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Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN620: Network TheoryBroadband Circuit Design
Fall 2014
Lecture 5: PLL Units & Noise Transfer Functions
Announcements, Agenda, & References
• HW1 is due Wednesday 9/24
• PLL Units• PLL Noise Transfer Functions
• Chapter 2, 3, 5, & 12 of Phaselock Techniques,F. Gardner, John Wiley & Sons, 2005.
• Chapter 1-3.4 of “Low-Power Low-Jitter On-Chip Clock Generation,” M. Mansuri, Ph.D. thesis, UCLA, 2003.
2
PLL Units w/ Dimensionless Filter (Non-Charge-Pump PLL)
3
Phase Detector
• Detects phase difference between feedback clock and reference clock
• The loop filter will filter the phase detector output, thus to characterize phase detector gain, extract average output voltage
• The KPD factor can change depending on the specific phase detector circuit
V/rad are units :filter loop less-dimension w/ PLL
PDK4
Dimension-Less Loop Filter
• Lowpass filter extracts average of phase detector signal
• No units for the dimension-less loop filter
( ) ( )
CRCR
sssF
2211
21
2
11
==
+++
=
ττ
τττ
Example: Passive Lag-Lead Loop Filter[Allen]
5
Voltage-Controlled Oscillator
• Time-domain phase relationship
VDDVDD/20
ω0 1KVCO
( ) ( ) ( )tvKtt cVCOoutout +=∆+= 00 ωωωω
( ) ( ) ( )∫ ∫=∆= dtdt tvKtt cVCOoutout ωφLaplace Domain Model
φout(t)( )
Vsrad102
VMHz12
Vsrad are units
6
⋅×=
⋅
ππ
VCOK
6
Loop Divider
• Time-domain model
( ) ( )tN
t outfb ωω 1=
( ) ( ) ( )∫ == tN
tN
t outoutfb φωφ 1dt1
[Perrott]
φout(t) φfb(t)
• The loop divider is dimension-less in the PLLlinear model
7
PLL Units w/ Dimensionless Filter (Non-Charge-Pump PLL)
less-unit isDivider LoopVs
rad are units
less-unit isFilter LoopV/rad are units
⋅VCO
PD
K
K
8
[Mansuri]
PLL Units w/ Impedance Filter (Charge-Pump PLL)
=π2CP
PDDIKK
9
Phase Frequency Detector (PFD)• Phase Frequency Detector allows for wide
frequency locking range, potentially entire VCO tuning range
• 3-stage operation with UP and DOWN outputs• Net difference in pulse width represents phase error• UP longer = Positive ∆φ• DN longer = Negative ∆φ
• Edge-triggered results in duty cycle insensitivity• The un-averaged PFD gain (ratio of net output pulse time width with
input phase error time) is typically “1” and dimension-less
10
Averaged PFD Transfer Characteristic
• Constant slope and polarity asymmetry about zero phase allows for wide frequency range operation
• The averaged PFD gain is typically “1/(2*pi)” with units of rad-1
UP=1 & DN=-1
[Perrott]
11
Charge Pump
• Converts PFD output signals to charge
• Charge is proportional to PFD pulse widths
( )
used isdetector phasedifferent a ifcan vary gain Thisrad
Amps 2
Gain Pump-Charge & PFD Total
radAmps
2 Gain Pump-Charge Averaged
Amps Gain Pump-Charge Averaged-Un
=
=
=
π
π
CP
CP
CP
I
II
12
Loop Filter
• Lowpass filter extracts average of phase detector error pulses
• The units of the filter are ohms
I
I
VCO ControlVoltage
C1
R
C2
Charging
Discharging
VDD
VSSF(s)
( )sRC
sRsF
+
= 1
1
( )
++
+
=
21
21
12
11
CRCCCss
RCs
CsF
w/o C2
w/ C2
13
[Mansuri]
PLL Units w/ Impedance Filter (Charge-Pump PLL)
=π2CP
PDDIKK
less-unit isdivider Loop
Vsrad are units , of units hasFilter Loop
radA of units hasgain combined Pump-Charge &Detector Phase Total
averaged if radA averaged,-un ifA are unitsGain Pump Charge
averaged if rad of units averaged,-un if less-unit is 1-
⋅Ω VCO
PD
K
KOnly Average
Once
14
PLL Noise Transfer Function
[Mansuri]
15
Input Noise Transfer Function
( ) 222 2
221
nn
nn
n
outn ss
sN
RCKKss
RCsNK
sHIN
IN ωζωζωζω
φφ
++
+=
++
+
==
( )
++
+
=
==
RCKKsss
RCsNKK
sK
vsT
oo
n
out
n
outn
ININ
IN2
1
φφφ
sKo
Input Phase Noise:
Voltage Noise on Input Clock Source:
[Mansuri]
)1 (assumes 2
:factorgain loop aw/ == PDVCOCP KN
RKIKπ
16
Input Noise Transfer Function
( ) 222 2
221
nn
nn
n
outn ss
sN
RCKKss
RCsNK
sHIN
IN ωζωζωζω
φφ
++
+=
++
+
==
( )
++
+
=
==
RCKKsss
RCsNKK
sK
vsT
oo
n
out
n
outn
ININ
IN2
1
φφφ
Input Phase Noise:
Voltage Noise on Input Clock Source:
( )
( )VCO0
VCOn
,10 ,12
26.1 ,253
1N ,12 ,rad 2
10
10*2 ,1 ,1*2
Parameters Simulation
ωωπ
ππµ
πωζπω
===
Ω==
===
===
bufdelay
VCOPD
VpsK
VMHzK
kRpFC
VGHzKAK
GHzMHz
17
VCO Noise Transfer Function
( ) 22
2
2
2
2 nnn
outn ss
s
RCKKss
ssHVCO
VCO ωζωφφ
++=
++==
( )
RCKKss
sKs
Kv
sT VCOVCO
n
out
n
outn
VCOVCO
VCO
++=
==
2φφφ
VCO Phase Noise:
Voltage Noise on VCO Inputs:
KVCO is different if the input is at the Vcntrl input (KVCO) or supply (KVdd)
[Mansuri]
18
VCO Noise Transfer Function
( ) 22
2
2
2
2 nnn
outn ss
s
RCKKss
ssHVCO
VCO ωζωφφ
++=
++==
( )
RCKKss
sKs
Kv
sT VCOVCO
n
out
n
outn
VCOVCO
VCO
++=
==
2φφφ
VCO Phase Noise:
Voltage Noise on VCO Inputs:
( )
( )VCO0
VCOn
,10 ,12
26.1 ,253
1N ,12 ,rad 2
10
10*2 ,1 ,1*2
Parameters Simulation
ωωπ
ππµ
πωζπω
===
Ω==
===
===
bufdelay
VCOPD
VpsK
VMHzK
kRpFC
VGHzKAK
GHzMHz
19
Clock Buffer Noise Transfer Function
( ) 22
2
2
2
2 nnn
outn ss
s
RCKKss
ssHbuf
buf ωζωφφ
++=
++==
( )
RCKKss
sK
RCKKss
ss
Ks
Kv
sT VCOdelay
buf
VCOdelay
buf
VCOdelay
n
out
n
outn
bufbuf
buf
++≈
++
+=
+
==
2
2
2
2
11
ω
ω
ω
ω
ωφφφ
Output Phase Noise:
Voltage Noise on Buffer Inputs:
Kdelay units = (s/V)
[Mansuri]
1+buf
VCOdelays
K
ω
ω
20
Clock Buffer Noise Transfer Function
( ) 22
2
2
2
2 nnn
outn ss
s
RCKKss
ssHbuf
buf ωζωφφ
++=
++==
( )
RCKKss
sK
RCKKss
ss
Ks
Kv
sT VCOdelay
buf
VCOdelay
buf
VCOdelay
n
out
n
outn
bufbuf
buf
++≈
++
+=
+
==
2
2
2
2
11
ω
ω
ω
ω
ωφφφ
Output Phase Noise:
Voltage Noise on Buffer Inputs:
( )
( )VCO0
VCOn
,10 ,12
26.1 ,253
1N ,12 ,rad 2
10
10*2 ,1 ,1*2
Parameters Simulation
ωωπ
ππµ
πωζπω
===
Ω==
===
===
bufdelay
VCOPD
VpsK
VMHzK
kRpFC
VGHzKAK
GHzMHz
21
PLL Noise Transfer Function Take-Away Points
• The way a PLL shapes phase noise depends on where the noise is introduced in the loop
• Optimizing the loop bandwidth for one noise source may enhance other noise sources
• Generally, the PLL low-pass shapes input phase noise, band-pass shapes VCO input voltage noise, and high-pass shapes VCO/clock buffer output phase noise
22
Next Time
• PLL System Analysis• Transient Response
23