Pulsed-IV Pulsed-RF Measurements Using a Large Signal Network Analyzer

Transcription

1 Pulsed-IV Pulsed-RF Mesurements Using Lrge Signl Network Anlyzer Seok Joo Doo*, Ptrick Roblin* #, Sunyoung Lee*, Dominique Chillot* + nd Mrc Vnden Bossche + *The Ohio Stte University, * + on leve from CEA, + NMDG, # roblin@ece.osu.edu Abstrct A new pulsed-iv pulsed-rf mesurement system using lrge signl network nlyzer (LSNA) is proposed to ddress the problem of desensitiztion fflicting conventionl pulsed-rf mesurement systems. Severl extrction methods using the entire spectrum mesured by the system re presented to extrct non-desensitized pulsed-rf S-prmeters of trnsistor. The comprison of the clculted S-prmeters using the lest-squre fitting in time domin with those using only the fundmentl tone revels the significnt increse in dynmic rnge chieved by the proposed mesurement scheme. Index Terms Pulsed mesurement, lrge signl network nlyzer (LSNA), desensitiztion. S I. INTRODUCTION LOW memory effects re low frequency dispersions originting from vrious physicl processes in trnsistors such s self- heting, prsitic bipolr trnsistor in FETs, nd trps[1]. It is essentil for successful device modeling to ccount for these slow memory effects in the cquisition of the DC nd RF device chrcteristics. To ccount for these slow memory effects, pulsed-iv pulsed-rf mesurement techniques hve been developed to reproduce the conditions under which trnsistor opertes for lrge RF signls []. Low-frequency memory effects hve indeed slow time response nd the fst pulsed-iv pulsed-rf mesurements mintin constnt temperture, body BJT voltge or trp stte in FETs like when there re excited by lrge RF signls. By bypssing slow memory effects, pulsed-iv pulsed-rf mesurements enble us to obtin relistic isotherml RF chrcteristics of the FETs. Most pulsed-rf mesurement systems using conventionl network nlyzers cn only cquire the center tone of the pulse spectrum nd this results in significnt loss of dynmic rnge due to the resulting desensitiztion of 0log(duty_rte). Typiclly 1µs durtion pulse with 1% duty rte is used to void low-frequency memory effects [3], nd the dynmic rnge of the network nlyzer for the pulse decreses by 40dB. The reduction in dynmic rnge is n importnt problem which ffects the ccurcy of the pulsed mesurement dt obtined, especilly for low duty rte pulse. Since the power provided by the center-tone does not reflect the ctul signl RF power pplied during the pulse, it is desirble for more tones in the RF pulse to be included into the chrcteriztion of the DUT. Fortuntely, it is possible to get lrge portion of the spectrum of the pulse signl with lrge signl network nlyzer (LSNA). Therefore the LSNA cn be expected to improve the dynmic rnge in the pulsed-iv pulsed-rf mesurement system [4]. The gol of this pper is to introduce the new pulsed-iv pulsed-rf mesurement system developed using the LSNA, nd present the nlysis used to recover the S-prmeters from the dt mesured. II. PULSED-IV PULSED-RF MEASUREMENT SYSTEM A. System Reliztion In generl in pulsed-iv systems both V gs nd V ds pulsed bises re pplied t the gte nd the drin of trnsistor. In the experiment reported here, pulse is only pplied t the drin nd constnt DC voltge is used t the gte. It is ssumed tht slow memory effects re dominted by self-heting nd the contribution of trps in the gte is neglected. For this entire study, we hve used the Infineon CLY 5 GAs FET s DUT. Fig. 1 shows the pulsed-iv pulsed-rf mesurement system implemented using the LSNA. Port 1 is used for the gte, nd port for the drin. The constnt V gs is pplied to the gte using the LSNA internl bis tee. An externl bis tee is used for the drin. A current sensor consisting of resistor is used to mesure the drin current with n oscilloscope. A FPGA digitl testbed synchronized to the LSNA [5] ws initilly used to synthesize bnd-limited pulsed-rf signl using n IQ modultor. This permitted to ddress the fct tht the modultion bndwidth of the LSNA is presently limited to 0 MHz. However the 0 MHz bndwidth limittion is not mjor concern for 0.33µs durtion pulse. The finl pproch dopted is to use the pulsed RF signl provided by RF signl RF Signl Genertor RF Trigger V gs Power Supply Lrge Signl Network Anlyzer V ds Pulse Genertor PORT1 RF+DC DUT Pulse Pulse PORT Bis Tee Current Sensor (RF) Fig. 1. Pulsed-IV pulsed-rf mesurement system. Osilloscope

2 stble. Note tht 104 pulses re cquired by the LSNA when using 95 Hz resolution. Fig. shows generted RF pulse with duty rte of 0.33% nd 0.33µs pulse durtion. In ddition to generte RF pulse, the RF signl genertor is lso triggering the V ds pulse genertor to overly the RF pulse upon the drin pulse signl t the proper moment. More detils on the timing for the pulsed-iv pulsed-rf signls re provided in Fig. 3. The drin pulse hs duty rte of 1% nd 1µs durtion which is short enough to chieve the trgeted isotherml condition. () Sinc function in frequency domin (mgnitude: white, phse: red). The min lobe consists of 601 tones with frequency spcing of 9.918KHz. B. Mesurement Bndwidth To remove the problem of desensitiztion in conventionl pulsed-rf system, it would be theoreticlly desirble to use the entire spectrum of the pulse to obtin higher mesurement ccurcy. In prctice, however, it is not possible to cquire the complete spectrum due to the 0MHz IF bndwidth limittion of the LSNA. Nonetheless by cquiring the spectrum in wide enough rnge we cn enhnce the mesurement ccurcy. According to Prsevl s theorem, the totl verge power of the signl x ( is the sum of the verge power in ech hrmonic component: 1 T T 0 x( dt = n= C n = n= 0 C n. (1) (b) RF pulse in time domin (pulse width: 0.33µs, duty rte: 0.33%) Fig.. LSNA pulse mesurements t modultion frequency of 9.918KHz nd resolution BW of Hz. With this setup the LSNA cptures 104 pulse periods for ech mesurement. where C n is Fourier coefficients, nd usully hs the form of sinc function if x ( is pulse signl. Thus the power included in the bndwidth rnge m is m power (%) = C n Cn 100 () n= 1 n= 1 Fig. 4 shows tht the min lobe of sinc function includes bout 90.3% of the totl verge power, providing enough power to essentilly suppress the desensitiztion experienced in the conventionl pulsed-rf system. It is lso preferble to focus only on the min lobe becuse of the noise floor level of Fig. 3. Timing for the pulsed-iv pulsed-rf system. Signls () nd (b) indicte pulses between the input nd output of the current resistor sensor for the drin (Fig.1), with pulse width of 1µs nd duty rte of 1%. The RF signl (c) which is pplied to the gte t the mesurement time from 0.6µs to 0.93µs, hs 0.33% pulse duty rte. genertor (Anritsu MG369A) s shown in Fig. 1. The pulse-modulted signl from this signl genertor is very Fig. 4. As the number of single-sidebnd (SSB) tones in sinc function increses, the included power lso goes up. With only min lobe (m=300), it is possible to obtin the 90.3% of totl verge power (duty rte: 0.33%, durtion: 0.33µs).

3 the LSNA. In other words, with resolution bndwidth of 1KHz the noise floor level of the LSNA is typiclly -70dBm up to 0GHz when the IF frequency is 10MHz [6], nd deep nulls of the sinc function re usully expected to be below the noise floor (see Fig. ()). III. S-PARAMETER ANALYSIS In this work the system is used to cquire the pulsed-iv pulsed-rf S-prmeters of RF trnsistor. For ech of the 601 tones ( i ) in the pulsed RF spectrum s shown in Fig. (), n S-prmeter cn be defined s Fig. 7. Amplitude nd phse of S 11 (ω i ) vrying tone index. These dt re fitted using the lest-squre lgorithm to recover S 11 (ω 0 ). S kl ( ω ) b ( ω ) ( ω ) ( = 0). (3) i = k i l i m l Fig. 5. Distribution of S-prmeters (green dots) for frequencies between 0.97GHz to 1.03GHz for single pulsed RF mesurement t 1 GHz. The blue + s indicte the S-prmeters of the center tone (t 1GHz) in the pulse, while the red o s represent the estimted S-prmeters recovered by the below lestsqure fitting lgorithm in time domin using ll tones in the 601 frequency rnge specified. Typicl S-prmeters resulting for ll ω i in the center lobe re shown (green dots) in Fig. 5. Note tht S ) nd 11( ω0 S 1( ω0 ) which re obtined by using only the fundmentl center tone (i=0) will nturlly be noisier due to the desensitiztion. In Fig. 6, it is shown tht the dynmic rnge of the center tone decreses by bout 50dB. To estimte the pulse-rf S-prmeters bsed on ll the 601 mesured tones in the spectrum, three different pproches re considered. A. Lest-squre fit in frequency domin Becuse the rw S-prmeters S kl ( ω i ) re usully noisy, n estimtion of the trend of the S-prmeters is needed. It is resonble to ssume tht the S-prmeters re bsiclly liner functions of the frequency within the bndwidth of the min lobe. Lest-squre fitting in frequency domin is then used to find the best fitting stright line. The stright lines fitting the mplitude nd phse of the S-prmeter Skl ( ω i ) re then used to predict S kl ( ω0 ) s shown in Fig. 7. However, s we move wy from the center tone in Fig. 7, S- prmeters re getting noisier due to the sinc nture of the spectrum of 1. To reduce the impct of the incresed noise on the edge, the lest-squre lgorithm is pplied only to reduced bndwidth corresponding to 401 tones. Fig consecutive incident wves for continuous wve nd pulse signl. The RF input power is 5dBm. In the cse of the pulse signl (duty rte: 0.33%), the center tone t 600MHz hs only -45dBm power, nd indictes the 50dB desensitiztion. B. Envelope convolution in time domin The signl envelope E [ ] of n rbitrry signl x ( cn be recovered from the mplitudes nd phses in the spectrum using E [ x( ] = I ( + Q (, (4)

4 Fig. 8 shows the recovered envelopes nd their convolutions. The times t which the convolutions E b ( ]* ( )] nd [ 1 t τ S d S dωi b 1( nd ( [ 1 1 t E b ( ]* ( )] rech their pek gives the group dely = nd τ S = d S1 dωi of the envelopes of 1 b reltive to the envelope of 1(. In ddition the rtios of E [ b1 ( ]* 1( ] nd E [ b ( ]* 1( ] reltive to E [ 1( ]* 1 ( ] give the mgnitudes of S 11 nd, respectively. S 1 Fig. 8. The top figure shows the envelope signls 1 (], b 1 (] nd b (] of the pulsed RF-signls 1 (, b 1 ( nd b ( which re reconstructed from 601 tones spectrum mesured. The bottom figure shows the convolution of 1 (], b 1 (] nd b (] with 1 (]. where I( nd Q( re in-phse/qudrture-phse components of the signl x (. After getting the envelopes E [ 1 ( ], b 1 ( ] nd E [ b ( ], subsequent convolutions between these signls cn be used to extrct the device group-dely (τ) nd the mplitude of S- prmeter using the following equtions. S τ Skl = d S dω (5) kl ( ω 0 ) b ( ]* ( ] ( ]* ( ] (6) kl = k l 50dB desensitized signl Fig. 9. Reconstructed RF signls 1 (, b 1 ( nd b ( for times corresponding to the center of the pulse. The fundmentl tones which re desensitized due to the pulsing, yield wek signls (dshed lines) with reltively negligible mplitudes. However the reconstructed signls using ll the 601 tones in the min lobe yields much stronger signls less sensitive to noise. i l l C. Lest-squre fit in time domin The RF signls in time domin cn be reconstructed from the mplitudes nd phses of the RF pulse spectrum. Fig. 9 shows the reconstructed RF signls 1(, b 1( nd b ( using Eq. (7). It is obvious tht the reconstructed signls (plin line) in the center of the pulse re much stronger thn those of the desensitized center-tone signl (dshed line). This results from the fct tht the reconstructed signls using 601 tones re keeping bout 90.3% of the originl verge power. Note tht the decresed dynmic rnge for pulse with duty rte of 0.33% is of bout 50dB compred to CW tone. A lest-squre fitting into cosine nd sine functions in the time domin of ( ), b ( ) nd b ( ) is used to extrct the 1 t 1 t t I nd Q components of the signl x ( : x = I cos( ω Q sin( ω ) (7) ( 0 0t This permit us to clculte in turns the mplitude nd phse of S kl ( ω0 ). As result of the lrger signl mplitude, reduced noise is expected in the S-prmeters. In ddition to considering the bove three nlyses, dditionl processing steps re required for extrcting S-prmeters from LSNA mesurements. In distinction to conventionl network nlyzer, the LSNA clibrtion is not used for trnsforming the termintion t port 1 nd into perfect mtch lods. It results tht the smll reflections from the coupler nd termintions introduce reproducible oscilltion fetures in the obtined S-prmeters if port 1 nd re ssumed to be perfectly mtched. To remove these unwnted fetures one cn lterntely send n excittion t port 1 nd. Obviously these two consecutive mesurements must be performed for the sme pulsing conditions. Then solving the four simultneous equtions obtined from Eq. (8) for both mesurements, gives the error-corrected S-prmeters. b b 1, p, p = S = S 11 1, p 1 1, p + S + S 1, p, p where nd b re incident / reflected components tht re reconstructed using the lest-squre fitting in time domin, nd p indictes the primry port (1 or ) excited. (8)

5 IV. EXPERIMENTAL RESULTS A. Pulsed-IV chrcteristics Fig. 10 shows the mesured pulsed-iv nd DC IV chrcteristics for the GAs FET. It is cler tht due to self-heting the DC IV chrcteristics include the negtive drin conductnce t high bis. Typiclly the FET used hs mximum therml resistnce ( R th ) of 35 K/W, nd the observed mximum temperture ( ) is 93 C for the T dev gte-source voltge of 0V. But, on the other hnd, the pulsed-iv chrcteristics bypss low-frequency dispersions s result of trps nd therml effects. Bsed on the pulsed-iv chrcteristics, DC quiescent point with Vds = 3V nd I ds = 350mA is used for S-prmeter mesurements. () S 11 nd S 1 Fig. 10. Comprison of pulsed-iv nd DC IV chrcteristics for V gs from -V to 0V in steps of 0.5V. DC IV curves re obtined by Keithley 400 semiconductor chrcteriztion system, nd the developed pulsed-iv system uses pulse signl with duty rte of 1.0%, durtion of 1µs for the drin with constnt substrte temperture of 5 C. The lod-lines for different RF power levels t 600MHz re lso shown. (b) S 1 nd S Fig. 11. Comprison of S-prmeters for the RF input power of -10dBm in frequency rnge from 600MHz to.6ghz with 10MHz step frequency. B. Comprison of S-prmeters Fig. 11 compres the S-prmeters obtined using four different methods: 1) Using only the center tone in the pulses. ) Using the min lobe in the pulses, nd by - pplying lest-squre fitting in frequency domin. - pplying lest-squre fitting in time domin. 3) Using continuous wve for single tone signl. These dt re ll verged over 5 consecutive mesurements. As expected, the S-prmeters obtined using center tones show bigger distortion due to the desensitiztion. The S-prmeters given by the other methods re smooth functions of frequency becuse both for the continuous wve nd for the pulse wve recreted from min spectrum lobe, the mesurements use 100% nd 90.3% of the verge CW nd pek pulse power, respectively. However, note tht the DC IV () S 11 nd S 1

6 (b) S 1 nd S Fig. 1. S-prmeters with the incresed RF input power chrcteristics in Fig. 10 re submitted to strong low-frequency dispersion due to self-heting. To pproximtely compenste for this self-heting effect in the continuous wve mesurement of the S-prmeters, the gte voltge ws rbitrrily djusted (by bout 0. V) to obtin the sme DC drin current s in the pulsed-iv mesurement. Fig. 11 lso shows tht the lest-squre fitting in time domin is better thn the fitting in frequency domin. In ddition, note tht the smll loops observed in S in Fig. 11(b) re reproduced in 5 subsequent pulsed RF mesurements. This reproducible feture indictes tht these loops most likely rise either from the bis-tee deembedding or residul clibrtion error. To get stisfctory results from center tone extrction, one my need to increse the output power of the RF signl genertor to compenste for the desensitized power t the risk of introducing non-liner effects (see self-bising shift of lod-lines in Fig. 10). Fig. 1 shows the S-prmeters for two different RF input powers. As the RF input power increses, the S-prmeters obtined from the center tone dt re notbly stbilized. But even the stbilized S-prmeters for the RF power of 5dBm show more distortion thn those extrcted by lest-squre fitting in time-domin from the min lobe for n RF power of -10dBm. It is verified in Fig. 13 tht the S-prmeters obtined by lest-squre fitting in time-domin show good consistency for wide rnge of RF input power. This is lrgely due to the fct tht the fmily of tones in the min lobe of the sinc spectrum provide enough energy to clculte ccurte S-prmeters. This mesurement benefits lso from the fct tht the LSNA mkes it possible to mesure very smll signls with power round -70dBm. () S 11 nd S 1 V. CONCLUSION A pulsed-iv pulsed-rf mesurement system using n LSNA is implemented to overcome the problem of desensitiztion. It is confirmed tht by using lrge number of tones in the spectrum one cn reduce the noise compred to using the center tone only, yielding more relible smll-signl mesurement of the DUT. This mesurement setup could lso be esily modified for the cquisition of pulsed-rf S-prmeters for pulsed-gte voltge to study trp effects in GN devices. Finlly this LSNA-bsed pulsed-rf system mkes possible the cquisition of the pulsed-rf hrmonic (nω 0 ) response of the device under vrious lod termintions. In tht regrd such non-liner pplictions would follow in the foot steps of the work done t IRCOM [7] except tht the mesurement scheme used in our pproch does not require ny modifiction of the LSNA hrdwre. (b) S 1 nd S Fig. 13. S-prmeters obtined by lest-squre fitting in time domin for three different RF input powers ACKNOWLEDGMENT We would like to thnk Jen Pierre Teyssier of IRCOM for fruitful discussions on pulsed RF mesurements. We re grteful to Dr. Gerld Witt of AFOSR nd the Stte of Ohio for the support they provided to the development of the Non-Liner RF Lbortory t OSU.

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