Characterization of Differential Interconnects s from TDR Measurements

Transcription

1 haracerizaion of Differenial Inerconnecs s fro TDR Measureens Inroducion Applicaion Noe Differenial signaling schees are a coon approach o achieving higher noise iuniy for criical signals in a high-speed digial design. Measureen and odeling of he ransission lines carrying differenial signals, however, pose several differen challenges ha need o be addressed in order o achieve an accurae picure of differenial signal ransission in digial syse design and siulaion. Differenial line signaling and analysis "Differenial signaling" eans ha wo ransission lines and wo signals are used o ransi a single daa bi fro a driver o a receiver. The lines are no independen; when he signal on one line is logical low, he signal on he oher line is logical high, and vice versa. In addiion, he wo lines are ypically laid ou quie close o each oher and exhibi coupling of signal energy fro one line o he oher o a varying degree. The reason we ay choose o sacrifice precious space on a circui board is o allow us o ransi he signals beween a driver and a receiver in a siuaion where a clean, reliable coon ground beween he driver and he receiver is no available. Exaples are when he pah beween he driver and he receiver raverses a large disance, or does no run over a reasonably coninuous ground plane. Driver + - 0, d 0, d Receiver + - Signal ine 1 (+) ogic 1 (high) ogic 0 (low) Signal ine 2 (-) Figure 1. Differenial signaling. Two signals are used o ransi a single bi of daa fro he driver o he receiver. When he signal on one line is logical high, i is logical low on he oher line, and vice versa. As an addiional benefi, differenial signaling schees provide increased iuniy o he coon-ode noise in he syse, because he receiver only sees relaive (differenial) volage beween he wo ransission lines in he differenial pair. In addiion, because he fields radiaed by each signal are of opposie polariy, hey cancel ou o significanly reduce he radiaed energy, he ain cause for elecroagneic inerference (EMI) beween devices. Differenial signals are also less sensiive o he aenuaion of he signal in he ransission ediu, because he receiver design ypically allows sufficien gain o reproduce he original signal [1]. Typical applicaions of differenial signals are low-volage differenial signaling (VDS), Fiber channel, disk drive flexible inerconnec, and a Rabus clock signal. Differenial line circui descripion Differenial lines are ypically roued fairly close ogeher. Because of he ineracion (coupling) beween he lines, propagaion of he signal hrough he differenial pair can no be described by a single ipedance, capaciance or inducance value. Insead, se of and arices is used o describe an elecrically shor secion of ransission line [2]: o o (1) where o +. The above quaniies are relaed o a physical circui by he diagra in Figure 2 of an elecrically shor secion of a ransission line pair, in which is he capaciance of one line o ground, and is he uual capaciance beween lines. The quaniies and are he inducance of one opyrigh TDA Syses, Inc. All Righs Reserved This applicaion noe was published in Microwave Journal, March 2000

2 line and he uual inducance beween lines, respecively. Elecrically shor siply eans ha he ie required for a signal o ransi he secion is uch shorer han he riseie of he signal i. Muliple secions of his ype ay be cascaded in order o represen a ransission line pair ha is elecrically long. In any applicaions, paricularly if a large nuber of he secions in Figure 2 are required o represen a circui, i akes ore sense o use differenial and coon ode ipedances and delays o describe differenial ransission line behavior. Differenial ipedance is defined as he ipedance easured beween wo conducors driven differenially, i.e., wih idenical, bu opposie-polariy signals as opposed o he ode ipedance, which is he ipedance of a single conducor (ransission line) in a differenial pair when he wo conducors are driven differenially [3]. Even ode ipedance is he ipedance of eiher conducor when he differenial pair is driven wih idenical, sae-polariy ( or coon ode) signals. oon ode ipedance is he ipedance of he wo conducors in a differenial pair, when boh conducors are driven wih a single coon ode signal, relaive o ground. In soe cases, he differenial ipedance alone is he paraeer of ineres o a board designer. Based on he differenial ipedance value, he or she can ake a firs cu a predicing he propagaion of he signal hrough he differenial pair. In addiion, he coon ode ipedance can help analyze he coon ode noise rejecion; if he coon ode ipedance is uch higher han he differenial ipedance, he coon ode rejecion will be high. For analysis purposes, a differenial pair is ofen described according o is - and -ode ipedances and delays, which are closely relaed o differenial- and coon-ode counerpars, and can be copued using he following equaions. Figure 2. An elecrically shor secion of a differenial ransission line pair o + Using he definiions above, i is easy o conclude ha differenial 2 (3) (2) wih he delays for differenial and ode, and coon and ode, being equal. Noe ha for he case in which no coupling beween he lines in he differenial pair is presen, boh - and ode ipedance values siply collapse o he characerisic ipedance of each line, given by o (4) In os pracical cases, he and ode delays will also be differen (Figure 3). Figure 3. Even and ode ipedance profiles displayed in he Ionnec sofware wavefor viewer. Boh and ode ipedances and delays are clearly differen + ( )( o + ) ( + )( o ) coon 2 2

3 Differenial line siulaion in a SPIE or IBIS siulaor Since a differenial pair, as we discussed above, is jus ypically a pair of closely roued syeric and coupled ransission lines, i would be logical o odel he as a syeric coupled pair. A coupled - arix is ypically used o describe such a coupled ransission line srucure. However, a large nuber of luped coponens will be required o siulae ransission lines ha are elecrically long, which is ofen he case when a differenial ransission schee has o be used. An alernaive, of course, is o use a disribued approach. Single line ipedance clearly is no enough o characerize he differenial ransission line pair, as one needs o ake line coupling ino accoun. Differenial ipedance alone is no enough eiher, since one also needs o ake ino accoun coon ode rejecion and propagaion. The soluion coes fro a relaively siple aheaical analysis of he differenial pair, which can also be viewed as a syeric coupled ransission line pair. The odel shown on Figure 4 and presened in [4] is an accurae represenaion of a coupled ransission line pair based on is and ode ipedances. I is a siple exercise in circui analysis o deonsrae ha his ransission line configuraion will presen wice he ode ipedance o a differenial signal, and wo separae ode ipedances o a coon ode signal. Since any signal can be decoposed ino is differenial and coon ode coponens, his odel will predic propagaion of any cobinaion of differenial and coon ode signals, accuraely represening he behavior of he differenial ransission line pair.,, - /2, /2, Figure 4. Differenial pair odel based on he and ipedance and delay values One disadvanage of his 4-line odel is is relaive coplexiy. In addiion, his odel is difficul o exend o a case of ore han 2 coupled lines. If i is known, however, ha he coon ode ipedance is significanly larger han he differenial ode ipedance, he coon ode signal will be osly rejeced, and ainly he differenial signal will be observed a he receiver end. The high coon ode rejecion will ake he difference in differenial and coon ode signal delay irrelevan due o he sall apliude of he coon ode signal, and will allow us o use a siplified odel illusraed on Figure 5. Here,, and uual can be found as uual 2 (5) Again, under he assupion ha, i can be easily shown ha his odel will accuraely represen boh differenial and coon ode signal propagaion hrough he differenial pair. Boh he four-line and hree-line odels can be easily uilized in a SPIE or IBIS siulaor, aking he exreely usable for high-speed differenial inerconnec odeling and siulaion. The negaive-ipedance line in he four-line odel ay be realized wih dependen volage sources, if care is aken o ensure ha he resulan odel represens a physically realizable srucure (discussed in he "Even and ode analysis" secion). The resuling accurae odels help he designer o achieve a beer undersanding of his differenial inerconnecs, resuling in higher perforance syse design. Obaining differenial line odel fro TDR easureens Differenial TDR easureen basics Differenial TDR easureens can coe in handy when i is difficul o achieve a good ground plane reference, or when a differenial line analysis us be perfored. A virual ground plane, creaed by wo TDR sources of he sae shape and opposie polariy arriving siulaneously a a DUT inerface, helps achieve he desired easureen resuls. We enioned previously ha TDR easureen accuracy suffers fro uliple reflecion effecs when uliple disconinuiies are involved in he easureen. The rue ipedance profile of he uual Figure 5. Siplified differenial ransission line odel 3

4 V V R source R source TDR Oscilloscope Fron Panel able: 0, d DUT, DUT V V refleced inciden able: 0, d DUT, DUT Figure 6. Differenial TDR easureen seup erinaion erinaion Fro a pracical odeling perspecive, he capacior and inducor values fro (6), e.g., ay be copued if a luped odel is desired. The above equaions also poin ou ha if he uual capaciance is o reain posiive, i is necessary ha > 0 (7) DUT can be obained, however, hrough an inverse scaering algorih repored by several auhors [5], [6]. Based on he inciden sep and TDR response of he syse, he uliple reflecions can be dynaically deconvolved fro he TDR response. Even and ode analysis The ipedance profile is obained fro a differenial TDR easureen wih wo TDR sources of opposie polariy, and he ipedance profile is obained hrough a easureen wih wo TDR sources of he sae polariy. In each case, only a single TDR channel needs o be acquired. A differenial reference shor is also used for copuing he ipedance profile. The reference shor wavefor is os easily obained by disconnecing he DUT and connecing he wo signals o ground in close proxiiy of each oher, or connecing he direcly o each oher and hen connecing he o ground. Once he and ipedance profiles of he differenial pair are obained, he odel shown on Figure 4 or Figure 5 can be easily copued. In addiion, he - arices can be easily exraced fro he and TDR ipedance profiles for a line wih consan ipedance using he following equaions: o ( ) ( ) (6) A odel ha does no saisfy his consrain should no be used, since i does no represen a physically realizable srucure, and could resul in inaccurae siulaions. Differenial line odeling exaple As an exaple, a differenial pair on an FR-4 board was easured and odeled. The ransission lines in his Device Under Tes (DUT) have SMA connecors as an inerface o a TDR oscilloscope. The lines are closely coupled (he spacing o he ground plane on he board was half he gap beween he lines). The ipedance profiles for he sae differenial pair are shown on Figure 3 above. Afer he daa was acquired fro a TDR oscilloscope, i was processed using he -line ipedance deconvoluion algorih in he Ionnec sofware. As was enioned previously, i is necessary o have wo wavefors o copue he ipedance profile: he DUT wavefor and he reference sep wavefor. The reference sep wavefor for a differenial easureen can be obained by connecing he signals on boh of he differenial TDR channels ogeher and hen o ground, if possible. When using cables wih SMA connecors, he easies way o achieve his easureen is o connec he cables ogeher using an SMA barrel inerconnec. Only he wavefor on one channel of a TDR insruen needs o be acquired; no addiional adding or subracing he wavefors in he oscilloscope is necessary. The resuling and ipedance profiles are shown on Figure 3, and based on hese ipedance profiles, one can easily exrac he odel for he DUT. Noe ha wihou he ipedance deconvoluion algorih, he ipedance profiles are subjec o he uliple reflecion effecs in TDR oscilloscopes and he ipedance readou values ay no be correc a each poin on he TDR race. Afer he ipedance profiles have been copued, 4

5 he ipedance profile wavefors are pariioned in he Ionnec sofware Syeric oupled ine odeling window (Figure 7). The disribued odel is he os appropriae in his case, since he elecrical lengh of he lines approaches or exceeds 1ns. Since we have assued ha in pracice here could be significan coon-ode propagaion on he pair, he ore accurae four-line odel is ore appropriae. Figure 8. Verifying he differenial line odel in Ionnec sofware. Siulaions wih boh and ode siuli are perfored using an inegraed inerface o a SPIE siulaor and he siulaion resuls are copared o he easureen daa Figure 7. Pariioning he ipedance profile wavefors in he Ionnec sofware Syeric oupled ine odeling window. When he odel is saved, he equivalen SPIE circui ha describes a odel depiced in Figure 4 is obained. A saple lising of such circui is given in Appendix I. To verify he creaed odel, a designer needs o creae a coposie odel in he Ionnec sofware. The coposie odel copleens he exraced DUT odel wih he source and erinaion ha eulae he TDR easureen source and erinaion. Using an inegraed inerface o a SPIE siulaor, he designer can siulae his coposie odel, and based on he resuling siulaion wavefors, verify he accuracy of he DUT odel. Boh and ode siuli us be used in siulaions in order o ensure ha he odel accuraely predics boh and odes of signal propagaion (Figure 8). One can see ha wih he excepion of he SMA connecors, which are no par of he differenial line, he odel accuraely predics he signal propagaion. In addiion, he connecors can be odeled using luped circui odeling capabiliy in he Ionnec sofware. Even and ode analysis can also be uilized for characerizaion of luped inerconnec srucures, when a single-ended signaling schees are uilized. Such srucures as highspeed connecors, BGA packages, highperforance ATE sockes can be easily odeled using and TDR easureens and equaions (6). Based on he and ipedance profiles, he - arices for he Figure 9. Applying he and analysis o copue - arices for he coupled lines in Ionnec sofware. Noe ha for long lines a large nubers of subsegens us be used o accuraely odel he line 5

6 coupled srucure are easily copued (Figure 9). onclusions We have deonsraed a echnique for exracing a disribued coupled line odel fro Tie Doain Reflecoery (TDR) easureens. This odel can be easily used in any sandard ie doain siulaor (SPIE or IBIS), and can accuraely predic he propagaion of a digial signal hrough a differenial ransission line pair. As a resul, he digial syse siulaion will predic he syse behavior ore accuraely, resuling in higher perforance syse design. Appendix I. Saple SPIE circui lising * Nae: Auoaically Generaed.subck Syeric ****** Pariion # TD92.3p TD92.3p ****** Pariion #2 l n c p l n c p c f k1 l1 l2 207 ****** Pariion # TD87.5p TD87.5p.ends Bibliography [1] ee W. Richey, "How o design a differenial signaling circui," Prined ircui Design Magazine, March 1999 [2] F. Roeo, M. Sanoauro, "Tie Doain Siulaion of n oupled Transission ines," IEEE Transacions on Microwave Theory and Techniques, Vol. 35, No. 2, February 1987 [3] "Differenial Ohs Measureen wih he Series Oscilloscope," Tekronix Technical Brief 47W-7520 [4] A. Tripahi, V.K. Tripahi, "A onfiguraion- Oriened SPIE Model for Muliconducor Transission ines in an Inhoogeneous Mediu," IEEE Transacions on Microwave Theory and Techniques, Vol. 46, No. 12, Deceber 1998 [5].A. Hayden, V.K. Tripahi, "haracerizaion and odeling of uliple line inerconnecions fro TDR easureens," IEEE Transacions on Microwave Theory and Techniques, Vol. 42, Sepeber 1994, pp [6].-W. Hsue, T.-W. Pan, "Reconsrucion of Nonunifor Transission ines fro Tie-Doain Reflecoery," IEEE Transacions on Microwave Theory and Techniques, Vol. 45, No. 1, January 1997, pp [7] M.M. McTigue,.P. Duff, "Differenial Tie- Doain Reflecoery Module for a Digial Oscilloscope and ounicaions Analyzer," Hewle-Packard Journal, Deceber Tie Doain Analysis Syses, Inc. All Righs Reserved SW Barbur Blvd., Suie 100, Porland, OR 97219, USA Telephone: (503) Fax: (503) E-ail: info@dasyses.co Web sie: The Inerconnec Modeling opany DIFF-1199 Daa subjec o change wihou noice