RFIC_Lecture_Note_No2_p13-p26 (Transceiver Architectures)

RFIC_Lecture_Note_No2_p13-p26 (Transceiver Architectures) -...

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ECE695F RFIC Prof. S. Mohammadi Transceiver Architectures Receiver Architectures Heterodyne Receiver Æ Signal frequency is transferred to much lower frequency to relax the Q requirement for filter LO ω Add LNA since mixers often have high noise figure IF ω LPF RF ω LNA mixer + + = GAIN MIXER LNA LNA NF NF NF Æ Problem of Image: LO and IF are chosen by considering the image frequency LO ω LPF IMAGE RF ω ω , RF ω IMAGE ω Could be a strong interferer LO ω IF ω IF ω So the image frequency must be attenuated LO ω Image Reject Filter IMAGE RF ω ω , LNA RF ω Filter transfer function IMAGE ω - 13 -
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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 ω 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 -
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ECE695F RFIC Prof. S. Mohammadi Use dual-IF topology to solve the problem of image 1 LO ω Image Reject Filter LNA Band Select Filter 2 LO ω 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 ) Æ very simple (IF = 0) * There is no problem of the image frequency Æ Image filter is not needed RF LO ω ω = * No IF filtering is needed (need only low pass filters) RF ω 0 LNA LPF RF LO ω ω = - 15 -
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ECE695F RFIC Prof. S. Mohammadi In case of phase or frequency modulation LPF LPF I (In - Phase) Q (Quadrature) LNA t LO ω cos t LO ω sin ) ( cos ) ( t t a θ ) ( sin ) ( t t b θ )) ( cos( ) ( )) ( cos( ) ( t t t b t t t a RF RF θ ω θ ω + + + Problem with DC offset in zero-IF Æ
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