RFIC_Lecture_Note_No2_p13-p26 (Transceiver Architectures)

RFIC_Lecture_Note_No2_p13-p26 (Transceiver Architectures) -...

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Unformatted text preview: ECE695F RFIC Prof. S. Mohammadi Transceiver Architectures Receiver Architectures Heterodyne Receiver Signal frequency is transferred to much lower frequency to relax the Q requirement for filter LO Add LNA since mixers often have high noise figure IF LPF RF LNA mixer + + = GAIN MIXER LNA LNA NF NF NF Problem of Image: LO and IF are chosen by considering the image frequency LO LPF IMAGE RF , RF IMAGE Could be a strong interferer LO IF IF So the image frequency must be attenuated LO Image Reject Filter IMAGE RF , LNA RF Filter transfer function IMAGE - 13 - ECE695F RFIC Prof. S. Mohammadi If image is too close (Small IF) realizing the filter (image-reject) would be difficult Large IF Q of IF filter must be high Channel selection becomes difficult choice of IF is a trade-off among three parameters- image noise- spacing between desired band and the image- loss of the image reject filter = Q IF channelBW 1 Drawback of heterodyne image reject filter is an external component LNA must drive an external component (typically ) 50 lower gain higher noise figure stability problem higher power dissipation LO Image Reject Filter LNA In FDD system if IF is high enough image can be removed using FDD filter LNA can be connected directly to mixer LNA mixer- 14 - ECE695F RFIC Prof. S. Mohammadi Use dual-IF topology to solve the problem of image 1 LO Image Reject Filter LNA Band Select Filter 2 LO Channel Select Filter IF 1 IF 2 Channel Select Filter Out of band signal Next channels image GHz IM3 : 2W 2-W 1 MHz kHz variable fixed 2nd IF ~455kHz High IF so that image rejection is easy Homodyne Receiver ( Zero-IF or Direct Conversion ) very simple (IF = 0) * There is no problem of the image frequency Image filter is not needed RF LO = * No IF filtering is needed (need only low pass filters) RF LNA LPF RF LO =- 15 - ECE695F RFIC Prof. S. Mohammadi In case of phase or frequency modulation LPF LPF I (In - Phase) Q (Quadrature) LNA t LO cos t LO sin ) ( cos ) ( t t a ) ( sin ) ( t t b )) ( cos( ) (...
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RFIC_Lecture_Note_No2_p13-p26 (Transceiver Architectures) -...

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