ECE264D_Lecture2

ECE264D_Lecture2 - Design of CMOS RF Frequency Synthesizers...

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Design of CMOS RF Frequency Synthesizers cture 2: Basic PLL and Components Lecture 2: Basic PLL and Components GANG ZHANG September 2009 © 2009 G. Zhang ECE264D Fall 2009
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Basic Phase locked loop Phase Detect Loop Filter VCO Divider N Reference clk VCO output + - Fref Fvco Fvco=Fref*N Page 2 ! Negative feedback system: ! Forward path: high gain, but gain not well defined; ! Feedback path: well defined division ratio (feedback factor), very good linearity; ! discrete time sampling at phase detector ! Phase signal is sampled at edge zero-crossing instants: divider, phase detector (charge pump), discrete-time signal processing; ! From loop filter to VCO, signal becomes continuous: sample and hold at loop filter; ! Mixed domains: phase, voltage(current, charge), frequency. . ECE264D RF Frequency Synthesizers © 2009 G. Zhang
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! Negative feedback is a fundamental mechanism everywhere in the universe. ! Exists in every aspects in everyday life: social, economical, evolution, mechanical,. . Negative Feedback: de-sensitivity of forward gain variation a β input + - output + Closed-loop gain: Page 3 ! The fundamental benefit/effect of Negative feedback is de-sensitizing of forward gain variation (self-correcting) ! Variation in “a” will give attenuated variation in “A” by a factor of (1+a β ), close to loop gain; ! This means any error (of gain, nonlinearity. .) in forward path gain will be attenuated; ! This does not apply to variation in feedback gain: β ! Good negative feedback: has to be stable first, ! High loop gain, esp. forward gain, moderate error in the forward path can be tolerated. ! Linear feedback path with minimal error; ECE264D RF Frequency Synthesizers © 2009 G. Zhang
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Phase detector Phase Detect Reference clk + - Fref Divider output output δφ δφ output δφ output ideal real Page 4 !
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ECE264D_Lecture2 - Design of CMOS RF Frequency Synthesizers...

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