Equalizationandclockrecoveryfora25-10-Gbs2-PAM4-PAM….docx

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1、IEEE JOURNAL OF SOLID-STATE CIRCUITS. VOL. 38. NO. 12. DECEMBER 2(X)32121Equalization and Clock Recovery for a 2.5-10-Gb/s2-PAM/4-PAM Backplane Transceiver CellJared L. Zerbe. Member. IEEE, Carl W. Werner, Member, IEEE. Vladimir Stojanovic, Member, IEEE、Fred Chen, Member, IEEE, Jason Wei. Member. IE

2、EE, Grace Tsang, Member IEEE, Dennis Kim, Member IEEE、William F. Stonecypher, Andrew Ho, Member, IEEE、Timothy P. Thrush, Ravi T. Kollipara, Member. IEEE.Mark A. Horowitz, Fellow, IEEE、and Kevin S. Donnelly, Member. IEEE0018-9200/03S17.00 2003 IEEEAbstractA folded multitap transmitter equalizer and n

3、mltitap receiver equalizer counteract the losses and reflections present in the backplane environment. A flexible 2-PAM/4-PAM clock data recovery circuit uses select transitions for receive clock recovery. Bit-error rate less than 10-i * and power equal to 40 mW/Gb/s has been measured when operating

4、 over a 20-in backplane with two connectors at 10 Gb/sIndex TermsAdaptive equalizers decision feedback equalizers, multilevel systems, pulse amplitude modulation, SerDes, serial links, transceiversI. IntroductionA. Backplane EnvironmentTHE backplane is a complex environment consisting of many compon

5、ents and represents a serious challenge to signaling rates above 5 Gb/s. As shown in Fig. 1, the signal path includes over 11 different components, each of which has its own inipedance variations In addition, there are up lo ten vias in the signal path, each having both a through and stub component,

6、 each thus presenting an additional potential impedance discontinuity and resonant pole As a result, the (ransier functions (S21s) of channels in this environment vary significantly, as can be seen in Fig 2 Al Nyquist frequencies below 2 GHz. (here arc some channel differences but the presence of vi

7、as and impedance discontinuities docs not have a significant impact. Above 2 GHz, channels vary significantly depending on the signaling layer (and thus the thru/stub ratio of the via), the trace length (and thus the skin and dielectric loss), and the dielectric niaierial. Achieving high data rales

8、across this variance of channel behaviors presents a significant challenge for high-speed serial links Often architectures which can achieve 10-Gb/s data rates with newer materials and connectors have also demonstrated operation in older legacy backplane environments at rates up to 6 Gb/s, thus demo

9、nstrating the similarity of (he two problems八 significant group of 10-Gb/s transceivers 1. however, were not designed for (his harsh electrical environment and thus arc often improperly suited for the variety of difficulties it presentsManuscript received April 12, 2003; revised June 25, 2003.J. L.

10、Zerbe. C. W. Werner. V. Stojanovic. E Chen. J. Wei. G. Tsang. D. Kim. V. F. Stonecypher, A. Ho, T. P. Thrush, R. T. Kollipara, and K. S. Donnelly arc with Rambus Inc. Ia)s Altos. CA 94022 USA (e-mail: ).M. A. 1 lorowitz is with Stanford University, Stanford CA 94305 USA.Digital Object Identifier 10.

11、1109/JSSC.2003.818572B. Worst Case SequenceAs can be seen in the raw single-bil response of Fig. 3, a single 200-ps pulse undergoes both serious loss and dispersion when sent down a backplane channel. In addition, it initiates reflections that can be a significant percentage of an equalized eye Fig.

12、 3 (inset) shows a zoom of the reflections plotted on a scale roughly equivalent to a single 4-PAM eye after transmit equalization. Because a transmit equalizer functions by attenuating the lower frequency components while operating in a peak-power constrained environment, the single-bit response is

13、 smaller after transmit equalization even though the intersymbol interference (ISI) has been reduced The total usable amplitude shown in Fig. 3 after equalization is slightly smaller than 3 辜 d. which is the distance between the peak sample and the next sample of the raw pulse response. While (he ma

14、gnitude of any of the individual channel reflections may not appear significanl when compared with the equalized eye height, the complete set of reflections can quickly become significant when combined in a worst case sequence. In such a sequence, the polarity of each of the sequence of bits is set

15、so that all of the reflections sum in the same direction onto a single victim bit. As there can be encroachment on an eye from either side of the voltage extremes, there are two such sequences for a 2-PAM eye, and six such sequences for a 4-PAM eye.The magnitude and importance of the worst case sequ

16、ences can be seen in Fig. 4. In Fig. 4(b), 2000 symbols of a simple 2-PAM pseudorandom bit sequence (PRBS) is run across a channel at 6.4 Gb/s. This sequence is then followed by the two worst case sequences (plotted in bold) for this particular channel, and the total result is then folded into an ey

17、ehe worst case sequences appear as encroachments into the eye sample point, and cause a readily discernable degradation in voltage margin at the sample point. Fig. 4(a) shows the probability distribution function (PDF) plotted on log scale of the distributions of the waveform voltages at the sample

18、point. The PDF was calculated using the technique of 2. This PDF can then be viewed as the probability that a given voltage margin at the sample point will occur. There is good alignment, as shown, between the upper encroaching worst case sequence sample voltage and the PDF voltage at 101 and also t

19、he mean of the eye and the peak of the PDF. It is interesting to note that the PDF distributions show smooth and continuous nonzero tails, which, while bounded, indicate that it will be extremely difficult to rely on coding to minimize the impact of these worst case sequences If coding were lo be us

20、ed to attempt toZERBE ct 1() dB of loss difference between the 2-PAM and 4-PAM Nyquist fundamental frequencies would be likely to benefit from 4-PAM signaling This can be understood from a simple first-order understanding of the relative eye sizes The transfer functions of two example backplane chan

21、nels and their resultant 2-PAM and 4-PAM eyes running at 6.4 Gb/s are shown in Fig 5 1( is interesting to note that both channels are from the same backplane with equal trace length and total via length. The only difference in these channels is the signaling layer and the ratio of through via to stu

22、b via. In Fig. 5(a), the transfer function is not very steep between the 4-PAM Nyquist frequency of 1.6 GHz and the 2-PAM frequency of 3.2 GHz. As expected, the 2-PAM eye has superior voltage margin in this case In Fig 5(b). the channel characteristics show a difference in lhe transfer function at 1

23、.6 and 3.2 GHz of almost 30 dB and. as expected the 4-PAM eye shows superior voltage margin in this case As these two channels are almost identical physically but so different electrically, this clearly demonstrates how there is no definitive answer to (he question of which is belter: 2-PAM or 4-PAM

24、. The only conclusion must be that each channels individual characteristics will detennine the answer to the question for that particular channel.This design supports both 2-PAM and 4-PAM operation via lhe Gray coded levels shown in Fig 6 The differential output driver can be operated in 4-PAM mode

25、as in Fig. 6(a) with a 2-bit input T1:0 Alternately, by simply setting the I-SB to0J0.15 XU0050ONSampling point-0.05rmWorst case pattern at sample pointV*2。0.511.50 -0.05 -0.1 -0.15ogoPdfx10-10secFig. 4. (a) 2-PAM PDF showing the probability of voltages at the sample point, (b) Eye diagram formed by

26、 2000 symbols of a PRBS sequence and overlayed with the worst case patterns. 6.4 Gb/s over 20-in backplanea a a(b)Fig. 5. (a) transfer functions of low loss and high-loss backplane channels (b) measured 2-PAM and 4-PAM signaling at 6.4 Gb/s over the channels. All four ploited on the same vertical an

27、d horizontal scales.T1：000 01 11 ； 1000； 102-PAM: T0=0T1：0TNTPHg. 6. Compatible 4-PAM and (b) 2-PAM modes via lhe use of Gray coded levels and LSB=卅 when in 2-PAM mode.zero, Gray coding allows the driver to ojerate in 2-PAM mode, as shown in Fig. 6(b). It is important to note that the 2-PAM mode is

28、a subset of 4-PAM operation, and thus the 4-PAM transmitter and receiver can be used throughout. When switching between 2-PAM and 4-PAM operation, the phase-locked loop (PLL) multiplier needs to be halved in order lo mainlain a consistent data mie.When considering whether to use 2-PAM or 4-PAM signa

29、ling, the effect of reflections must also be carefully considered, as the size of lhe minimum eye relative (o the inaximum transition has decreased by 2/3. This can be under- slood by referring to Fig. 3, where (he minimum eye size for 4-PAM is 1 事(b and 3 * d in 2-PAM. The worst case reflection mag

30、nitude, however, docs not decrease as the maximum swing remains constant to (hat of a 2PAM swing. Thus, the0.50.40.30.20.10Channel2468RX DATARX Equalizer5-17 tops after mainPick any 5 tapsTX Driver/Equalizer : 5 taps1 (pre)4-1(main)*3(po8t)i1TX DATA 、Jap 1 ,5tap RRFig. 7. S) Block diagram showing ho

31、w transmit and receive equalizers arc combined to make a rangerc5triclcd DFE(b) Equalizer ranges overlayed with an unequalized single-bil response. Each dot is a symbol sample point.impact of reflections on the 4PAM receive eyes can be very destructive In complex backplanes, some channels may have l

32、ow highlrequency loss and can tolerate 2-PAM signaling. Other channels may have higher loss and lower reflections and thus will be better suited for 4-PAM operationB. Equalization ArchitectureApproaches to solve IS1 are well known. the most common of which is equalization 6J. In the backplane link e

33、nvironment, the question becomes how to perform effective equalization at very high performance with very low cost in area and power. While (he use of multiple signaling levels and transmit equalization can be effective in minimizing the effects of dispersion 3, 4, transmit-only equalization is an e

34、xpensive way to combat the effect of reflections which can potentially be more destructive to multilevel signaling. Decision-feedback-based receive equalization (DFE) can be effective when dealing with configuration-dependent relleclions This work uses both transmit and receive equalizers and clock

35、recovery* circuits for operation in a backplane environment with these issues The transmit and ceiver equalizers are combined to make a range-restricted DFE with effective ranges, as shown in Fig. 7. Since dispersion varies as a function of many properties in backplanes, flexibility in the transmit

36、equalizer, both in number of taps and in (ap sellings, is highly desirable One completely flexible extreme would involve the use of a digital filter and a digital-to-analog converter (DAC) 7, while the simplest extreme is two-tap pre-emphasis 8. Any technique must be evaluated for additional inserti

37、on loss as well as power and complexity.C. Transmit EqualizationA simple thcnnoinctcr-coded 2-PAM/4-PAM transmitter structure is shown in Fig. 8. Prc-dccodcd data is sent to threeEqualizing 5-Tap 2P/4P Tx : total gate = 15W/LFig. & Five-tap 2-PAM/4-PAM equalizing Iransmitlcr without equalization (or

38、iginal).different output differential-pair drivers which can be selected lo achieve any of the 4-PAM levels. In order to extend this to a five-tap equalizing transmitter, one simple method is to replicate the original driver five times over and feed each driver with individual symbol-delayed inputs

39、of the original data, as shown in Fig. 8 (inset) In order for each tap to have the same range and resolution of the original tap, each replicated driver must be just as large and have a DAC just as fine as the original transmitte匚 Consequently, this simple approach would result in a 5x. incrense of

40、lhe diffusion capacitance on the output pad and a similar increase in power and areaThe five-tap merged differential transmitter/equalizen shown in Fig. 9(b), leverages the fact that the transmitter is peak-power constrained due to ouipul differential pair saturation margin. Thus, only 1/5 of the eq

41、ualizing iransmitlcr (or total gate klC equal to the original single-tap transmitter) will be active at anyauelu&as JOAPOsE0AI 占 deHpe 右急 peQ(a)TPElw,Variable DelayEqualizerFig. 10. Five-tap adjustable receive equalizer including variable delay for removing output clockio delay.Fig. 9. (a) Original

42、live-tap 2-PAM/4-PAM equalizing transmitter, (b) New shared equalizing transmitter.given time Rather than keep (his device overhead, a single transmitter is divided into segments that can be shared by any of the taps. However, the use of this approach alone limits the resolution of the output driver to be the inverse of the number of segments into which the transmitter is split. For example, for 16 parts, the transmitter would only have a resolution of 4 bits. This would result in ha