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Transcript of 665 RF VCO
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Introduction to
RF VCO Design
Sung Tae Moon
Nov. 2004
Analog and Mixed-Signal Center
A M S C
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Contents
n Introduction
n VCO design procedure
n Quadrature generators
n Measurement
n Inductor measurement using microprobe
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Introduction to VCO
n VCO stands for Voltage Controlled Oscillator.
n VCO is an Oscillator of which frequency canbe Controlled by external Voltage stimulus.
Vc Vo
control voltage output signal
VCO
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VCO in Frequency Synthesizer
n One of major applications for VCO is a frequencysynthesizer.
n Frequency synthesizer provides sinusoidal/pulse
signals at predetermined frequencies that isprecisely controllable by digital words.
n Frequency synthesizer is a core building block ofany system that has to work at multiple frequencies
such as wireless communication transceivers.
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VCO in Frequency Synthesizer cont.
n Frequency synthesizer usually consists of aVCO, a PLL and a frequency divider.
Vc
Vo
phase lock loop
output signal
VCO
PLL
%N
frequency divider
fref
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Requirements for VCO Design
n Frequency tuning range
q Tuning range must cover the entire band of operation.
n Phase noise
q Close to the oscillation frequency due to spontaneous jitter.
n Harmonic distortion
q Spectral impurity of the signal
n Signal powerq Must be high enough to drive the load.
n Power consumption
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VCO for Bluetooth transceiver
n Frequency tuning range : 2.402 ~ 2.479GHz
n Phase noise : -128dBc/Hz@3MHz
n Harmonic distortion : less than 20dB
n Signal power : more 0dBm
n Power consumption : less than 8mA
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Procedure 1. Specification Study
n Relatively low tuning range : 3.3%
n High frequency of oscillation : >2.4GHz
n Very high phase noise requirement
LC tuned oscillator is most suitable
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Procedure 2. LC Tuned Oscillator
n Oscillation frequency istuned by resonant frequencyof LC tank.
n Cross-coupled transistorswork as a negativeresistance that sustainsoscillation by compensatingloss in the LC tank.
n Frequency is controlled byvarying the capacitance ofthe tank.
gm
L C R -R
LC
M1 M2
LCo
1
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Procedure 3. On-chip Inductor
n Specifications
q L = 2nH
q Q > 5
n On-chip spiral inductormust be simulated withEM(Electro-Magnetic)simulator such asASITIC.
R
LQ
RL
ASITIC EM simulator
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Inductance and Q
areaMetal1
widthMetal
areaTotalareaMetal
_
resonantself
f
Q
L
n Q can be improved by increasing metal width.
n
To keep L same, total area has to be increased.n Increased metal area reduces self resonant
frequency
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Procedure 4. MOSFET Varactor
n Back gate controlled PMOS varactor
n Accumulation mode
n Depletion mode
VBG
< 0G
p+ p+
n
B
G
B
Cox
G
B
0 < VBG
< Vth
G
p+ p+
n
B
G
B
Cox
G
B
Cd
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Procedure 4. MOSFET Varactor (cont.)
n Inversion modeV
BG> V
th
G
p+ p+
n
B
G
B
Cox
G
B
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Procedure 4. MOSFET Varactor (cont.)
n Accumulation/Depletion only varactor
n Inversion only varactor
G
n+ n+
n
B
G
VB = VDD
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Procedure 5. Active Elements
n gm of the cross-coupled MOS pairs must behigh enough to compensate the loss of thetank.
n It is a good idea to have plenty of margin indesign.
n The length of the transistors must be
minimum in order to minimize parasitics.
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Procedure 5. Active Elements (cont.)
n Sources of the loss
q Quality of L (QL)
q Quality of C (QC)
q Output impedance of thetransistor. (go)
n Gm of the transistor must
be larger than total loss.n is safety margin for
starting oscillation.
gm
C
1/go
-RL
RL
RC
3
1,
1
>
==
++>
Co
C
L
oL
C
o
oL
om
CRQ
R
LQ
Q
C
LQgg
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Series-parallel conversion
n Only valid close to resonant frequency
C
L
RS
CL RP
2
2)(
L
P
P
oS
Q
R
R
LR ==
SL
S
oP RQ
R
LR
22)(
==
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Phase Noise
n Phase noise is uncertainty of centerfrequency of VCO output
n The spectrum looks as if it has finite power in
certain frequency offset away from the centerfrequency
n In time domain, phase noise is also referred
to as timing jitter
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Phase Noise (cont.)
n Signal amplitude
n Noise power
n Phase noise
SLtail
S
otailPtailA RQI
R
LIRIV
22)(
===
2
2
2
2
2
2
4
4
=
=
oSLm
o
L
Pmn RQgkT
Q
RgkTv
2
222
288
==
o
tail
m
A
n
LQI
gkT
V
vPN
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Phase Noise (cont.)
n Signal power can be increased either byhigher Q or by higher L
n Only high Q improves phase noise
n High power dissipation also improves phasenoise
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Quadrature Signal Generation 1
n Two identical coupledoscillators
q Immune to mismatch -the coupled oscillators
synchronize to exactlythe same frequency
q Large area and powerdissipation
I +I
+QQ
n Lam, C.; Razavi, B. A 2.6 GHz/5.2 GHz CMOS voltage-controlledoscillator, ISSCC, 1999, pp. 402-403
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Quadrature Signal Generation 2
n RC-CR network
q Low power : passiveelement only
q Sensitive to mismatch
q Amplitude mismatch
INV
2OUTV
1OUTV
n
Orsatti, P.; Piazza, F.; Huang, Q., A 20-mA-receive, 55-mA-transmit, single-chipGSM transceiver in 0.25 CMOS, JSSC, vol 34, Dec. 1999 , pp. 1869-880
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Quadrature Signal Generation 3
n Polyphase network
q Low power
q Amplitude matching
q Insertion loss
I
+I
+Q
Q
+IV
IV
n
Parssinen, A.; Jussila, J.; Ryynanen, J.; Sumanen, L.; Halonen, K.A.I., A 2-GHz wide-banddirect conversion receiver for WCDMA applications, JSSC, vol. 34, Dec. 1999, pp. 1893-903
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Quadrature Signal Generation 4
n Divide-by-two circuit
q Relatively immune tomismatch
q Requires 2x frequency
oscillation which leads tohigh power dissipation
Latch
Latch
INV2OUTV 1OUTV
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Layout of Bluetooth VCO
MOSDrivers
Varactor
Spiral Inductors
Phase shifter
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Chip Microphotograph
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Another Layout (for 802.11b+BT)
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PC Board
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Using Spectrum Analyzer
Device Under Test Matching Network
Spectrum Analyzer
FSE-K4 Phase noisemeasurement software
on PCB
n A matching network to make the output impedance of
the VCO to match 50 is recommendedn The phase noise measurement can be automated by
using FSE-K4 software
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Testing Results
n Since the frequency of oscillation is too highfor any oscilloscope available in the lab, thespectrum analyzer is the only option for
testing the circuit.n Measured parameters:
q Frequency tuning range : 2.37 ~ 2.72GHz
q Signal power : -12dBmq Phase noise : -130dBc/Hz @ 3MHz
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Testing Results
n Tuning Range
q 2.37 ~ 2.72GHz
n Phase noise
q -130dBc/Hz @ 3MHzoffset
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Inductor Measurement using Microprobe
n Measurement instruments
q Probe Mount Station
q Microprobe (150mpitch)
q Network Analyzer (HP 8719ET)
q SMA Coaxial cable
SpiralInductorPads for
microprobes
G S G
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Calibrating Network Analyzer
Calibration
Standards(open, short, 50,
through)
Probe
Station
Network
Analyzer
Coax
n Calibration is required to compensate the effect ofmicroprobes, coaxial cables and network analyzer itself.
n The effect of pads for microprobe landing cannotbecalibrated out. De-embedding after measurement isrequired.
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Measurement Setup
2221
1211
ss
ss
s-parameters
Chip(TSMC 0.35u
Spiral inductor)
Probe
Station
Network
Analyzer
Coax
n The device under test is measured after calibration
n Network analyzer extracts the s-parameters
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s-parameter to y-parameter Conversion
21122211
2112221122
2121
12
12
2112221111
)1)(1(
)1)(1(
2
2
)1)(1(
ssss
Zssssy
Z
sy
Z
sy
Z
ssssy
o
o
o
o
++= ++=
=
=
++
=
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Modeling Inductor from y-parameter
z
L
Y1
Y2
Y3
1i 2i
1v
2v
31
02
222
1
01
221
1
02
112
21
01
111
1
2
1
2
YYv
iy
Yv
iy
Y
v
iy
YYv
iy
v
v
v
v
+==
==
==
+==
=
=
=
=
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Conclusion
n Basic concept of VCO is discussed.
n Design procedure of a 2.4GHz VCO forBluetooth application is presented.
n Testing result of the circuit is provided.