Agenda for SCT links meeting 14:00 Wednesday, December 18 room 40-SS-B01 20' John Wilson - irradiation studies of LEDs and PINs. 20' Juerg Beringer - LED/VCSEL irradiation studies. 15' Stefan Marconi - STP noise tests - status and first results. 10' Tony Weidberg - Status of DORIC/LDC. 15' Tony Weidberg - Plans for realistic data links in the SCT (for TSP). ********************************************************************* John Wilson presented work done by Stephen Oglesby on irradiation studies of 3 GEC packages (consisting of 2 "GEC" LEDs and one PIN diode). The packages were irradiated to a fluence of 6*10^13 p/cm^2; the fluence was limited to avoid ruining the devices, as they are needed for the 1997 TSP optical links. LEDs: Data were shown for 4 LEDs. Pre-irradiation light output for the LEDs varied from 1 to 9 microwatts of optical power with a 10mA drive current. The devices were biased with 10mA during irradiation, and held at -7 degrees C. Post-irradiation values dropped to about 10% of initial light output. After removal from the PS they were kept at room temperature and annealed at 25 mA for 8 days. After annealing the LEDs recovered to 15-30% of initial output, with the better performance associated with the LEDs with higher optical power. Extrapolating these results to a fluence of 2*10^14 p/cm^2 one expects a degradation before annealing to about 2% of initial light output, consistent with the Bern results. PINs: 3 PIN diodes were also studied. They were irradiated with a -10V bias at -7 degrees C. The response of the pin to light from an ABB-Hafo LED (which was not in the beam) was measured. A power meter was used to check the stability of the ABB-Hafo LED. The responsivity of the LED in A/W was determined from a linear fit to the PIN current versus incident optical power. The measurements of current versus power showed deviations from linearity which need further study. The slopes of these linear fits were then studied as a function of time/fluence. The 3 devices showed rather different behaviours. More study is needed. However, one can set a lower limit on the degradation of responsivity after annealing at 0.2 A/W, which is 40% of the expected initial value. Unfortunately, the initial values for these PINs were not measured due to lack of time before the irradiation run. The Birmingham group proposes a neutron irradiation of these devices at the Dynamitron in January. * * * Juerg Beringer presented a large volume of data on irradiation of GEC PINs and VCSELs from Sandia, a U.S. government laboratory. These devices were measured using the measuring machine at Bern, which has been previously described. The irradiation of the devices described below took place at room temperature. LEDs: There were 88 GEC LEDs irradiated under proton fluences of 2 - 4*10^14/cm^2, and 64 under neutron fluences of 3 - 10*10^14/cm^2. Half of the devices were pulsed at 20mA during irradiation, the other half were unbiased. The GEC LEDs irradiated at 2*10^14 showed initial degradation of RLO (relative light output) to about 2%. Annealing studies were carried out at a variety of currents. The optimum annealing current seems to be 50mA; LEDs annealed at this current recovered to an RLO of 20+-5%. Average RLO values for LEDs annealed at 20, 30, and 40 mA were 4%, 5%, and 9%, respectively. A degradation in LED performance was seen when currents of 80-100 mA were used. Devices irradiated at 4*10^14 showed almost no light after irradiation, and almost no annealing so far at 50mA; measurements are continuuing. VCSELs: VCSELs in 20-fold arrays (of which 2*8 can be operated in the Bern scanning machine) were obtained from Sandia. There were 4 different production runs, giving VCSELs with threshold currents of 0.5 mA, 2mA (2 runs), and 10mA. There were 144 devices irradiated under protons and 112 under neutrons, with fluences as for the LEDs described above. Half of the VCSELs were pulsed (at 2 or 4 mA) during irradiation, the other half were left unbiased. The effect of irradiation is to raise the threshold current. The pulsed VCSELs showed only slight degradation, even after 4*10^14 p/cm^2. Those unpulsed showed rapid annealing to nearly full RLO. The problem of coupling the VCSEL light to a fiber has been studied. Initial results were poor; the amount of light coupled to the fiber varied strongly with details of fiber routing. Two effects have been found: the intensity of the light emitted by the VCSEL varies with angle/position, and this pattern varies with drive current. The total light emitted, however, seems to scale with current; it is therefore critical to couple all the light into the fiber. The solution found at Bern is to bring the VCSEL into contact with the fiber (after aligning to 20 microns or better). A second problem seen involves back reflections into the VCSEL cavity. These can be controlled with a filter. The performance with these modifications is good. It is clear that packaging developed for the use of VCSELs must take these effects into account. Lifetime tests: Two GEC LEDs (at 2.2*10^14) and one ABB Hafo LED (at 7.4*10^13) died during or after irradiation. Long term studies of LEDs and VCSELs have begun. The devices are held at 50 degrees C and DC biased. The number of devices in the study are 144 ABB-Hafo LEDs, 56 GEC LEDs, and 8 VCSELs (of the 2mA threshold variety). After roughly one month of test no degradation has been observed. The aging acceleration factor has yet to be calculated, but is perhaps of the order of 100. Information on the irradiation studies in Bern can be found at http://www.lhep.unibe.ch/atlas/ledlife/welcome.html * * * Stefan Marconi presented the status of the twisted pair option. The PC board for performing noise and transmission tests was designed by Peter Lichard of the TRT group, and was delivered one week ago. Random data are generated at half the clock frequency (i.e. a 40 MHz clock leads to 40 Mb/s of generated data). The data are sent through LVDS drivers over 10 meters of stp cable, received, and compared to the (delayed) data sent across the board. There are 4 bundles of 24 stp cables forseen, one of which was fabricated in time for the meeting. The board has a provision for introducing AC and DC common mode signals between the driver and receiver circuits. After some initial debugging the board works correctly at 40 Mb/s, and should work at 80 Mb/s with better termination of the on-board signals and better delays. An initial bit error rate test at 40 Mb/s yielded BER<10^-11. Studying scope traces of the received data suggest that 80 Mb/s transmission will be feasible. Tests to be performed starting in January include BER, testing noise on unused cables in the bundle, studying the noise radiated to SCT and TRT electronics (one will use existing DAQ setups at CERN for these tests), and testing of common mode effects. Arno Beer has been working on the cable on connector specifications. The connector chosen by the TRT has 66 pins, providing for 27 twisted pairs, a common shield, and 6 low voltage power cables. The connector is 4.5cm wide and 3.5cm long. The miniature twisted pair cables will also undergo modification to improve the contact between the cable shield and the drain wire, and to add an overall plastic coating to avoid the unwravelling of the shield. Pictures and further details on the noise tests, cables and connectors can be found on the web: http://wwwcn.cern.ch/m/marconi/www/public/stp/mainpage.html * * * Tony Weidberg gave the status of the DORIC and LDC (led driver) chips. The chips will be submitted to the AMS 0.8micron BICMOS process (but only npn transistors will be used) on January 22. The processing will take 12 weeks, giving chips in mid April. PIN diodes with faster rise times have been ordered. The PINs will have a lower responsivity (roughly 2/3 of previous PINs) but should have a 10-90% rise time of <1 ns at -5V bias. Several GEC packages with 3 of these PINs (instead of the normal 2 LEDs and 1 PIN) have been ordered, and should be ready for tests in February. Five optical links will be provided for the 97 TSP program. They will consist of (existing) GEC packages of LEDs and PINs, multimode fiber, DORIC, LDC, a clock and control driver (to multiplex the signals going down to the module) and a low noise transimpedance amplifier (obtained from Pierre Jarron) with a separate discriminator on the counting room end. A simple interface board will be necessary near the module, both for connecting the low-mass power cable to 50-way cables and for mounting the optical components. A separate set of twisted pair links will also be provided for the TSP. * * * These minutes can be found from web page http://www.cern.ch/Atlas/GROUPS/INNER_DETECTOR/sctnew/Electronics/links/