适用于WLAN接收机的高线性电流模式混频器设计

Design of High Linear Current Mode Mixer for WLAN Receiver

摘要:

针对无线局域网接收机对低成本和线性度的定制化需求,设计了一款适用于IEEE 802.11 b/g/n/ax标准WLAN接收机的高线性度电流模式混频器；采用零中频接收机架构,电流模式混频器的电路结构主要包括跨导级放大器,混频开关级和跨阻放大器；通过跨导级两种工作状态的转换和跨阻放大器反馈电阻的两种取值变化实现了混频器的四档增益可调；混频开关级选用双平衡无源混频电路以提供良好的线性度；为了解决零中频接收机存在的直流失调问题,加入了一种电流注入式的直流失调校准电路,进一步提高了混频器的线性度；对跨阻放大器中的跨导运算放大器电路进行优化设计以提高其带宽,使跨阻放大器的输入阻抗足够小以保证混频器的线性度；基于180 nm RF CMOS工艺,借助Cadence软件对混频器进行仿真:当本振频率为2.4GHz时,四档增益分别为38dB、32dB、27dB和21dB,中频带宽可达20MHz；噪声系数在高增益的情况下为8.46dB,输入三阶交调点在低增益的情况下可达13.72dBm；仿真结果表明,在较宽的中频带宽下,电流模式混频器取得了良好的线性度性能,满足WLAN接收机的定制化需求。

Abstract:

A high linearity current mode mixer for IEEE 802.11 b/g/n/ax standard WLAN receivers is designed to meet the customized requirements of low cost and linearity for wireless LAN receivers. Adopting a zero intermediate frequency receiver architecture, the circuit structure of the current mode mixer mainly includes a transconductance stage amplifier, a mixing switch stage, and a transimpedance amplifier. By switching between two operating states of the transconductance stage and changing the feedback resistance of the transimpedance amplifier, the four level gain of the mixer can be adjusted. The mixing switch stage adopts a dual balanced passive mixing circuit to provide high linearity. In order to solve the DC offset problem in zero intermediate frequency receivers, a current injection DC offset calibration circuit has been added to further improve the linearity of the mixer. Optimize the design of the transconductance operational amplifier circuit in the transimpedance amplifier to increase its bandwidth, ensuring that the input impedance of the transimpedance amplifier is small enough to ensure the linearity of the mixer; Based on the 180 nm RF CMOS process, the mixer is simulated using Cadence software. When the local oscillator frequency is 2.4GHz, the four gain levels are 38dB, 32dB, 27dB, and 21dB, respectively. The intermediate frequency bandwidth can reach 20MHz; The noise figure is 8.46dB under high gain conditions, and the input third-order intermodulation point can reach 13.72dBm under low gain conditions; The simulation results show that the current mode mixer achieves good linearity performance under a wide intermediate frequency bandwidth, meeting the customization requirements of WLAN receivers.
Keywords: current mode mixer; linearity; transconductance amplifier; transimpedance amplifier; zero intermediate frequency receiver; DC offset calibration