RFIC_Lecture_Note_No13_p217-p235 _Mixers_

RFIC_Lecture_Note_No13_p217-p235 _Mixers_ - ECE695F RFIC...

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Unformatted text preview: ECE695F RFIC Prof. S. Mohammadi - - 217 Mixers Down-conversion LNA RF IF Amp IF Mixer LO VCO Up-conversion RF IF PA VCO LO important aspects of mixer RF IF G C = * Conversion gain (loss) = G C mixers can be passive so they may show loss rather than gain (sometimes passive mixers are better in terms of isolation, linearity, NF) conversion gain can be in terms of voltage, current or power consider a multiplier function : ( ) ( ) ( ) ( ) [ ] t t AB t B t A LO RF LO RF LO RF + + = cos cos 2 cos cos IF high frequency term that will be filtered out amplitude LO of B A AB G C 2 1 2 2 = = = voltage you expect that conversion gain be a function of LO power! ECE695F RFIC Prof. S. Mohammadi - - 218 * Noise figure NF definition is tricky because the noise present in both and band will be down-converted to IF frequency RF image LO RF image side bands IF IF most frequently the data (signal) we are interested is at frequencies. RF But the noise comes from both and frequency bands this type of noise figure is called single side band NF RF image if data is actually on and then we can define double-side-band NF RF image DSB NF is typically 3 dB lower than SSB NF NF of mixers are typically much higher than LNA s if it was low we would not have used LNA typically SSB NF : 10 ~ 15 dB The reason it is high 1) Noise from frequencies other than RF frequencies will contribute 2) mixers often use switching elements a switch transistor operate between active (saturation) and off state in linear/sub-threshold state, the transistor is very noisy ECE695F RFIC Prof. S. Mohammadi - - 219 * Mixer linearity mixer itself is a non-linear element, so what does the linearity of a mixer means? RF IF G C = this gain should be constant as you go to high input power (RF ) gain starts to drop (saturation) we can define * 1-dB compression point * IP3 IF output power RF input power 1dB comp. point IP3 3rd order IM term ECE695F RFIC Prof. S. Mohammadi - - 220 * Isolation in mixers RF IF LO ideally you want @ IF terminal only IF and not RF or LO not a big deal since you can filter RF or LO @ RF terminal only RF...
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RFIC_Lecture_Note_No13_p217-p235 _Mixers_ - ECE695F RFIC...

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