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Amplifier A
OSCILLATORS
Fundamentals. Linear Aspects: Oscillation conditions.
Frequency
Selective
Network
o sx
+
+
ox
fx
)s(A)s(1)s(A
A1A
x
xA
s
of ===
)s(L1)s(A)s(1)s(D ==
1)j()j(A)j(L ooo == Barkhausen Criteria
Note that for the circuit to oscillate at one frequency the oscillation
criterion should be satisfied at one frequency only; otherwise the
resulting waveform will not be a simple sinusoid.
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Ring Oscillators The main difficulty for using submicron CMOS ring
oscillators in wireless communication systems is theirrelatively poor phase noise response.
Precaution is required to achieve as low phase noise aspossible from CMOS ring oscillators.
Dominant noise sources in IC environment arecommon-mode signals in nature (e.g. power supplynoise, substrate-coupled noise).
Fully differential design is a must!
Courtesy of Fikret Dulger
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Ring Oscillators
++-
180
H(j)
(a)
+++
360
H(j)
(b)
+++
0
H(j)
(c)
Various views of oscillatory feedback system.
Vout
CLM1
RD
VDD
One-pole feedback Two-pole feedback system
CLM1
RD
Vout
CLM2
RD
VDD
E F
CLM1
RD
VoutCLM2
RD
VDD
E F -1
Ideal
Two-pole feedback system plus an inverter
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( )3
0
3
0
1
+
=
s
AsH
Three-stage ring oscillator
VoutCLM3
RD
VDD
G
CLM1
RD
CLM2
RD
E F
03 =osco
60tan 1 =
o
osc
( )( ) ( )
( )( ) 3
0
3
0
3
0
3
0
3
0
3
0
3
0
1
11
1As
A
s
As
A
sVsV
in
out
++ =
++
+
=
1
1
32
0
3
0 =
+ osc
A
20 =A
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Modifications to the current-starved VCO to set minimum and maximum frequencies.
R
M5 M7VinVCO M1M8
M2
M3
M4
VDD
M9M6
VDD
First stage of VCO
Current starved VCO.
M5VinVCO M1
M2
M3
M4
VDD
M6
VDD VDD VDD
ID4
ID4
Osc out
Single ended Inverters
(delay elements)
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(a)
(b)
+ -
- +
X1
Y1
+ -
- +
X2
Y2
+ -
- +
X3
Y3
+ -
- +
X4
Y4
(a) Five-stage single-ended ring oscillator, (b) four-stage differential ring oscillator
XY
VDD
M1
R1 R1
M2
ISS
M3
R1 R1
M4
ISS
M5
R1 R1
M6
ISS
t
VDDVX
VY VDD + R1 ISS
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Fully-Differential Ring Oscillator
Vo1
(in phase) Vo2
(quadrature)
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One potential delay element
Vin-
Vdd
frequency
controlIb
Vbias
Vbias
Vout
Vin+
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Replica Biasing needed for the DelayElements
Vref
Vdd
Vdd
Vbias
( off-chip )
( to delay
elements )
frequency
controlIb
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Frequency Control Frequency of oscillation, ( fosc), is controlled through the
bias current of a delay element ( Ib). Maximum peak-to-peak voltage, ( Vpp), is controlled
through the replica biasing.
Total output capacitance of a delay element, ( Cout
),directly affects the frequency of oscillation.
outpp
bosc
CV
If
.
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Chip Photomicrograph
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Test Results
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Source coupled voltage-controlled oscillators (also known as source coupled multivibrators).
VinVCO
VDD
ID
Output Output
M6M5
M1 M2
M3 M4
C
VinVCO
VDD
Output Output
M6M5
M1 M2
M3 M4
C
ID
a) NMOS b) CMOS
M1 (off) M2 (on)C
ID IDID
X Y
Simplified schematic of source coupledoscillator, M1 is on and M2 is off.
t t time
Output
Y
X
M2 onM1 off
M1 onM2 off
M2 onM1 off
M1 onM2 off VDD-VTHN
VDD-2VTHNVDD-VTHN
VDD-2VTHN
VDD-3VTHN
Voltage waveforms for the NMOS source coupled VCO.
Coupled Multivibrators