance indicators is analyzed in detail.Second,this paper research how to reduce phase-shift variation without reducing attenuation accuracy.In order to achieve a high-performance attenuator,this paper proposes a combination of an ultra-low power,high linearity passive attenuator and a low insertion loss,low noise active attenuator.The hybrid attenuator is analyzed and optimized for its gain variation,phase-shift variation and insertion loss.Active attenuator achieve gain control based on a current-steering structure,introducing additional cascode and on-chip passives to reduce phase-shift variation.In order to meet the inter-stage matching,the input and output matching use source degenerate inductance technology and inductance peaking technology to achieve impedance matching,respectively.The passive attenuator is based on the π type attenuation structure and deep N-well process.The switch MOS transistor adopts body floating technology to reduce the insertion loss,and the parallel signal branch adopts capacitor low pass network for compensation to reduce the phase-shift variation.Based on SMIC 55 nm RF CMOS process,a Ka-band low phase-shift variation CMOS6-bit digital attenuator is designed and implemented by using a combination of 5-bit active attenuator and 1-bit negative attenuator in this paper.After several iterations and optimizations of the layout,the post-simulation results are obtained.The attenuation step of the attenuator is 0.5d B,and the dynamic attenuation of 31.5d B is achieved.The RMS gain error is less than 0.28 d B,and the RMS phase error is less than 3.5°.The input/output return loss is better than 10 d B,the insertion loss is less than 1.5d B,and the input 1d B compression point of the reference state is-1.46 d Bm@30GHz.The circuit simulation results show that the attenuator achieves high attenuation accuracy and low phase-shift variation,which proves the effectiveness of the hybrid attenuator combining active attenuator and negative attenuator.