Design And Realization Of Image Rejection Filter In The Wireless Receiver

Image rejection filter is an important part in the RF front-end circuit of wirelesscommunication system, It is used for rejecting the image sigals, and the design willaffect the treatment effect to the useful signals, then, the reception performance of thewhole system will be affected directly. There are three types of image rejection filters inthis thesis, passive RC polyphase filter, active polyphase filter and the complexband-pass filter,which has functions including image rejection and frequency selection.Image rejection filter studied in this paper is an important part of a monolithicallyintegrated wireless receiver. The goal of the work is to achieve a image recjection filterwhich can work in the high frequency with high efficiency, high integration and highimage rejection ratio.Polyphase filter includes passive polyphase filter and active polyphase filter. Apassive polyphase filter uses the RC architecture, the design is simple and easy toimplement, a high image rejection ratio can be achieved. However, because of the signalloss, buffer circuits are necessary between the stages or at the final output whilemulti-stage cascade.An active polyphase filter with the structure including activedevices and capacitors is based on the principle of passive polyphase filter, namely, theimage signal is cancellated by shifting the phase. The overall advantage of the activefilter is low power consumption, a small chip area and a high gain of the useful signal.As a result of the high input impedance of every stage, the active polyphase filter avoidslarge power loss effectively because the gains across every stage do not decrease whilemulti-stage cascading. four-stage cascaded passive and active polyphase filter designedin the work can get a image rejection of-60dB in the range of30MHz to60MHz.A complex band-pass filter coude provide channel selection and image rejection ina wireless receiver by passing useful signals and rejecting image sigals.The passive polyphase filter designed in this study is integrated in the wirelessreceiver chip taped out with TSMC-0.18μm process. Therefore,we can put the passivepolyphase filter in the whole channel to test its performance. By observing thedifference between the useful signal and the image signal, we can get the imagerejection ratio of the RF front-end. The test result is-30dB, meeting the needs of the RFfront-end chip.