Agenda for SCT links meeting 10:00 Friday, September 13 room 1-1-025 10:05 Discussion of boundary between links and ROD - R. Kowalewski 10:30 Electrical properties of PIN diodes - T. Weidberg 10:50 LED/VCSEL radiation studies at Bern - J. Beringer 11:30 Status of DORIC and LED driver design - D. White 12:05 Updated costing - R. Kowalewski 12:20 Discussion of participation in TSP - T. Weidberg 13:00 Status of STP noise test setup - R. Kowalewski 13:15 lunch *********************************************************************** The meeting began with the discussion of where the boundary between the responsibilities of the ROD and those of the SCT links lies. This discussion was motivated by some questions Alex Grillo had circulated to the SCT links and ROD groups about two months ago. Since Alex was present he summarized his concerns and goals regarding the structure of the SCT electronics work. He sees the need for a contact person for each bit of electronics in the SCT. These contacts are clearly defined for the SCT links and RODs. He felt that there were several items, however, which were hard to categorize at present, given the uncertainties in what the hardware (especially on the ROD end) will look like. These are the interface to the ATLAS TTC and DCS systems and the encoding of the SCT command protocol. He forsees dealing with these items separately from the links and the ROD until such time as it becomes clearer what the hardware dictates, at which point the organizational structure would be modified. He underlined that there was no problem with groups working on the SCT links to also work on other areas of SCT electronics. There was some discussion of where the items listed above logically belong. While no conclusions were reached, Alex was able to explain his approach to the electronics organization and members of the links group were able to make their various viewpoints known to him, which were the purposes of the discussion. * * * Tony Weidberg showed some results on the performance of the PIN diodes in the GEC package. Measurements of rise time versus bias voltage showed the following: Bias (V) 10-90% rise time (ns) 5 15 10 8 15 6.5 These measurements overestimate the true rise time somewhat due to amplifier oscillations; more careful measurements need to be done. The slow rise time at reasonable bias voltages is thought to be due to carrier mobility. The rise time needs to be improved upon in future versions, which essentially means that GEC has to purchase faster PINs for use in their package. There are no new GEC packages currently on order. Some discussion followed on the need to get faster PINs and to perform irradiation studies on PIN diodes. It was not immediately clear who would be able to test PINs in large numbers (the Bern emitter tester is not set up to study PINs), and when faster PINs would be found. The PINs currently in the GEC packages (there are a handful available) can be irradiated as a first step; Birmingham has expressed interest in doing this. Action item: Birmingham to test PINs in GEC packages. Bob and Tony to follow up on procuring faster PINs in larger numbers and identifying a means of testing them. * * * Jurg Beringer gave a report on irradiation studies of VCSELs and LEDs carried out at Bern. Since the June SCT links meeting Bern has obtained 2 16-fold arrays of low-threshold (2mA) VCSELs and 3 16-fold arrays of high-threshold (10mA) VCSELs from Sandia Labs, and 320 LEDs from GEC. One array of each type of VCSEL and 56 GEC LEDs were irradiated at the CERN PS to a fluence of about 2*10^14 p/cm^2. The VCSELs produce far more light than LEDs (factors of 5 or more) at a given current. The effect of radiation damage on the VCSELs is to increase the threshold current, from 2mA to about 5mA for the low threshold VCSELs. After annealing with 3mA for 2.5 days, the low threshold VCSELs recover almost fully. The behaviour of the high threshold VCSELs depends on whether or not they were pulsed (at 18mA) during irradiation. Those pulsed showed a degradation in threshold current from 10mA to 13mA, while the thresholds of those off during irradiation went from 10mA to >20mA. In neither case was there significant annealing after 2.5 days at 15mA current. The low threshold VCSELs are the only ones under serious consideration due to their low power consumption. The initial tests on radiation hardness are therefore quite encouraging. Problems were experienced with the VCSELs when trying to couple their light into multimode fibers; significant amounts of noise were observed and the signal amplitude at the receiving end of the fiber was quite sensitive to details of fiber routing. Emmanuel Monnier commented that similar problems were seen by the Marseille group in coupling fibers to VCSELs, but that these problems could be dealt with by bringing the fiber to within 50 microns of the VCSEL and using long fibers (70m in their case). The length of fiber coupled to the VCSEL in the SCT would be only a few meters to the first connector. The coupling question needs further study. If VCSELs are chosen as the emitters for SCT links a package needs to be developed. The GEC LEDs have a much larger spread in light output for a given current, and require a larger drive current (about 20mA) than the ABB Hafo LEDs (10mA). The spread would be smaller if the LEDs had been pre-tested by GEC. 56 GEC LEDs were exposed to 1.8 - 2.6 *10^14 p/cm^2. All showed large decreases in light output (down to 2% of pre-irradiation levels). Annealing currents of 20mA to 50mA were applied over a period of 9 days. While the best device recovered to 22% of initial light output, several remained at about 5%. Contrary to expectations, the best device had been off during irradiation, and the LEDs pulsed with 18mA during irradiation showed poorer behaviour. These results are not encouraging. The annealing studies will continue, at currents of 30mA, 40mA, and 50mA. In a separate run, 56 ABB Hafo LEDs were irradiated with 8*10^14 n/cm^2. The LEDs were not pulsed during the irradiation. The relative light output immediately after irradiation was <5%. With a 10mA annealing current the light output on one of the LEDs recovered to 20% after 8.5 days and was still rising. Another LED recovered to 40% after a few hours of annealing at 30mA and remained steady thereafter. Some anomalies were observed, however; a LED initially annealed with 10mA up to 58% after a few days was then biassed with 30mA and showed a DROP in light output down to 35%. It was pointed out that the variation in LED damage would require individual tuning of driver currents. Jurg concluded that * VCSELs appear to be radhard and that reliability studies will begin when a large number of devices is received (late September). Coupling of VCSEL light into fibers needs further studies. * Initial results on GEC LEDs under proton irradiation indicate they are less radhard than ABB Hafo LEDs and require higher annealing currents; however, the annealing is not yet complete. * Neutron irradiation of ABB Hafo LEDs give similar results to proton irradiation * Long term studies will be done operating LEDs/VCSELs for months at higher temperature. * * * Dave White showed the status of the DORIC and led driver (LDC) design. A project specification is available on the SCT electronics pages on WWW. The amplifier and comparator stages were shown, as well as a block schematic for the whole chip. The large stray capacitance of the PIN in the GEC package has made the design more difficult but Dave feels that he has this under control. He has taken care to filter supply lines as well. The simulation indicates the chip reaches stable operation within 500ns after the clock is started. The physical size of the chip will fit easily into the space reserved for it (2.0mm x 2.5mm) on the optical hybrid board proposed for use with the GEC package.. The DORIC will be passive, and will use the 50% point of the leading and falling edges to define the extracted clock (the use of leading or falling edge to define the clock changes each time an odd number of ones is sent in the biphase mark scheme). The driver circuitry off detector must therefore be able to adjust these 50% points, accounting for differences in rise and fall times for each channel. This could only be avoided by having a phase-lock loop in the DORIC, which the procurers of the chip refuse to allow. The layout is proceeding. The chip will be designed in AMS 0.8 micron BICMOS, but use only bipolar npn transistors, and is hoped to be radiation tolerant. This will be tested once the chip is available. The submission will be in late October, and the chip should be available for testing in early 1997. The design of the LDC is less well advanced. Submission should also take place in October. * * * Bob presented an updated costing based on the same assumptions as shown at the June meeting, but using the updated fiber prices as shown by John Dowell in the June meeting, and including alternate quotes for radhard fibers from Oxford. The numbers appear on the SCT link WWW pages. The conclusion is that an all-electrical link is a factor of 2 or 3 cheaper than an all optical link. Hybrid electrical-optical solutions, which are no longer being actively considered, are intermediate in cost. The cost per module (without contingency) for complete twisted pair links is approximately CHF 100, while for a full optical system it is in the range CHF 225-340. * * * Tony presented a list of reasons for providing realistic low-mass links for the TSP of summer 1997. There is a desire on the part of the rest of the INDET community and of the calorimeters to avoid excessive material in the TSP. In addition, this is an opportunity for the SCT links group to gain experience working with links in a real experimental environment. Steiner Stapnes was present and said he expects to have low mass modules without heavy support cards for the 97 TSP, and that he sees low mass links as being very desirable. He said a working ABC was a critical item, but was possible. The 1997 TSP will probably consist of about a dozen modules. At this point the desirability of providing links for the 97 TSP was discussed. Most present agreed that we would gain useful experience in doing so. There was a consensus that the main aim of this group is to evaluate the link technologies and make an informed choice based on radiation studies, noise tests, cost, etc., and that the TSP effort should not compromise these studies. The timescale of the 97 TSP is such that the links must be ready by June to allow time for testing in conjunction with the modules and DAQ at H8 prior to beam. Given this short timescale the components needed for the links must be obtained soon. Any links made for the 97 TSP are therefore unlikely to be identical to what will ultimately be used in Atlas. The final link technology would presumably be used for the 98 TSP. Bob emphasized that the 97 TSP will therefore not be the test to decide between STP and optics. The suggestion that a full set of optical links and a full set of STP links be produced was well received. Tony then listed the bits needed for an optical link which could be built in time for the 97 TSP: * LEDs and PIN in the GEC package (available) * DORIC and the LED driver chip, LDC (D. White design, submission Oct 96) * an opto-hybrid board for DORIC, LDC, GEC package (Dave White) * VME clock and control driver - adapt existing board (Roy Wastie) * Fujikura rad-hard fibers (available) * data receiver - sensitive dc-coupled preamp from P. Jarron's group available (rise time 7ns). Separate discriminator needed. * interface to bias card for data and TTC (Maurice Goodrick). Tony is employed by CERN for the next year and will request engineering help from CERN. In addition to these items the question of whether one uses connectors in the TSP and whether one uses bare fiber or ribbon cable are to be decided. Several people felt that putting in realistic connectors and realistically packaged cables was important. Tony will further investigate what resources are needed to build these links. STP links can also be produced, consisting of fine STP cables, a repeater, and standard gauge cables. Bob will look into what is needed in detail. Action item: Tony and Bob will report back to the group with more detailed information about what is needed for TSP links. * * * Bob presented the proposed test setup for fine STP cables. The test setup was designed by Philippe Farthouat and Peter Lichard of the TRT group, and generalizes an existing test setup for bit error rate studies on single low-mass STP cables to 4 bundles of 20 signal cables plus 4 dummy cables each. The block diagram of the circuit can be seen in a brief document which will be attached to the copies of the transparencies. Provision is made for testing bit error rate, cross talk, and radiated noise, and for generating both ac and dc common mode signals between the data generation and comparator ends of the circuit. The board will be fabricated at CERN and be available in October. * * * These minutes can be found from web page http://www.cern.ch/Atlas/GROUPS/INNER_DETECTOR/sctnew/Electronics/links/