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Ultra Wideband Oscillators

Itra Widebandmany wireless transceivers are battery-powered, it isthe voltage swing is maximized Beyond this swing level, raising the current willhe phaseil waste pow3 Single Transistor Oscillatornost popsitive feed back provided by an active deviceIn hartley oscillator the lc network has two inductors andance The Nmosamplifier is connected in a common gate configuration The capacitance C3 has one portother port connected to L2 There is no way to replace thisapacitance with the load capacitaMLIL當Fig 1 AC equivalent circuit a) Colpitts VCO with a load inductor b)Hartley OscillatorThe single-switch vCO (SS-vCO)and the double-switch VCO (DS-vCO)pologies used in the design of integrated oscillators Figure 2 shows the simplifiedschematic of both topologies

The transconductance in both circuits, which is set by the biasTo control the negative resistance and hence set the oscillation amplitude, a tail curremployed( transistor M3 in Fig 2(a)which reduces the oscillation headroom, affects thel[n the ss-vCo two integrated inductors or a single center-tapped differential inductor mayassociated with the transcondularger in the Ds-vco than in theCO Thetances reduce both the tuninoscillation frequency The oscillation amplitude of the DS-vCO, for identical resonators andqual power consumption, is anticipated to be twice as large as in the SS-vCo [2] Thus, DS-ntechopen

Ultra wideband oscillators7 3 Integrated Passive Inductorsaffects the phase noise performance and determines the power dissipAs the pid the number of metal layers is increased, the quality (Q)of thepassives is generally enhanced Typicues of a standard on -chrange of a few nH with a Q ranging from 2-10( for frequencies below 6GHz), depending onpared to capacitors and resistors However, planar induc an area inefficient structure,ctors often need large loore widely implementdue to their flat Q and thestandard processes Typical on-chip spirinductor structures are shown in Figure 12, which consist of multiple squares, octagonal, orcircular spiraling turns formingMaking an inductor wider decreases the series resistance, and hence has a positive effect ontain extent [12 However, this would increase the coilapacitance and further reduce the resonant frequency The maximal width of the metal(usually it is about 15-30 um) is usually established using an optimization forThe spiral is generally implementedost available metal layer berger thickness than lower metal layers which helps redutivelower parasitic capacitance to the substrate, top metal layers give rise to higher selfparallel to reduce resistance

Again, this technique effectively brings the coil closer to theiameter of the inductor depends on how wide the inductoretermines the area the inductor covers For a given inductor area,until the entire space is occupied Nevertheless, this is not recommended because of lossnstraints and the fact that inner turns only slightlythe overall inductance Thtiral inductors are rarely filled to their maximum number of turns, and increasing theinductance is typically achieved by increasing the coil radiuscomputations require the use of field solvers However, an approximate estimate, suitablequick hand calculatioed in [12t whichdeviate aboutin comparison with fieldagonal or circular spiral,whereas circular spirals is known to provide somewhat higher Q factor The octagonalpirals are used as the next best alternative [4]often circular geometriesported by many layout tools and not permitted in many technologiesAnother popular technique which provides a much more compact layout is to utilize aIn addition the differential structureand associated losses [14] These benefits can also improve the selfntechopen

Itra Wideband回◎(ctegrated inductors: (a)square, (b) octagonal, and (c)circular spirals 14Fig 13, A pair of single-ended inductors(a)and a differential inductor(b) with similar totale used on the same chip the distance between coil centers shouldbe at least two timethan the coil diameter for each couple of coilsanticipated to increase Q in two or three times, is using copper (Cu) alloys instead ofAl)alloys The second measure is removal of the substrate under the coilby two or three times

Themicromachined Cu inductors may have Qs as high as 50, and allow realization of bandpassfilters with the insertion loss which is better than -5dB at the frequency about 6GHdashiellhich has beenthe Q of ther, since it reduces theling to the lossy substrate [13] Thialso reduces the noise coupled from the substrate at the penalty of reductionresonant frequency of the inductoruch inductors aremanufacertainparameters in RF circuit design and in many cases the major bottleneck of entire systemsntechopen

Ultra wideband oscillators732 Inductor Moddown with the technology [2] Aside from their large physical dimensions, integratedinductors are usually described by simple lumped equivalent networksFigure 14(a)shows a lumped T-model for an integrated inductor L, describes the seriesresistance of the metal latance between the traces, In silicon technology the fairly conductivethat resonates with the inductor rel the resistive path in the substrate which alsothe Q of the inductor Csub models the capacitive coupling from metal to substratewhich redntegrated inductor(b) wideband lumped equivalentThe t-equivalent network is awband model only valid in the close vicinity of thatrticular frequency and is not suitable in wideband designs [4]he network sant characteristics of the coig an expanded lumped equivalent network

As aresult, its validity holds over a much wider frequency range and it is better suited fordebond design analysis [4]The Q value of ideal indtved with the incteqtThis is howeverbecause the parasitic capacitance and substrate lossesshow their significance at higher frequencies [12The series dc resistance of the inductor is the dominant loss contributor at low frequencies(<1GHz) At higher frequesistance rises considerably,ae piral This makes the inner turns of the spiral less effective than the outer turns and theresistandximity effects due to fields from adjacent turns result in a similar frequency-dependentn and corresponding loss increase Also, the florcurrentsin the substrate translates to additional losses which are a strong function of the substrateesistivity and become significant as frequencyntechopen

Itra WidebandAs a result, Q initiallynearly with frequency since the loss is dominated by the coil'sdc series resistance Eventually, skin and proximity effects as well as substratebecome dominant Thus, Q gradually peaks to a maximal value, and beyond whica fast decline as frequenimpact of the above effects on the inductor performance is very complicated, and therefooftware tools must be used for its perfof interest in rf circuits, sinceadductorsmical in terms of the fabrication technology andonnecting two transconductors with resistive15-18 This realizati(Figure 15(b)which the induquality factoproved Also, thetance R between M, and M3 significantly increases the inductance [16]

TheDue to low-inductance value and narrow frequency range of the above topology, improvedcascode structure employing activetors in the fe16(a)and Figure 16(b), respectively This active inductor exploits a tunable feedbackresistance, implemented by connecting the resistor(R) in parallel with a transistor Thete-source voltage(Vune)in this transistor controls the total effective resistance( re) Thisreduces the output conductance and improves the quality factIZin15(a) Schematic of cascode-grounded active inductor with a feedback resistance(b)ntechopen

Ultra wideband oscillatorsThe equivalent capacitance, inductance, and resistors are [19Fig, 16(aTAlb)Equivalentt model of active inductoro3C2C(R,g2+1)m32m3Cg2(Rg2+1)8m18m28m3+@gm28m3 C2g ds2+r, gdsIn(12), the effect of feedback resistor is shown by(R,8ds2 +I), which is designedthan unity

Decreasing Rey by the help of R resulquality factor and the input impedance of TAI can be obtained from Intechopen

Itra Wideband(R1+RG1+(OL /R4)(+Re Ge -@ leo Ce)+ja(Re Ce+Le Geo)Note that in conventional TAI topology, Vi=constant(see Figure 16(a) As can beuations(10),(11), and(13), active resistor has direct effect onHowever, this increase in Ro will degrade the quality factor Tobe utilized as the extra tuning voltage to control the gas of12 Thus, theinductance and quality factorachieved by cother enhancement of quand inductance, one can utilize the transistor m5th feedback resistanhown in Figure 17(b)[18]

This transistor, whichn the cut-off region,CMOS technology, a tunable resistance from 10022 to 16 ks may beachieved [18 for Vtune12v to Vune"0 4V, as illustrated in Figure 17(a)tion of effective resistance versus tuning voltages (b) Proposedresistance with parallel MOSFET (c)Variation of capacitance in proposed active resistanceThe values of each component of equivalent circuit model are expressed be

low (Cequivalent capacitance of M2aC, Regas? Re antechopen

Ultra wideband oscillators20CefrRoCRgm1gds2gds3(CPC(17gml 8m2 8m3 +0 Cgm1gm?cKeff Sds?gm28 m3Cn order to have Leg greater and reg smaller than other conventional inductors, the followinglations should be satisfiedgCgvaractorents of voltage-controlled oscillators(VCOs)merit fotunability(Cmax/Cmin), CV linearity fn general, two types of varactors have been developed for the RF CMOS processes, MOSaccumulation mode capacitor(MOS varactor) and CMOS diodeMOS diode varactors are basically reverse-biased p-n junctions which can be implementedusing the availabn-diffusions andp-wells [4] These varactors exhibit tunabilityDe used where fine tuning of capacitanced

Also, they provide better linearity than MOS varactorshe mos varactor canaractors, Thisdesxcellent tunabilitydfficiently high Q factor The performance of this varactor improves with technologyHA)junction varactor has beenin the literature with a nearlylinear C-V tuning ratio of 31 and a Q exceeding 100 at 2 GHz[8]ntechopen

Ultra wideband oscillatorsCO exhibits better phase noise performance compared to the SS-vCO However, theformer requires a larger supply voltage than the latter, due to the additional stacking of theP1P2DDVtuneM2M3Fig 2 Simplified circuits schematic of (a) the SS-VCO and(b) the DS-vCOThe most critical performance specification for an oscillator is its spectral purity

Inoscillator, the spectrum has power distributed around the desired oscillation frequency (n,known as phase noise, in addition to power located at harmonic frequencFig 3 Practical oscillateport circuits connected together Consider the linear feedback model depicted in Figure 4ntechopen

Itra Wideband4 Basic Oscillator feed backhe overall transfer function frol(s)a(s) 1-a(sf(s)Thistput without any input as long as the quantity a(s)f(s),e the loop gain, is one and the phase shift around the loop is zerop gain magnitude greaternonlinearities in the amplifier will reduce the magnitude to exactlyoperationAssuming a(s) has zero phase shift, we can implement f(s) as a resonator, realized with aparallel LC tank, having zero phase shift at the desired oscillation frequencAnother wew an oscillator is to brexactly balanced by a negative resistance -Ra of thee circuit, thenegative resistance compensates the losses in the resonator and steady-state oscillation isActiveCurcuitRFig

5 Two One-port networks view of an oscillator51 One-Port view of phase noiseFigure 6 shows an equivalent one-port model of an LC oscillator, in which in(o) denotes alin the circuit Suppose thentechopen

Ultra wideband oscillatorsAssurlinear time-invariant behavior, total noise power density Pn(o)/Ao can becalcuPn(o thA(o)41()Z(ahere, Z(o)I is the tank's magnitude responseRIActiveFig 6 One-port model of an LC oscillatoractivethe oscillator Second, thde as narrow as possible, i e a high qualityfactor(Q)should be employed for the5

2 Two-Port View of phase noiseReturning to the two-port model shown in Figure 4,RLC tank as shown in Figure 7(a) The magnitude and phaof such(b) As discussed befre need zero degrees net phase shifteedback loop(any integer multipledegrees) Since noise sources in the oscillatcircuit willtemporary phase shifts in the feed back loop, the instantaneous oscillationfrequency will be changing such that the tank produces a compensating phase shift, keepingthe total phase shift around the loop equalThus, phasshort-term instability in the frequency of oscillation [3]The phase noise denoted by L(Aolis defined asL[△o}=10logwhereAw)represents the single sideband power measured in a 1-Hz bandwidthand located at a frequency offset Ao from the oscillation frequency oo P, represents the totalanous sio pes, as pical plot of L(Aolis shown in Figure 8 Note the existence of regions ofntechopen

Itra WidebandAs mentioned above, to reduce the phase noise the magnitude response of the tank shouldbe as sharp as possible, ie it should have a veryfactor QZrFig

7(a) Parallel RLC tank(b) Magnitude and phase responseL{△o}↑Fig 8 General appearance of single-sideband phase noal lc tanks contain adWo=1/vLC, the tank impedance is purely resistinthe phase of the impedancentechopen

Ultra wideband oscillatorsresponse is exactly zero At frequencies below(above)parallel RLC network is mainly inductive(capacitive) For series RLC networks, thisThe resonators quality factor, Q, is generally defined asEnergy storedAverage power dissipatedThe quality factor, which indicates the ability of the tankain energy, oftenC VCOs Also, Q indicates the steepness of the impedancethe sharpness of the peak impedance at (o Therefore, Q can also be described byQhere, AO-3dB is the-3db bandwidth of the impedance response

Clearly, a larger Q resultsin a higher rejection of spectral energy away from the resonant frequency, leadinpurity of the oscillator outputt resonance, the Q of the RlC networks is given byRWorc (parallel rlc)(series RLC)and parallel RLC networks is apparentn wide-band COs, the eqshows the simulated Q of a standard availablCMOS technology [2observed that the Q is linearlAiming for a wideband vCO operating between 3-6Ginterest to haveQ at the highest frequencnd may be reduced with the gain in Q However, the variations in Q cawantedffects on the output amplitude This issue will be explored in more detail in sectionntechopen

Itra Wideband1046811214161820of MeritMost oscillator designers usually report a figure of merit(FoM) value for their specificy used FoM in the RF communitypower-trequency-tuningormalized( PFTN) figure of merit(FOM), as defined in[4, 6FoM=10 loO,max-aand L(Aol is the phase noise measured at an offset Ao from theAlso, (xmaxnd aomin denote the high-side and the low-side frequencies of the tuning range, respectivelIn CMOS VCO circuits, finding the optimal layout for both the passive and active devices isitical to achieving the best possible performance Oneing impact of therasitics in the device layout with technology scaling Thereforminimizes the parasitics and the noise sources, is very important This section is devoted to thesesues, including the integration of spiral inductors with high quality factor, active inductorspacitors, varactor, resistors, and transistors for realizing the ultimate goal of vCo design

71 ResistorsFigures of merit for resistors are sheet resistance, tolerance, parasitic capacitance, andoltage and temperature coefficients In)s technologv resistrmed from thethe gate polysilicon, the source/drain active areas, and metalPolresistors are often usedtegrated circuits for theirvoltage and temperature A low-doped p-type polysilicon resistor is used for applicatintechopen

Ultra wideband oscillatorswiring high resistance Despite its good parasitic capacitance, this resistor exhibits a 25%doped p-type polysilicon resistors are preferred inmost cases because of their good matching and low parasiNon-salicide high resistance poly has a sheet resistance between 800-1200 ohm/Non- salicide P+(N+) poly resistance has a sheet resistance of 280-455(95-180)ohm/squareThese non-salicide poly resistors can be used for high-frequency circuits Salicide P+/Npoly resistance has a sheet resistance of 2-15 ohm/square, with small parasitic capacitanceParaso% tolerSalicide p+/nt diffusionInce has a sheetth large parasitic Non- salicide P+(N+)diffusion has a sheetresistance of 110-190 (60-100)ohm/ square Non-salicide diffusion resistors are only suitabfor low-frequency circuits, eg they are often used for ESD protectionBecause of their strong dependence on voltage, they are usually used to feedAnother high-performance resistor is formed of a thin metal film, further above the substratein the wiring levels [9]

It has a sheet resistance of 0025 to 0 115 ohm/square, with severalattractive features such as low tolerant7 2 Capacitorst5Motal 4Fig 10 Four Stacked Lateral Flux Capacitors Fingers with dark cross-sections are connectedone port The remaining fingers are connected to the other port [101ntechopen

Itra Widebandpacitors can be realized二from any two differentbtained using theerformanceTo achieve large capacitance per unit area, it is common to use several sandwiched-typecapacitors and connect them in parallel (Figure 11) In order to obtain two capacitors with and dummy devicemployed Matchedwith relatively high-Q can also be implementedltance applications (005-05pF)

With more metal layersa modernlossy metal-insulator-metaltra-thin laver of silicon nitride sandwiched alorintermediate metal layer Theirtypical Q exceeds 100 at 1GHz, with a relativelcapacitance(1% or less)[4MIM capacitors and Metal finger capacitors cannetworks, where R represents the series loss from the finite resistance of the metal platesetalMetal3AMetal3C3Metal2MetalCross-SectionFig 11 Cross-Section of a Vertical Mesh Capacitor (left) and side view(right)(10]ntechopen