Keysight Technologies Simulation and Measurement-based X-parameter Models for Power Amplifiers with Envelope Tracking

Transcription

1 Keysight Techologies Siulatio ad Measureet-based X-paraeter Models for Power plifiers with Evelope Trackig White Paper This iforatio is subject to chage without otice. Published i US, ugust 3, EN

2 Siulatio ad Measureet-based X-paraeter Models for Power plifiers with Evelope Trackig Haedog Jag 1, drew Zai 2, Tibault Reveyrad 3, Patrick Robli 1, Zoya Popovic 2, ad David E. Root 4 1 The Ohio State Uiversity, Colubus, OH 2 Uiversity of Colorado, Boulder, CO 3 XLIM-UMR CNRS, Lioges Cedex, Frace 4 Keysight Techologies, Ic., Sata Rosa, C bstract Static X-paraeter (XP) odels for RF power aplifiers (Ps), derived fro both siulatios ad oliear vector etwork aalyzer (NVN) easureets, are ivestigated for the predictio of P perforace uder dyaic sigal coditios such as i evelope trackig (ET). The istataeous M-M, M-PM ad PE predictios of XP odels extracted fro siulatio are copared uder ET dyaic sigal coditios to two types of circuit odels usig evelope siulatio. XP P odel is extracted for a peak 8W GaN class-f -1 ET P fro NVN easureets with autoated bias cotrol. By applyig a costat gai shapig table derived fro the XP odel to the drai supply voltage, the average PE is iproved fro 4% to 57% for 3.84 MHz WCDM sigals at 2.14 GHz copared to fixed drai bias operatio. Idex Ters X-paraeters, evelope trackig, loadpull, power aplifiers, supply odulatio. I. INTRODUCTION High data rate couicatio requires spectrally efficiet coplex odulatio schees such as WCDM ad LTE, resultig i sigals with high peak to average power ratios (PPR). These dyaic sigals sigificatly degrade the average P efficiecy whe operated at a fixed bias coditio. The evelope trackig (ET) architecture has show sigificat iproveet of the average efficiecy by dyaically adjustig P supply voltages accordig to look-up tables (LUT), or shapig tables, that specify appigs fro iput RF tie-varyig evelopes to drai cotrol sigals o the P [1]. Noliear characterizatio ad odelig of the trasistor that ca predict P perforace uder varyig bias ad load is eeded for ore coplex trasitters, such as those that eploy evelope trackig [2]. siplified ET block diagra is show i Fig. 1 with a oliear RF P that ca be viewed as a three-port device, with the third port beig the supply iput. Most trasistor ad P odels are ot desiged with dyaic bias take ito accout. ssuig the device uder test (DUT) is quasi-static to the varyig supply voltages ad the evelope tracker is ideal, the coo P perforace figure of erits (FOMs) such as M-M, M-PM ad PE, ca be deteried usig the covetioal static Evelope Detectio I I 2 + Q 2 Delay alig Q I Q Evelope Shapig RF up-covert Driver Evelope Tracker RF P Fig. 1. Siplified ET-P syste block diagra with a oliear three-port RF P. characterizatio at ultiple supply voltages. The goal of this work is to develop a odel for this applicatio that is easy to extract fro siulatios ad stadard easureets for the shapig table desig, ad validate that it gives good estiates of the P perforace uder dyaic bias operatio. I this work, bias depedet static X-paraeter (XP) behavioral odels [3][4] are developed fro siulatios ad fro oliear vector etwork aalyzer (NVN) easureets to odel the oliear RF P for use uder dyaic sigal operatio with supply odulators. Properties of the bias depedet static X-paraeter odels are suarized i sectio II. I sectio III, the FOMs of the static odel are copared to those of circuit odels usig dyaic iput sigals. I sectio IV, a static XP odel is extracted fro a S-bad class-f -1 GaN P. shapig table is desiged fro the odel ad used to defie dyaic drai cotrol sigals to iprove efficiecy. The iteded FOMs based o the static odel are easured uder the dyaic ET coditio usig the supply odulator fro [1]. Furtherore, the perforace predictio of a odel with varyig loads is critical for LUT desigs ad P perforace optiizatios uder atea isatch coditio [5]. The load depedecy predictio of the XP odels icludig haroic loads [6] is validated by coparig the XP odel, extracted fro NVN easureets i 5Ω, to tie-doai loadpull data fro a idepedet easureet syste. The the /13/$ IEEE

3 Fig. 2. Two differet XP odels were extracted fro a circuit odel. (1) XP odel was extracted fro the etire P (blue) (2): XP odel was extracted oly of the trasistor (red) ad the ebedded i atchig (IM, OM) ad bias circuitry (B/C). effect of the load variatio o the shapig table desig is preseted. II. BIS DEPENDENT XP MODEL I the frequecy doai, the scattered th haroic copoet at port p is expressed by (1) at static RF coditios at cotrolled DC bias coditios [3][4][6]. The odel is characterized over a large set of drai voltages ad a wide rage of RF powers of the iput drivig sigal. For tievaryig bias ad odulated RF iput sigals, the XP odels approxiate the P behavior through the quasi-static approxiatio by evaluatig (1) with tie-varyig arguets whe siulated i the evelope doai. B = X + + ( F ) X X 11 ) P S ),, 11 ) T ) ) ( p (, 11 P P + The additioal S ad T ters liearly estiate the effect of the icidet th haroic ad its cojugate fro the port q to the scattered haroic of iterest, effectively predictig the P perforace with varyig load earby the load where the X-paraeters were extracted. This has bee deostrated to be accurate for power aplifier applicatios where perturbatios are relatively sall copared to the large drivig sigal [6]. Extractio toes were applied i the easureets for the idetificatio of those ters. The aplitude of the toes was selected 16 db lower tha the ai large toe for proper liear approxiatio. Evely distributed four phases were used to acquire the cojugate copoet. Phases of respective haroic copoets are oralized relative to the iput large sigal by the P ter. * III. SIMULTION-BSED XP MODELING I this sectio, two types of static XP behavioral odels are geerated fro P odels i the gilet DS siulator usig cotiuous wave (CW) RF excitatio at differet static drai bias coditios. The the XP odels ad the uderlyig circuit odels are copared uder odulated RF sigal coditios, first uder fixed bias coditios ad secod uder (1) M-M (db) M-PM ( ) dyaic supply odulatio coditios applied usig a draicotrol LUT based o the XP odel. The P circuit odel is based o a silico MOSFET trasistor odel, biased at V gs =1.75 V for class B operatio with 27.9 db output power at 1 db copressio. XP odels were extracted i two differet cases, as show i Fig. 2. I case (1), the etire atched aplifier, icludig the trasistor odel ad bias ad atchig etworks, is coverted ito a XP odel. The iput power was swept fro -1 to 26 db to odel the P i saturated operatio. The drai voltage was swept fro 1.5 to 6 V for ET desig resultig i a three terial oliear odel. I case (2), the bare trasistor odel was coverted ito a load-depedet XP odel, ad the circuitry for bias ad atchig are ebedded aroud the XP trasistor odel. The iput power rage ad loads used for the XP extractio of the bare trasistor are differet fro that of the whole P. The iput power rage for the bare trasistor XP extractio was fro -2 to -1 db. Istead of 5Ω, the PE (%) Fig. 3. M-M, M-PM siulatio results for LTE UL sigals at fixed bias coditios. The XP odel for the whole aplifier (gree), XP odel of bare trasistor (blue) ebedded i the atchig ad bias circuitry of the P, the circuit odel (red). 6 X-par(ET) Ckt(ET) 4 Ckt(Fxd) Fig. 4. Siulatio results for a LTE sigal ad a CMOS P uder ET operatio copared to fixed bias coditio operatio. Both XP odel ad circuit odel show iproved efficiecy. The XP odel estiates the ET perforace iproveet over static bias very closely to that of the circuit odel. histogra of the LTE sigals is show for referece /13/$ IEEE

4 load see fro the trasistor, Γ , was estiated fro the circuit odel ad applied to the drai of the trasistor. lso, the load was swept aroud the selected poit with.1 step for the agitude ad 3 for the phase. The dyaic M-M, M-PM evelope siulatio results usig LTE uplik sigals at fixed bias coditios for the XP odels ad the uderlyig circuit odels are show i Fig. 3. The sigals were geerated usig 16 QM odulatios with 5 MHz badwidth. Whe the whole P circuit odel was replaced by the XP odel as i case 1 (gree), o gai ad phase spreadig is observed for the tie-varyig sigals as with the detailed circuit odel (red). However, the average gai ad phase, which correspods to the static oliearity of the XP odel, is well-represeted. Whe oly the trasistor was replaced by the XP odel, case 2 (blue), the istataeous sigal follows well the results of the circuit odel (red). The spreadig ca be cocluded to coe priarily fro the bias ad atchig etworks i this exaple. It should be oted that oly the iput sigal iduced dyaic effects are preset with the fixed bias while both iput ad supply iduced dyaics are preset uder ET. LUT for ET operatio of the P was desiged usig static gai curves as fuctios of drai voltage predicted by the XP odel of the etire P. The evelope siulatio results of the XP odel uder ET operatio ad fixed bias coditios with the above LTE sigals are copared to the circuit odel i Fig. 4. The average PE siulated, usig the XP odel, was 43.3% (ET) copared to 31.5% (fixed bias) which is very close to the results obtaied fro the circuit odel of 42.4% (ET) copared to 31.7% (fixed bias) for the sae average output power of 24 db for a fair copariso. DS exaples showig evelope trackig ay be dowloaded fro [7]. IV. MESUREMENT BSED X-PRMETER EXTRCTION ND EXPERIMENTL VLIDTION X-paraeters of a class-f -1 GaN HEMT 8-W P [8], based Gai (db) PE (%) load load B 8 SWP eas 4 XP odel load B load Fig. 5. P gai ad PE versus power at two differet loads at fixed V dd =28 V. Predictios fro the NVN easureetbased XP odel (red), verificatio by easured results (blue) usig a sapler-based SWP tie-doai loadpull syste. Load : Γ Load B: Γ ). o a TriQuit TGF223-2 trasistor, were easured at 2.14GHz i CW ode usig a NVN. The NVN autoatically characterized ad geerated a X-paraeter odel as a fuctio of drai voltage ad RF power. The X- paraeters were easured i a oially 5Ω syste. Five haroics of the scattered waves, agitudes ad phases, were easured. The drai voltage varied over the rage 12 to 3 V. uxiliary aplifiers were used for both iput ad output to boost the source power. Exteral couplers were used with additioal atteuators cosiderig peak powers. The iput power was swept fro to 25 db. The extractio toes were selected to be 16 db lower tha the peak operatig power for both iput ad output with 4 phases. I order to verify the XP odel extracted fro the NVN frequecy doai easureet istruet, the DUT was easured at various bias ad load coditios o a idepedet tie doai loadpull syste - a VTD (ow gilet) sapler-based SWP-X42. I Fig. 5, gai ad PE versus output power curves are copared at two differet loads, specified i the captio, at a fixed drai bias of 28V. The XP odel, geerated fro easureets ito fixed ipedace of oially 5Ω was able to track the sigificat load-depedet perforace characteristics of the aplifier, without the eed for a load tuer. Give the fidelity of the load-depedet XP odel predictios, additioal load variatio effects o the LUT desig were ivestigated. Fig. 6 illustrates just how uch the correspodig LUT for costat gai ET operatio depeds o load, for four differet coplex reflectio coefficiets of Γ.1. The P perforace uder ET coditios, based o the static XP odel, was easured usig a WCDM sigal as show i Fig. 7. The test sigal, with duratio of 1 s ad 3.84 MHz badwidth, was geerated ad the LUT was applied to the sigal to geerate the drai supply cotrol sigal. This cotrol sigal was fed to the supply odulator usig a arbitrary wavefor geerator. The origial WCDM sigal was up-coverted by a ESG ad fed to the iput of the P through a liear driver aplifier. I-phase ad quadrature phase data fro iput ad output ports were recorded. The operatio, at fixed bias coditios of 26 V, 28 V ad 3 V, V dd (V) j j P i Fig. 6. The costat-gai LUT depedece o fudaetal load, coputed fro the easured ad validated XP odel, is quite sigificat, eve for reflectio coefficiet agitudes as sall as /13/$ IEEE

5 25 RB 1 Sigal Geerator Matlab RB 2 Driver Evelope Tracker RF P Sigal alyzer Gai (db) M-PM ( ) Fig. 7. The basebad WCDM sigal was geerated by arbitrary sigal geerator (RB) 1 ad fed to a RF sigal geerator. RB 2 geerates shaped drai cotrol sigal ad fed to the evelope tracker. sigal aalyzer recorded 1 s log basebad coplex data. was also easured for copariso. The sigal sources were sychroized by sharig a 1 MHz referece. The easured average PE uder ET coditio excludig the supply odulator was 57.8 % with db output power while the PE of the three fixed bias cases showed 4.3 % i average with 3.6 db average output power. The easured average drai voltage durig the ET operatio was 21V, which is well below the fixed bias coditios. The desiged average output power was 28.8 db. s show i Fig. 8, ET operatio showed ore gai copressio above 18 db tha the fixed bias coditios. The M-PM characteristics uder ET operatio show wider spreadig. We expect that soe of this spreadig ca be attributed to icreased eory effects uder dyaic biasig differet fro the assuptio of ideal liear operatio. systeatic characterizatio of the supply odulator could be developed to cosider the iperfect odulator characteristics ito the desig. V. CONCLUSION Siulatio-based ad easureet-based frequecy doai static XP odels were ivestigated specifically for evelope trackig power aplifiers. The evelope siulatio ad easureet results show good quatitative agreeet for the static oliearity of the P versus power ad drai voltage, ad also as a fuctio of load. ew result is the load-sesitivity of the lookup table predicted by the XP odel that was idepedetly validated by tie-doai loadpull easureets. It is expected that the static X-paraeter based ed-to-ed easureet-based desig flow should eet the coercial specificatios, at least for aplifiers ot exhibitig sigificat log-ter eory effects. s future work, liitatios of the quasi-static approach ca be explored by treatig the RFP as a ixer, with the bias pi treated as a secod large sigal icoesurate with respect to the RF iput toe. other approach would use the P i Fig. 8. The easured ET (gree) gai ad phase variatios are copared to the fixed bias (cya: 3V, pik: 28 V, blue: 26V) coditios with WCDM dow lik sigal. The supply voltage does ot vary for iput power levels below the black dashed lie. dyaic extesios of X-paraeters [9] applied to both the RF ad the supply ports. CKNOWLEDGEMENT This work was supported by a Grassroots grat fro gilet Techologies ad i part by ONR uder the DRP MPC Progra N The authors thak. Howard, J. Hor, R. Bieracki, M. Marcu, P. Cai,. Cogata, ad J. Xu for support ad valuable discussios. REFERENCES [1] J. Hoverste, S. Schafer, M. Roberg, M. Norris, D. Maksiovic, ad Z. Poppovic, Co-desig of P, supply, ad sigal processig for liear supply-odulated RF trasitter, IEEE Tras. Microwave Theory & Tech., vol. 6, o. 6, Jue 212. [2] P.M. sbeck. H. Kobayashi, M. Iwaoto, G. Haigto, S. Na, L.E. Larso, ugeted behavioral characterizatio for odelig the oliear respose of power aplifiers, IEEE MTT-S Itl Microwave Syp. Dig., 22 [3] J. Verspecht, ad D.E. Root, Polyharoic distortio odelig, IEEE Microwave Magazie, vol. 7, o. 3, Jue 26. [4] D. E. Root, J. Verspecht, D. Sharrit, J. Wood, ad. Cogata, Broad-bad, poly-haroic behavioral odels fro fast autoated siulatios ad large-sigal vectorial etwork easureets, IEEE Tras. Microwave Theory & Tech., vol. 53, o. 11, Noveber 25. pp [5] R. Paul, L. Sakey, L. Corradii, Z. Popovic, ad D. Maksiovic, Power aageet of widebad code divisio ultiple access RF power aplifiers with atea isatch, IEEE Tras. o Power Electroics, Vol. 4, No. 4, pr. 21 [6] J. Hor, D. E. Root, ad G. Sipso, GaN device odelig with X-paraeters, IEEE CSICS Oct. 21. [7] evelev+trackig+to+iprove+efficiecy [8] M. Roberg, J. Hoverste, ad Z. Popovic, GaN HEMT P with over 84% power added efficiecy, Electroics Letters, Vol. 46, No. 23, Nov. 21 [9] J. Verspecht, J. Hor, L. Betts, D. Guya, C. Gillease ad D.E. Root, Extesio of X-paraeters to iclude log-ter dyaic eory effects, IEEE MTT-S It. Micro. Syp. Dig. Jue /13/$ IEEE