1 1 High Frequency Voltage Controlled Ring Oscillators in Standard CMOS Yalcin Alper Eken PhD Candidate in School of ECE GaTech July 7 th , 2003 2 Agenda § Integrated VCO types § Ring oscillator theory § Important characteristics of ring oscillators § Frequency § Noise § High frequency low noise ring oscillators § Prototype Chip § Performance Comparison § Applications/Summary/Conclusions
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High Frequency Voltage Controlled Ring Oscillators in Standard CMOS
Yalcin Alper EkenPhD Candidate in School of ECE
GaTech
July 7th, 2003
2
Agenda
§ Integrated VCO types§ Ring oscillator theory§ Important characteristics of ring oscillators§ Frequency§ Noise
§ High frequency low noise ring oscillators§ Prototype Chip§ Performance Comparison§ Applications/Summary/Conclusions
2
3
Integrated VCO Types
§ LC Oscillator
§ Ring Oscillator
4
Resonator
Amplifier
VCO Types : LC
§ High Q resonant element
§ Expensive to implement
§ Require more die area
§ Reduce integration density
§ Extra steps
§ Secondary effects
§ Eddy currents
§Magnetic coupling
LC Oscillator
3
5
VCO Types : Ring
§ Less expensive to implement
§ Wider tuning range
§ Multiple output phases
§ Low Q
Ring Oscillator
6
Ring Oscillator Theory
4
7
Ring Oscillator Operation in Time Domain
§Odd number of inversions
§ T = 6*Td or 2N*Td for N stage
§ fosc = 1/(6*Td) or 1/(2N*Td) for N stage
At t = t 1
At t = t 1+TdAt t = t 1+2Td
At t = t 1+3TdVinitial
GndVinitial
Gnd
Vinitial
Vdd
Gnd
Vdd
X1 X2 X3
8
S-domain Analysis : Ring Oscillator
(s)A(s)A(s)AsA
(s)(s)...A(s)AAL(s)
N21N
N21
====
=
...)( that assuming
frequency noscillatio theat
and
:Criterion Barkhausen
1)(2
)( 00 ===∠N
jANk
jA ωπ
θω
AmplifierA(s)
Frequency Selective Network
α (s)
X(s) Y(s)
5
9
Ring Oscillator Linear Model
+−
=ω
ωRCj
RgjA m
1)( functiontransfer Stage
2
2cos
1
≥
≥
≥
Rg
Rg
Rg
m
m
m
stage-4For
stage-3For
:t requiremen Gainθ
RC
RC
RC
1
3
tan
=
=
=
0
0
0
stage-4For
stage-3For
:Frequency
ω
ω
θω
0=φ θπφ += θπφ 22 +=
stages of # oddfor
0
)(
)(
=
+=
+=
NN
Nπ
π
θπφ
Nπ
θ =
10
Differential Ring Oscillators
A1+
- +
-A2
+
- +
-A3
+
- +
-A4
+
- +
-
§ Better immunity to common-mode disturbance
§ 50% duty cycle
§ Improved spectral purity
§ Even/Odd number of stages
6
11
Important Characteristics of Ring VCOs
§ Frequency
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Frequency Tuning - I
Load Control -I
Load Control - II
Current ControlDrive
Strength Control
swingL
controlosc
control
swingLd
VNCI
f
I
VCT
2=
=
7
13
Frequency Tuning - II
Feedback Control
Coupling Control
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Frequency Increase : Multipliers
8
15
Frequency Increase : Subfeedback Loops1
Implementation with N = 5, i = 2
X1 X2 X3 X4 X5
5-Stage Main-Loop
3-Stage Subfeedback Loop
1 L. Sun, T. Kwasniewski, and K. Iniewski, “A Quadrature Output Voltage Controlled Ring Oscillator Based on Three-Stage
Subfeedback Loops,” Proc. Int. Symp. Circuits and Systems, Orlando, FL, 1999, vol. 2, pp. 176-179.
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Important Characteristics of Ring VCOs
§ Noise
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17
Phase Noise : Leeson’s Model
2
0
22
}{
∆
=∆ω
ωω
QPFkT
LS
Single Sideband Oscillator Phase Noise in Leeson’s Model
Q of LC Oscillators
CMOS) (standard
10≤Q
Q of a ring oscillator?
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Ring Oscillator Q : Razavi
220
2
+
=
ωφ
ωω
dd
ddA
Q
2
0
22
}{
∆
=∆ω
ωω
QPNFkT
LS
Q of a ring oscillator
Modified Leeson’sequation
4.12
3.1433
≅
≅
: Q stage-4
: Q stage-3
10
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Application of Harjani's Equation
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Time (nsec)
Sw
ing (V
)
Sine CurvefitOutput Signal
Phase Noise : Harjani
>>
∆
<<
∆=∆
πωω
π
πωω
ω
3*8
)(27
512
3*8
)(9
64
}{20
3
202
ddpp
pp
dd
ddpp
pp
VVfor
VFkTRV
VVfor
VFkTR
L
Equation from : L. Dai, and R. Harjani, “Design of Low-Phase-Noise CMOS Ring-Oscillators,” IEEE Trans. Circuits Sys. II, vol. 49,