Baldinger, R

Eichler, R

Grab, C

Lüders, S

Meier, B

Streuli, S

Szeker, K

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Compact Frontend–Electronics and Bidirectional 3.3 Gbps Optical Datalinkfor the H1–ExperimentLu¨ders S., Baldinger R., Eichler R., Grab C., Meier B., Streuli S., Szeker K.Institute for Particle Physics, ETH Zu¨rich, 5232 Villigen PSI, SwitzerlandStefan.Lueders@desy.deAbstractThe upgrade of the multi–wire proportional chamber(CIP) of the H1–experiment at HERA (DESY) increasesthe number of channels to 9600. These channels have to beread out within the time between two bunch crossings of96 ns and made available to the z–vertex trigger. With theextremely tight spatial conditions at the rear CIP endflangea fast and compact bidirectional read–out electronics is re-quired, keeping the power consumption and the amount ofdead material in the H1–experiment to a minimum.This contribution presents a solution using 40 identicaloptical link modules, stacked on top of each other in groupsof five and each transferring the trigger information with adata rate of 4832 Mbps (an effective rate of 4625 Mbps)via optical fibers out of the H1–experiment.I. INTRODUCTIONWith the year 2000 upgrade of the HERA electron–proton collider at DESY, an increase in luminosity by a fac-tor of five is anticipated. The expected higher backgroundrate, predominantly proton–wall and proton–gas reactions,necessitates a redesign of the H1–experiments’ central in-ner multi–wire proportional chamber (CIP) to provide highbackground rejection efficiency of the z–vertex trigger[1, 2, 3].The new CIP [4] is built of five concentrical cylinders(layers) with radii from 157 mm to 193 mm. In the azimu-thal angle, each layer is equally subdivided into 16 seg-ments, at which each segment consist of 120 separate pads 1along the symmetry axis (z–axis). These 9600 channelsprovide space points which define the direction of tracksneeded for the z–vertex trigger.To retain high acceptances for the CIP and neighbour-ing detectors in the H1–experiment, the available spacefor mechanics and electronics is limited at the backwardendflange2 of the CIP to a 130 mm long open cylinderwith inner and outer radii at 150 mm and 200 mm, respec-tively. These extremely tight spatial conditions have to be1In fact, the CIP uses a projective geometry requiring 119 pads on theinnermost layer, and 112, 106, 99 and 93 pads on the following layers,respectively. But for symmetry reasons, each optical link module will becapable to handle 120 pads.2The term “backward” labels the end of the H1–experiment pointingin the direction of the electron beam, i.e in the  z direction.shared between frontend electronics (on–detector electron-ics), their suspension and cooling, low and high voltagepower cables and gas supply lines (figure 1). Therefore oneFigure 1: On–detector cards mounted on the CIP. To the leftthe five layer CIP with gas tubes and HV cables. Pluggedon five on–detector units stacked on top of each other andsupported by cooling blocks.on–detector electronics unit is limited in size to 130 mmlength, 249 mm width at most and 8 mm height. Its powerconsumption has to be kept to a minimum to avoid toomuch heat dissipation inside the H1–experiment becauseof a limited water cooling.Furthermore, to cope with these tight spatial con-straints, only optical fibers allow high transmission rateswhile reducing the number of cables to a minimum. Aread–out with copper cables as presently done would in-crease the present volume by a factor of ten, consumingeven more space at the endflange and producing an un-wanted high contribution to the dead material. Opticalfibers are not likely interferring escaping particles due toits small radiation length compared to copper cables.Via 40 identical optical link modules, the pad infor-mation is read out every 96 ns (the time between twobunch crossings) i.e. with a rate of 10.4 MHz (HERAclock signal) to the trigger electronics located outside theH1–experiment. Thus the total digitized trigger infor-mation per module sums up to an effective data rate of4625 Mbps according to 2120 pads from two adjacentsegments of one layer (double segment). Vice versa, theon–detector electronics component must be synchronizedwith the global HERA clock signal — requiring bidirec-tionality — and analog signals from each pad should be ac-cessible for remote monitoring purposes. The ETH Zu¨richgroup has already built an optical read–out for the H1–experiment, which has been successfully operated since1995 [5].Each optical link module consists of an on–detectorunit, two optical hybrids, optical fibers and a receiver unit.The on–detector electronics unit amplifies the signals fromthe pads, discriminates and serializes them to 416 bitwords as explained in section II. Two optical hybrids, per-forming opto–electrical (re)conversion, maintain the opti-cal link between on–detector electronics and the receiverelectronics. Their functional design is presented in sectionIII. Section IV. describes the receiver electronic unit whichretrieves the multiplexed signals and distributes them to thetrigger electronics. The performance of the optical linkelectronics is presented in section V.II. ON–DETECTOR ELECTRONICS UNITEach on–detector unit collects the charge from 2120pads, amplifies and digitizes this information and performsa two step multiplexing (four– and 16–fold). After electro–optical conversion, these light pulses are sent to the receiverunit (figure 2).For each segment, the charge on the pads is read–outvia micro coax cables to the rear endflange of the CIPand passed to a pair of analog read–out chips : CIPix [6].Each CIPix chip amplifies, discriminates and fourfold mul-tiplexes the signals from 60 pads. Synchronously with thefourfold HERA clock signal, its 15 digital output channelsgive four successive words with 15 bits each. The First-Word bit tags the first of these words and will allow to main-tain the synchronization with the HERA clock signal at thetrigger electronics. Together they form the 16 bit triggerword. An EmptyDataSet–signal, generated in the case ofmissing inputs on all 64 input pads of the CIPix, serves asthe EmptyDataSet bit. This 17th bit will also be used for avery fast cosmic trigger.A 16 fold multiplexer serializes the trigger word and ad-des a four bit encoding word proceeding the trigger word.CIPixCIPixMUXPLLMUXMUXPINVCSELHERA clk10.4 MHzanalog testanalog testHERA clk10.4 MHz41.6 MHz41.6 MHzMUXdata wordsdata wordsOn-Detector Electronics Unit832 MbpsHIMI2C120 Pads120 PadsSegment nSegment n+1CIPixCIPix16 bit 832 Mbpstrigger wordstrigger words16 bitFigure 2: Signalflow of the on–detector electronics unit.This encoding scheme saves an additional clock signal linebetween multiplexer and demultiplexer. The four differen-tial high–speed data channels — one for each CIPix — aretransmitted by the optical hybrid to the receiver electronicsunit with a data rate of 4832 Mbps (16 bit trigger wordplus 4 bit encoding word). Thus information from 120 padsis transmitted with an effective data rate of 4625 Mbps.In addition, the analog signals are branched off before en-tering the CIPix discriminator to monitor the CIP. Theseanalog test signals are also transmitted.The overall synchronization is done with the globalHERA clock signal received by the optical hybrid. A lowjitter phase–locked–loop (PLL) unit generates a fourfoldHERA clock signal, distributed to the multiplexer and —in addition to the HERA clock signal — to the CIPix chip.For compactness, one optical hybrid serves a doublesegment and is mounted on one of the two separate halvesof the on–detector unit. The other half holds the PLL unitand provides the power supplies. Due to the curvature ofthe CIP, the high–speed data channels, analog test signalsand HERA clock signal of the other segment need to bebridged to the optical hybrid via a thin four layer flex–capton print.III. OPTICAL HYBRIDThe optical hybrids constitute the interface of the elec-trical and the optical regime.Following the dataflow from the CIP to the trigger elec-tronics, the optical hybrid on the on–detector unit (HIM)acts as a driver for the outgoing data words and analog testsignal channels and as a receiver for the incoming HERAclock signal. The optical hybrid on the receiver unit side(DeHIM) acts vice versa.Each HIM / DeHIM pair serves one double segment,i.e. transmittes four multiplexed data channels, two analogtest signal channels and two HERA clock signal channelsvia an optical fiber array with eight fibers. Commercialsolutions fail, because of the asymmetric bidirectionalityand the very tight spatial boundary conditions. The outerdimensions of the optical hybrid are 41 mm  33 mm 6 mm at most.A driver IC matches the signals of the four data chan-nels to vertical cavity surface emitting laser diodes (VC-SEL diodes). The VCSEL convert the current-modulatedelectrical signal into power-modulated optical pulses. After40 m optical fibers, PIN diodes reconvert the optical backto electrical signals. These are amplified by a receiver chipand produce four differential data signals. The specifica-tions for the used VCSEL and PIN diodes are given in table1.Table 1: Specifications of the VCSEL and PIN diodes.Optical spec’s : VCSEL PINWavelength 850 nm, 850 nmmultimodeBamprofile roundBeamdivergence <10ÆActive area diameter 18mSense area diameter 100mPout/ Response > 0.5 mW > 0.5 A/WElectrical spec’s : VCSEL PINUoperating 1.7–2.3 VUreverse > 10 VSerial impedance typ. 30Ilaser 3-4 mAIdark < 40 nACtotal < 0.7 pF @ 1.7 Vrise/fall < 250 ps 100 psCrosstalk > 30 dBMechanical spec’s : VCSEL PINOperating temp. < 85 ÆCChip thickness 150mPitch diode/diode 250mIn case of the HIM, six VCSEL and two PIN diode ar-ray dies are aligned with a precision of 5m with respectto two guiding pins and to each other keeping a pitch of250m in order to match the pitch of conventional opticalfiber array connectors (MTP connectors). The guiding pinsadjust the connector to the diode arrays. On the DeHIMside, six PIN and two VCSEL diode dies are aligned withthe same accuracy.Because of a distance of less than 9 mm between twolayers, it is not feasible to mount the MTP connector abovethe VSEL / PIN array. Even if the connector is reduced toits inner core (the ferrule), adjusting the ferrule perpendic-ular to the hybrid leaves no space to properly fix the con-nector to the optical hybrid. In addition, it complicates theinstallation of the fibers directly at the endflange. Thus thisrequires to mount the ferrule parallel to the optical hybridi.e. parallel to the z–axis. While the diodes send the lightperpendicular to the die, the 62.5 / 125m fibers are bentwithin 2 mm of height by modifying the ferrule and by us-ing special fibers. The performance of the transmission lineremains stable. The attenuation at each deflection lies be-low 2.00.2 dB.Each optical hybrid is embedded in an aluminum cas-ing, to provide robustness and handiness, to avoid electricalinduction from outside and to shield the VCSEL and PINdiodes from dust. Brackets at the end of the aluminum cas-ing give a proper mechanical connection of the hybrid withfiber tails of 700 mm length (350 mm at the DeHIM) andprevents outside stress to derange the precise mechanicaladjustment of the ferrules to the diodes.IV. RECEIVER ELECTRONICS UNITThe receiver electronics unit provides the signals offour adjacent pads i.e. four successive trigger words andthe EmptyDataSet bits to the trigger electronics (figure 3).MUXMUXMUXPINDeHIM41.6 MHzdata wordsHERA clkHERA clkanalog test41.6 MHzHERA clkPLLVCSELtriggermonitoringelectronicsMUXReceiver Electronics Unit832 Mbps 16 bittrigger wordsFigure 3: Signalflow of the receiver electronics unit.The DeHIM receives the high speed data signals andpasses them to four demultiplexers regaining the triggerwords. Latches feed the remaining fourfold signals and theEmptyDataSet bit to the backplane that connects both trig-ger and receiver electronics.The incoming HERA clock signal is received via thebackplane and directed to the on–detector PLL unit via theDeHIM. It is also passed to the receiver boards’ low jit-ter PLL unit producing the fourfold reference clock signalused by the demultiplexer. This fourfold clock signal andthe FirstWord bit are passed to the trigger electronics forsynchronisation of different receiver units.At the receiver units’ frontend, the following signalsare available for monitoring purposeses : the 16 bit triggerword, the differential analog test signals, the HERA clocksignal and the fourfold HERA clock signal.V. PERFORMANCEThe optical link modules will be operated at z =  1mwell within the 1.16 T magnetic field. They will thus be ex-posed to radiation and not be accessible from outside with-out major effort. Therefore, the modules need to be testedbeforehand in the H1 environment for long–term stability,reliability and robustness.Prototypes of the module are operated since end of1999. Neither break–down of any of the used componentsnor a decrease in the power output of the VCSEL diodeshave been observed.The lasering of the used VCSEL diodes typically startsat a current of 4.5 mA (figure 4). With appropriate settings0 2.5 5 7.5 10 12.5 15 17.5 20050010001500200025003000I(Photo) [µA]I(Laser) [mA]U(Laser) [V]U(Laser)I(Photo)00.511.522.53Figure 4: Output power ( IPhoto) and characteristic curve(ULaser) of a VCSEL diode.for the average and modulation current of the HXT 2000,the working range i.e. the digital signal levels have beenoptimized. They vary beween -49.14.5dBm for logical”low” and -3.70.8dBm for logical ”high”, leading to adynamic range in optical output of 45 dB. The crosstalklies above 20 dB. The high response gain of the PIN diodesdrives the HXR 2004 receiver into saturation. Thus noise issuppressed.Several bits of the data words have been accumulatedfor the eye–diagram (figure 5). Their density distributionis given by the grey scale. The rising and falling edges arewell separated. The measurement of the zero–crossing ofthe rising edge results in a jitter of the data words of 61 ps.For quantitative test, 16 bit random generated bit pat-terns simulate the trigger words at the multiplexers’ input.After transmission via the full 40 m link and after demulti-plexing, these patterns are compared with the original in-Figure 5: Eye–diagram accumulated for several bits fromthe data words. Shown is the pulse height as a function oftime. The small histogram gives the jitter; the grey scalethe density distribution.put to measure the bit error rate. Over a period of tendays, three errors occured resulting in a bit error rate be-low 10 14. This lies far below a tolerated rate of 10 9,i.e. one error per second3. All errors could be related toinstabilities in the external power supply.Problems with the synchronization between the 16 foldmultiplexer and demultiplexer have been seen, if the trig-ger words imitate the bit pattern of the encoding schemefor some hundred periods. The demultiplexers’ Clock DataRecovery unit locks on the comprising bit pattern insteadof the encoding word. In the operational mode of concernfor the H1–experiment, this would require the same patternof 60 pads of one segment (in a very special arrangement)repeated over many bunch–crossings. This is expected tobe highly improbable.From the analog signals, a delay time between CIPixinput pads and receiver electronics unit frontend of 230 nshas been measured. This is dominated by the optical fiberswith a delay of 200 ns.The total power dissipation at the CIP end flange (i.e.the sum of all 40 modules) is about 310 W, low enough fora water based cooling. A summary of the specifications ofone optical link module is given in table 2.The on–detector unit has been operated in a magneticfield from 0 to 2 T.The optical output of the VCSEL diodes, the threshold ofthe CIPix, the analog pulse heights, the noise level andthe total power consumption of the on–detector electron-ics have shown no variations within the measurable preci-sion. The jitter of the HERA clock signal remains stable,while jitter of the fourfold HERA clock signal increasesfrom 49 ps to 53 ps with a phase shift of 18 ps. Thus nolosses in the performance of the optical link due to the mag-netic field are expected.3The inefficiency of the z–vertex trigger will be dominated by the in-efficiency of the CIP.Table 2: Specifications of one optical link module.Optical specifications :Digital “high” -3.70.8 dBmDigital “low” -49.14.5 dBmDynamic range 45 dBCrosstalk >20 dBElectrical specifications :Jitter HERA clock 43 psJitter fourfold HERA clock 49 psJitter data words 61 psDelay time 230 ns (fibers: 200 ns)Bit error rate < 10 14Power dissipation (digital) @ +3.3 V @ -3.3 VOn–detector unit (with HIM) 5464 mW 560 mWReceiver unit (with DeHIM) 3696 mW —Power dissipation (analog) @ +4 V @ -4 VOn–detector unit (with HIM) 1300 mW 430 mWReceiver unit (with DeHIM) 290 mW 270 mWThe radiation at the CIP endflange and thus at the on–detector electronics is estimated to be less than 4 krad peryear. The VCSEL diodes have been irradiated to a fluenceof about 21014 n/cm2, but no measurable change in eitherthreshold or efficiency has been seen [7]. After exposurewith 200 Gy  4% from a 60Co source, the optical fibertails and the short distance cables have shown no change inthe optical behaviour. The same is expected for the 36 mlong cables.An exposure to radiation of the complete on–detectorelectronics unit will be done at the Paul–Scherrer–Institutestest–beam in September 2000. But no decrease in the per-formance is expected.VI. ACKNOWLEDGEMENTSThe Optical Link project within the ETH Zu¨rich is partof the H1 CIP upgrade project headed by U. Straumann,Universita¨t Zu¨rich. It has been supported by the ETHZu¨rich and the Swiss National Science Fundation. Specialthanks go to S. Steiner, Universita¨t Zu¨rich, for providingfigure 1.VII. REFERENCES[1] H1 Collaboration, Nucl. Instr. and Meth. A386 (1997)310.[2] K. Mu¨ller et al., Nucl. Instr. and Meth. A312 (1992)457.[3] S. Eichenberger, H1–collaboration internal report H1-IN-096(11/1988), unpublished.[4] M. Cuje et al., H1–collaboration internal report H1-IN-535(01/1998), unpublished.[5] W. Erdmann et al., Nucl. Instr. and Meth. A372(1996) 188.[6] D. Baumeister et al.,http://wwwasic.ihep.uni-heidelberg.de/h1cip/, 1999.[7] T. Weidberg, ATLAS STC Links Group, 1998.

Compact frontend-electronics and bidirectional 3.3 Gbps optical datalink for the H1-experiment

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