introduction to rf vco design -...
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Introduction toRF VCO Design
Sung Tae MoonNov. 2004
Analog and Mixed-Signal Center
A M S C
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Contents
n Introductionn VCO design proceduren Quadrature generatorsn Measurementn 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 can
be 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 frequency synthesizer.
n Frequency synthesizer provides sinusoidal/pulse signals at predetermined frequencies that is precisely controllable by digital words.
n Frequency synthesizer is a core building block of any 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 a VCO, 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 rangeq Tuning range must cover the entire band of operation.
n Phase noiseq Close to the oscillation frequency due to spontaneous jitter.
n Harmonic distortionq 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 ~ 2.479GHzn Phase noise : -128dBc/Hz@3MHzn Harmonic distortion : less than 20dBn Signal power : more 0dBmn 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 : >2.4GHzn Very high phase noise requirement
à LC tuned oscillator is most suitable
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Procedure 2. LC Tuned Oscillatorn Oscillation frequency is
tuned by resonant frequency of LC tank.
n Cross-coupled transistors work as a negative resistance that sustains oscillation by compensating loss in the LC tank.
n Frequency is controlled by varying the capacitance of the tank.
gm
L C R -R
LC
M1 M2LC
o1
≈ω
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Procedure 3. On-chip Inductor
n Specificationsq L = 2nHq Q > 5
n On-chip spiral inductor must be simulated with EM(Electro-Magnetic) simulator such as ASITIC.
RL
Qω
≈RL
ß ASITIC EM simulator
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Inductance and Q
area Metal1 widthMetal
area Totalarea Metal
_ ∝∝∝
resonantselffQL
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 varactorn 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 mode VBG > Vth
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
GVB = VDD
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Procedure 5. Active Elements
n gm of the cross-coupled MOS pairs must be high enough to compensate the loss of the tank.
n It is a good idea to have plenty of margin in design.
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 lossq Quality of L (QL)q Quality of C (QC)q Output impedance of the
transistor. (go)
n Gm of the transistor must be larger than total loss.
n α is safety margin for starting oscillation.
gm
C
1/go
-RL
RL RC
3
1,
1
>
==
++>
αω
ω
ωω
α
CoC
L
oL
C
o
oLom
CRQ
RL
Q
QC
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
RR
LR ==
ωSL
S
oP RQ
RL
R 22)(
==ω
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Phase Noise
n Phase noise is uncertainty of center frequency of VCO output
n The spectrum looks as if it has finite power in certain frequency offset away from the center frequency
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
SLtailS
otailPtailA RQI
RLI
RIV 22)(
===ω
22
2
2
22
44
∆=
∆=
ωω
γω
ωγ oSLm
o
L
Pmn RQgkT
QRgkT
v
2
222
2 88
∆==
ωωγ o
tail
m
A
n
LQIgkT
Vv
PN
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Phase Noise (cont.)
n Signal power can be increased either by higher Q or by higher L
n Only high Q improves phase noisen High power dissipation also improves phase
noise
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Quadrature Signal Generation 1
n Two identical coupled oscillatorsq Immune to mismatch -
the coupled oscillators synchronize to exactly the same frequency
q Large area and power dissipation
−I +I
+Q−Q
n Lam, C.; Razavi, B. “A 2.6 GHz/5.2 GHz CMOS voltage-controlled oscillator”, ISSCC, 1999, pp. 402-403
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Quadrature Signal Generation 2
n RC-CR networkq Low power : passive
element onlyq Sensitive to mismatchq Amplitude mismatch
INV
2OUTV
1OUTV
n Orsatti, P.; Piazza, F.; Huang, Q., “A 20-mA-receive, 55-mA-transmit, single-chip GSM transceiver in 0.25 CMOS”, JSSC, vol 34, Dec. 1999 , pp. 1869-880
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Quadrature Signal Generation 3
n Polyphase networkq Low powerq Amplitude matchingq 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-band direct 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 circuitq Relatively immune to
mismatchq Requires 2x frequency
oscillation which leads to high 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 NetworkSpectrum Analyzer
FSE-K4 Phase noisemeasurement software
on PCB
n A matching network to make the output impedance of the VCO to match 50Ω is recommended
n 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 high for any oscilloscope available in the lab, the spectrum analyzer is the only option for testing the circuit.
n Measured parameters:q Frequency tuning range : 2.37 ~ 2.72GHzq Signal power : -12dBmq Phase noise : -130dBc/Hz @ 3MHz
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Testing Results
n Tuning Rangeq 2.37 ~ 2.72GHz
n Phase noiseq -130dBc/Hz @ 3MHz
offset
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Inductor Measurement using Microprobe
n Measurement instrumentsq Probe Mount Stationq Microprobe (150µm pitch)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, 50Ω,
through)
ProbeStation
NetworkAnalyzer
Coax
n Calibration is required to compensate the effect of microprobes, coaxial cables and network analyzer itself.
n The effect of pads for microprobe landing cannot be calibrated out. De-embedding after measurement is required.
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Measurement Setup
2221
1211
ssss
s-parameters
Chip(TSMC 0.35u
Spiral inductor)
ProbeStation
NetworkAnalyzer
Coax
n The device under test is measured after calibrationn Network analyzer extracts the s-parameters
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s-parameter to y-parameter Conversion
21122211
2112221122
2121
1212
2112221111
)1)(1(
)1)(1(
2
2
)1)(1(
ssss
Zssss
y
Zs
y
Zs
y
Zssss
y
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
YYvi
y
Yvi
y
Yvi
y
YYvi
y
v
v
v
v
+==
−==
−==
+==
=
=
=
=
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Conclusion
n Basic concept of VCO is discussed.n Design procedure of a 2.4GHz VCO for
Bluetooth application is presented.n Testing result of the circuit is provided.