Pll Charge Pump
description
Transcript of Pll Charge Pump
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Charge Pump PLL
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Outline• Charge Pump PLL
– Loop Component Modeling– Loop Filter and Transfer Function
• Loop Filter Design• Loop Calibration
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Charge Pump PLL• The charge pump PLL is one of the most
popular PLL structures since 1980s • Featured with a digital phase detector and a
charge pump• Advantages
– Fast lock and tracking– No false lock
PhaseDetector
ChargePump
LoopFilter VCO
N-Divider
fi fo
fo
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Phase Detector• Gives the phase difference between the input
clock signal and VCO output signal• Different types
– Nonlinear (such as Bang-Bang)– Linear (such as Hogge’s Phase Detector)
• Linear PD output a digital signal whose duty ratio is proportional to the phase difference – In Hogge’s PD, if the phase difference is θe , the
output digital signal duty ratio is
2e
C. Hogge, “A Self-correcting clock recovery circuit”, Dec, 1985
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Typical Phase Detector and Waveform
Y. Tang, et., al., "Phase detector for PLL-based high-speed data recovery," Nov. 2002
CircuitStructure
OutputWaveform
When locked
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Charge Pump• Convert a digital signal into current
UP
DN
Iup
IdnPI
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Loop Filter• Low pass filter
– 1st order– 2nd order (higher roll-off speed at high
frequency)– 3rd order & higher
)(1)(
21212
1
CCsCRCssRCsF
Ip VC
C1
R
Ip VC
C1
RC2
1
1)(sC
RsF
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VCO• Tuning gain KVCO is the most important
parameter• Usually coarse tuning and fine tuning
sKVCO
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CP PLL loop modeling
PhaseDetector
ChargePump
LoopFilter VCO
fi fo
fo
θi θo
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2nd Loop Transfer Function• Using a 1st order LPF: Active PI type• Open-loop transfer function
• Closed-loop transfer function1
22
11)(
Cπs
)(sRCVCOKpIsoG
1222
122
πCVCOKpI
π
RVCOKpIss
πCVCOKpI
π
RVCOKpIs
(s)cG
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3rd Loop Transfer Function• Using a 2nd order LPF• Let m=C2/C1• Open-loop transfer function
• Closed-loop transfer function)1(21
3122
)21(2
213
)11(2)(
msmRCs
CVCOKpIRVCOKpI
s
CCsCRCs
sRCVCOKpI
soG
122)1(21
3
122)(
CVCOKpIRVCOKpIsmsmRCs
CVCOKpIRVCOKpIs
scG
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Comparison• When m becomes 0, the 3rd order loop
degenerates into 2nd order loop• 3rd order loop gives an extra high frequency
pole, which increases the high frequency roll-off in jitter transfer
• 3rd order loop is widely used and can be treated as 2nd order loop for simplification
• Unfortunately, the 3rd order loop shows different jitter transfer from the 2nd order loop
• We focus on 3rd order loop
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Simplification of 3rd Order Loop• Define natural frequency ωn & damping ratio ξ
• Then totally 3 loop parameters: ωn, ξ &m• Simplified transfer function
122
CVCOKpI
n
22 VCOp
n
RKI
223
2
2)1(22)(
nnn
nnc
ssmsm
ssG
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LPF Design Consideration• 3-dB frequency – easy to control• Roll-off speed– easy to meet with 2nd and 3rd
order transfer function• Jitter transfer (jitter peaking)
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Jitter peaking of 2nd order loop• Jitter peaking can be reduced or
eliminated by increasing the damping ratio– Eliminated when damping ratio ξ >1
• Large damping ratio leads to slow closed-loop response
• Usually suggested ξ=5 to meet the jitter peaking spec
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Jitter peaking of 3rd order loop• Usually believed to be similar as the 2nd
order loop• Actually quite different from the 2nd order
loop case• Jitter peaking always exists even with very
large ξ• Need to be treated carefully
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Jitter peaking is dependent on ξ and m
• m=0 (2nd loop) jitter peaking can be
reduced or eliminated by using large ξ
• m>0 (3rd loop) ξ is quite small, increasing
ξ will decrease the jitter peaking;
ξ is larger than a threshold value ξm, increasing ξ will increase the jitter peaking
Jitter peaking versus damping ratio and capacitance ratio
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How to achieve the minimum jitter peaking
• For given m, there exists the minimum jitter peaking
--the minimum jitter peaking can be viewed as a function of m: JP(m)
• The minimum jitter peaking under a given m is achieved only by using a proper ξ
--ξ should be a function of m: ξm(m)
JP(m)
ξm(m)
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Sampling effect of phase detector • The phase detector has sampling effect,
especially when its rate is not much higher than the loop cut-off frequency
• Approximate TF of phase detector :
21e-1)(
P-sT
P
PD sTsH
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Jitter Peaking w/ PD Sampling Effect• It causes the jitter
peaking worse when ξ is very small, jitter
peaking decreases when ξ increases;
when ξ becomes larger than ξm, jitter peaking increases with ξ;
when ξ is larger than ξm2, jitter peaking decreases when ξ is increased further
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JP(m) and ξm(m) with sampling effect
JP(m) with sampling effect ξm(m) with sampling effect
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Tables of JP(m) and ξm(m) for practical design
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Design procedures of charge pump PLLs
for jitter transfer characteristic optimization 1. Decide the maximum tolerated jitter peaking and find
capacitance ratio m using JP(m). 2. Use ξm(m) to find the optimal damping ratio value ξm;3. Decide ωn according to the application, choose
reasonable KVCO, and calculate Ip, R, C1 and C2;4. Use time domain simulation to verify that the expected
jitter transfer performance can be achieved
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Design example
• Target: to design a 2.5GHz CP PLL, meet the jitter specification
• Design parameters: m=0.005 and ξ=5.0
• Simulation result: jitter peaking is only 0.078dB
Jitter transfer characteristic of the designed PLL
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More Discussion on Loop Transfer Function
• The above discussion suggests to use very small m to meet the jitter peaking
• However, if m is too small, the effect of the second capacitor can even be ignored
• Compromise should be made between jitter peaking and other performance
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Charge pump PLL calibration• Purpose: make the loop transfer
characteristic meet the spec • Calibration types:
– Component calibration– Loop calibration
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Charge Pump Calibration• Purpose: minimize the mismatching
between UP and DOWN current• Method: switch small current sources
UP
DN
Iup
Idn
UP
DN
Iup
Idn
…ICAL
ICAL ICAL ICAL
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Charge Pump Calibration Procedure
• Use the UP or Down current to charge/discharge a capacitor
• Compare the time difference and calculate the calibration code
UP
DN
Iup
Idn
Vref
Comparator
Counter
Ref CLK
R/S
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VCO Coarse Tuning• Purpose: to speed frequency tracking• Method: make use of the coarse tuning
functionality of the VCO• When extreme high frequency range is
desired, double VCOs can be used to help achieve fine frequency tuning resolution
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VCO Coarse Tuning Procedure• Apply different coarse tuning voltage
(output from a low resolution coarse tuning DAC)
• Measure VCO output frequency respectively– Compare to the reference frequency
• Write the desired DAC code into register
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Time Constant Calibration• Purpose: calibrate the loop transfer
function time constant so that the 3-dB frequency meets the spec
• Method: switch small CAL capacitors
…CCAL
CCAL CCAL CCAL
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Time Constant Calibration Procedure
tRCVreftVX )(
Vref
Comparator
Counter
Ref CLK
RVref
R
C
Vx
RCfCounter ref #
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Loop Gain Calibration• Purpose: calibrate the loop transfer gain to
the desired value• Method: switch different charge pump
output current (KVCO is not changeable usually)