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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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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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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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.

Documents

665 RF VCO