1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer...
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Transcript of 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer...
![Page 1: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/1.jpg)
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Charge Pump PLL
![Page 2: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/2.jpg)
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Outline
• Charge Pump PLL– Loop Component Modeling– Loop Filter and Transfer Function
• Loop Filter Design
• Loop Calibration
![Page 3: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/3.jpg)
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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
![Page 4: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/4.jpg)
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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
![Page 5: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/5.jpg)
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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
21
![Page 6: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/6.jpg)
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Charge Pump
• Convert a digital signal into current
UP
DN
Iup
IdnPI
![Page 7: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/7.jpg)
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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
CCsCRCs
sRCsF
IpVC
C1
R
IpVC
C1
RC2
1
1)(
sCRsF
![Page 8: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/8.jpg)
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VCO
• Tuning gain KVCO is the most important parameter
• Usually coarse tuning and fine tuning
s
KVCO
![Page 9: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/9.jpg)
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CP PLL loop modeling
PhaseDetector
ChargePump
LoopFilter VCO
fi fo
fo
θi θo
![Page 10: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/10.jpg)
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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
![Page 11: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/11.jpg)
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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
C
VCOKpIRVCOKpIs
CCsCRCs
sRCVCOKpI
soG
122)1(21
3
122)(
CVCOKpIRVCOKpI
smsmRCs
CVCOKpIRVCOKpI
s
scG
![Page 12: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/12.jpg)
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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
![Page 13: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/13.jpg)
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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
C
VCOKpI
n
22 VCOp
n
RKI
223
2
2)1(2
2)(
nnn
nnc
ssmsm
ssG
![Page 14: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/14.jpg)
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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)
![Page 15: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/15.jpg)
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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
![Page 16: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/16.jpg)
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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
![Page 17: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/17.jpg)
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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
![Page 18: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/18.jpg)
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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)
![Page 19: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/19.jpg)
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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
![Page 20: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/20.jpg)
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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
![Page 21: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/21.jpg)
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JP(m) and ξm(m) with sampling effect
JP(m) with sampling effect ξm(m) with sampling effect
![Page 22: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/22.jpg)
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Tables of JP(m) and ξm(m) for practical design
![Page 23: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/23.jpg)
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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
![Page 24: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/24.jpg)
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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
![Page 25: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/25.jpg)
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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
![Page 26: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/26.jpg)
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Charge pump PLL calibration
• Purpose: make the loop transfer characteristic meet the spec
• Calibration types:– Component calibration– Loop calibration
![Page 27: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/27.jpg)
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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
![Page 28: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/28.jpg)
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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
![Page 29: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/29.jpg)
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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
![Page 30: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/30.jpg)
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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
![Page 31: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/31.jpg)
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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
![Page 32: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/32.jpg)
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Time Constant Calibration Procedure
tRC
VreftVX )(
Vref
Comparator
Counter
Ref CLK
RVref
R
C
Vx
RCfCounter ref #
![Page 33: 1 Charge Pump PLL. 2 Outline Charge Pump PLL –Loop Component Modeling –Loop Filter and Transfer Function Loop Filter Design Loop Calibration.](https://reader035.fdocuments.us/reader035/viewer/2022081504/5518a0e7550346c31f8b488a/html5/thumbnails/33.jpg)
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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)