Introduction to RF VCO Design Sung Tae Moon Nov. 2004 Analog and Mixed-Signal Center A M S C
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Introduction toRF VCO Design
Sung Tae MoonNov. 2004
Analog and Mixed-Signal Center
A M S C
2
Contents
n Introductionn VCO design proceduren Quadrature generatorsn Measurementn Inductor measurement using microprobe
3
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
4
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.
5
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
6
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
7
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
8
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
9
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
10
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
11
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
12
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
13
Procedure 4. MOSFET Varactor (cont.)
n Inversion mode VBG > Vth
G
p+ p+
n
B
G
B
Cox
G
B
14
Procedure 4. MOSFET Varactor (cont.)
n Accumulation/Depletion only varactor
n Inversion only varactor
G
n+ n+
n
B
GVB = VDD
15
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.
16
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
17
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
18
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
19
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
20
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
21
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
22
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
23
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
24
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
25
Layout of Bluetooth VCO
MOSDrivers
Varactor
Spiral Inductors
Phase shifter
26
Chip Microphotograph
27
Another Layout (for 802.11b+BT)
28
PC Board
29
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
30
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
31
Testing Results
n Tuning Rangeq 2.37 ~ 2.72GHz
n Phase noiseq -130dBc/Hz @ 3MHz
offset
32
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
33
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.
34
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
35
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
−++=∆∆
+−+=
∆−
=
∆−
=
∆++−
=
36
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
+==
−==
−==
+==
=
=
=
=
37
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.
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