Voltage Controlled Ring Oscillator with Wide Tuning Range

8/6/2019 Voltage Controlled Ring Oscillator with Wide Tuning Range

1/34

Voltage Controlled Ring Oscillator with Wide Tuning Range

and Fast Voltage Swing

*

Nicodimus Retdian, Shigetaka Takagi Nobuo FujiiTokyo Institute of Technology Tokyo Institute of Technology2-12-1 Oookayama, Meguro 2-12-1 Oookayama, Meguro

Tokyo, Japan Tokyo, Japan

nico, [email protected] [email protected]

ABSTRACT

Anewdesignofavoltagecontrolledringoscillatorisproposed.Theproposeddesignallowsanimplementationofalowfrequencyringoscillatorusingrelatively

smalldevicesandlessstages.Avoltagecontrolledringoscillatorwithtuningrangefrom40Hzto


2/34

380MHzisachievedusingtheproposedmethod.Inaddition,theproposedcircuitenablestheoutputvoltagetoswingfaster

thantheconventionalone.

Keywords

ringoscillator,voltagecontrolledoscillator,inverter

1. INTRODUCTIONAvoltagecontrolledoscillator(VCO)isoneof

theimportantbasicbuildingblocksinanaloganddigitalcircuits[1],[2],[3].Forexample,aVCOis


3/34

themainbuildingblockinphaselockedloop(PLL)andclockgeneratorcircuits.Therearesomanydifferentimplementations

ofVCOs.OneofthemistheringoscillatorbasedVCO,whichiscommonlyusedintheclockgeneration.

Theconventionalringoscillator

basedVCOusesvariablebiascurrentstocontrolitsoscillationfrequency.However,whenthebiascurrent


4/34

isquitesmall,thevoltageswingoftheVCOwillbecomeslower(longerrise/falltime).Thatisnotdesirable

insomeapplications.Inaddition,increasingbiascurrentwillmakethevoltageheadroomsofthecurrentsourceMOStransistorsbecomenarrow.Thus,asaturation

*

TheauthorsarewiththeDept.ofCommunicationsandIntegratedSystems,Graduate


5/34


6/34

willdescribetheconventionalvoltagecontrolledringoscillator.Itwillbethenfollowedbythebasicideaandimplementation

oftheproposedcircuitinsection3.Finally,simulationresultsandconclusionswillbegiveninsections4and5respectively.

2. CONVENTIONAL CIRCUITA

conventionalvoltagecontrolledringoscillatorcircuitisshowninFig.1whereNisanoddnumber


7/34

andshowsthenumberofinverterstages.AssumethatthegatetosourceparasiticcapacitancesCGsoftheNMOS

andPMOStransistorsareequalasisshowninFig.2.Herethefrequencyoftheoscillationcanbefoundas

1

fosc=(1)

2Nt

wheretisthedelayofoneinverterstage.


8/34

UsingFig.2,thedelayofeachinverterstagewillbegivenby

Z

Ictrl

Vosc=dt(2)

CG

VoscCG

t=(3)Ictrl

whereVoscistheoscillationamplitude.Substituting(3)

into

(1)willgiveIctrl

fosc=.(4)2NVoscCG


9/34

HeretheoscillationfrequencyisdeterminedbythecurrentIctrl,thenumberofstagesN,theamplitudeVoscand

theparasiticcapacitanceCG.Foragivennumberofstages,theoscillationfrequencycanbecontrolledbyvaryingcontrolcurrentoramplitudeonlysinceCG

isafixedparameter.Controlschemebyvaryingthecurrentiscommonlyused.

Equation


10/34

(4)givesanillustrationthattheoscillationfrequencycanbetheoreticallytunedforawiderangebychangingthe

valueofcontrolcurrent.However,itwillnotworkproperlyforaverysmallcurrentsinceitisdifficulttokeepmatchingbetweenthevalues

oftheupperandlowercontrolcurrentswhentheyaretoosmall.Inaddition,thesmall


11/34

biascurrentwillmakethevoltageswingslow.


12/34

VDD

Ictrl

Ictrl

Ictrl

Ictrl Ictrl IctrlVSS

N

Figure

1:Conventionalvoltagecontrolledringoscillator.

VDD

Ictrl IctrlIctrl IctrlVSSCGCGFigure2:Delayapproximation.

3. PROPOSED CIRCUIT3.1 Basic CircuitTheproposed

circuitschemeisshowninFig.3.HereavariableresistorRVisaddedattheinput


13/34

terminalofeachinverter.TheinverteritselfismadefromPMOSandNMOStransistorsasisshowninFig.4.

ThedelayofeachstagetpcanbecalculatedfromFig.5.SincetheMOStransistorsineachinvertercanbeassumedasswitches,itcan

bereplacedbyaresistance1/GMasisshowninFig.5.IfthetransconductancesGMs


14/34

andparasiticcapacitancesCGsofNMOSandPMOStransistorsareequal,thedelayofeachinverterstagetpthen

willbeapproximately

CG(1+GMRV)

tp=.(5)

GM

ForaverylargeRVsuch

thatGMRV.1,thedelaywillbedeterminedbythetimeconstantofRV


15/34

CGregardlessofthevalueofGM.AssumingRV=0willgive

CG

tp

=,(6)

GM

whichisadelayforasimpleinverterstage.Finally,the

N

Figure

3:Proposedvoltagecontrolledringoscillator.

oscillationfrequencycanbefoundas


16/34

GM

fosc=.(7)

2NCG(1+GMRV)

Equation(7)showsthatthe

oscillationfrequencydependsonthevaluesoftransconductanceGM,resistanceRVandcapacitanceCG.However,GMandCGaredeviceparametersandassumedto

beconstant.Asaconclusion,theoscillationfrequencycanbecontrolledbychangingthevalueof


17/34

RV.Theoretically,choosingabigvalueforRVwillenabletheimplementationofaringoscillatorwitha

lowoscillationfrequencyusingsmallsizedevicesandlessstages.

IN

OUT

IN OUTFigure4:Invertercircuit.

CG

1/GM RVRV

VOUT

CG

VDD

Figure5:Delayapproximation.


18/34

3.2 Circuit ImplementationTheproposedmethodisimplementedinthecircuitshowninFig.6.Herethevariableresistanceisimplemented

usingPMOSandNMOStransistors.Bothtypesarerequiredtoenablethefullswingbetweenpowersupplies.TheoverallcircuitisshowninFig.7.Here

theresistancesoftheMOStransistorsarecontrolledbytheirgatevoltage.ChoosingaproperMOS


19/34

transistorsizeswillguaranteethematchingbetweentransistors.

4. SIMULATION RESULTSThesimulationisperformedusinglevel28,0.6m

CMOSprocessparameters.Circuitsusedinthesimulationarethree-stagevoltagecontrolledringoscillatorsshowninFigs.8and9.TheW/LratiosoftheMOS

transistorsareshowninTable1.ThepowersupplyVDDforbothcircuitsis3V.


20/34


21/34

OUTIN OUTINVDD

VP

Ictrl

VN

Ictrl

Figure6:Circuitimplementationoftheinverter

circuit.

M1M4M1M4M1M4M2M3M2M3M2M3M1M4OUTVPVNFigure7:Circuitimplementationof

theproposedvoltagecontrolledringoscillator.

4.1 Transient CharacteristicsBothtransientsimulationresultsoftheconventional


22/34

andtheproposedcircuitsareshowninFig.10.Theoscillationfrequencyforbothofthemaresetto370MHz.

ThecontrolcurrentIctrloftheconventionalcircuitis134AwhilethecontrolvoltageVctrloftheproposedcircuitis3V.

Next,Fig.11

showsthetransientsimulationresultoftheproposedcircuitwhenthecontrolvoltageis1V.The


23/34

oscillationfrequencyis6MHz.Thecontrolcurrentoftheconventionalcircuitis1.415Aforthesameoscillationfrequency.Here

theproposedcircuithasafastervoltageswingthantheconventionaloneforbothhighandlowoscillationfrequency.Incaseoffosc=6MHz,

therise/falltime(10%~90%ofvoltageswing)fortheconventionalandtheproposedcircuits


24/34


25/34

ofeachinverter.Asaresult,theproposedcircuithasafastervoltageswingthantheconventionalone.

4.2 F-V CharacteristicSimulationresultofthef-vcharacteristicoftheproposedvoltagecontrolledringoscillatorisshowninFig.13.Theoscillationfrequencyforcontrolvoltage

of0.4Vand3Vare40Hzand380MHzrespectively.Hereafrequencytuningrangefromtens


26/34

ofhertztohundredsofmegahertzisobtained.Inadditionimplementationoflowfrequencyoscillatorusingsmallsizedevices

isalsopossible.

Figure8:Conventionalthreestagesvoltagecontrolledringoscillator.

Table1:NMOStransistorsW/Lratios

W/L[m/m]

W/L[m/m]M13/0.6Ma9/0.6M29/0.6Mb4.5/1.2M34.5/1.2Msp3/0.6M41.5/1.2Msn


27/34


28/34

atuningrangefrom40Hzto380MHzforcontrolvoltagebetween0.4Vto3V.Furthermore,themaximumoscillationfrequency

oftheproposedcircuitdependsonthedevicesizes.Furthermore,theproposedcircuitisapplicableforalowersupplyvoltagebecauseofitssimplestructure.

Figure13showsthattheproposedcircuithasalinearindBcharactertisticfor


29/34

controlvoltageslessthan1V.ThisisbecausetheMOSTresistorsareintheweakinversionregion.Forcontrol

voltagesbiggerthen1V,theMOSTresistorsareinthelinearregionandtheoscillationfrequencyisincreasedby20MHzfor0.1Vincrementofcontrol

voltage.Theexperimentalchipisstillunderdevelopment.

6. REFERENCES[1]A.HajimiriandT.H.Lee.The


30/34

DesignofLowNoiseOscillators.KluwerAcademicPublishers,1999.[2]A.RezayeeandK.Martin.Athree-stagecoupledringoscillatorwith

quadratureoutputs.InProceedingsofInt.Symp.onCircuitsandSystems.IEEE,2001.[3]D.A.JohnsandK.Martin.AnalogIntegratedCircuitDesign.JohnWileyandSons,New

York,1997.


31/34

VDD

Vctrl

OUTMspMsnMaMbMsp MspMsn MsnMa MaMb MbVctrlFigure9:Proposedthree

stagesvoltagecontrolledringoscillator.

1

IVDDVOUT0

3Voltages [V]

100

2

Currents [uA]

conventionalproposed

344n 345n 346n 347n 348n 349n 350n 351n 352n 353n 354n

3

0

Time [s]

2

Figure12:Currentdissipation(fosc=6MHz).


32/34

1

Voltages [V]

0

Time [s]Figure10:Transientcharacteristic(fosc=370MHz).

108

0.51 23

Frequency [Hz]106

104

conventionalproposed

3

Voltages [V]

2102

1

Vctrl [V]

Figure13:F-Vcharacteristic

simulationresultsof

0

theproposedcircuit.

41.3u 41.4u 41.5u 41.6u 41.7u

Time [s]


33/34

Figure11:Transientcharacteristic(fosc=6MHz).


34/34

Voltage Controlled Ring Oscillator with Wide Tuning Range

Documents