The Effect And Application Of Passive Devices On Image Reject In Downconversion Mixer

The downconversion mixer is a significant circuits in most communication systems.It's used to convert the radio frequency(RF) signal down to an intermediate frequency(IF) by mixing the RF signal from the low noise amplifier(LNA) with the local oscillator(LO) signal.It is a very important building block because its performance affects the system performance. Building block specifications are desired to achieve high image rejection ratio(IRR),low noise figure(NF),high gain,moderate linearity,high sensitivity and low power consumption.There are three fundamental ways to realize image rejection in CMOS circuits: Heterodyne Receivers,Direct-Conversion Receivers and Image-Reject Architectures.In this paper,a downconversion mixer of Image-Reject Architecture for RF CMOS applications is presented.,in which a Resistance-capacitance(RC) ployphase filter network is used to achieve high image rejection ratio for a wideband RF input signal.This kind of network not only can provide an efficient way for wideband quadrature signal generation with reduced sensitivity to components mismatch,but also,polyphase network represents a passive implementation of complex analog filters used for image signal suppression.The general behavior of this class of networks,as well as their distinct property of discrimination between positive and negative sequences,are discussed.This network is so important ,because its performance settles the IRR of the downconversin,that a particual detailed analysis of the network about how the resistance and capacitance's the variability and mismatch affect the IRR of the downconversion mixer is investigated for one-,two-,and three-stage networks in this paper,and a hign IRR mixer is designed.The downconversion mixer has been implemented in a Chartered 0.25μm CMOS process,and the input RF 1.22GHz,LO 1.176/1.184GHz,the output IF is 36/44MHz.With the RC polyphase filter used in the mixer,following simulation results are achieved:IRR at less 49dB in 36-44MHz IF bandwidth,voltage gain 38dB,noise figure(NF) no more than 9.2dB.