***** Minutes revised on 15-Apr-1997 concerning NIEL calculations ***** Agenda for SCT links meeting The meeting was held on Thursday, February 27 at 14:00 in room 40-R-D10. 14:00 Irradiation studies. Stephen Oglesby 15' 14:20 Irradiation and lifetime studies. Juerg Beringer 25' 15:00 Results from prototype optical link Tony Weidberg 10' 15:15 Shielded twisted pair tests. Stefan Marconi 15' 15:30 Coffee 16:00 VCSEL packaging John Dowell 10' 16:10 DC coupling scheme for optical links Tony Weidberg 15' 16:30 Planning for links technology choice Bob Kowalewski 20' 17:00 Working meeting for links section of TDR 60' ********************************************************************* Stephen Oglesby presented results on neutron irradiation of GEC packages containing PINs and LEDs. 4 LEDs in 2 packages and 3 PINs in 3 packages were irradiated at the Birmingham dynamitron with 1 MeV neutrons at dose rates of 1-5*10^9 n/cm^2/s for a total fluence near 1.7-2.1*10^14 n/cm^2. All of the devices had been previously irradiated to 5.5*10^13 p/cm^2 (24 GeV protons) and subsequently annealed. The LEDs recovered to relative light outputs (RLO) of 35-40% after the initial proton irradiation. Annealing was done at room temperature and with currents of 10 to 50mA. A DC current of 10mA was maintained throughout the irradiation period. The RLO (normalized to the value after annealing the proton induced damage) dropped to about 15% after the neutron irradiation; annealing of these devices is in progress. Stephen calculated damage constants of 6.53*10^-14/cm^2/proton and 1.15*10^-14/cm^2/neutron; the ratio of n:p is 1:5.7. He calculates and expected ratio using NIEL stopping powers of 1:5.1. **revision as of 15-Apr-1997 Juerg and Stephen discussed the difference between the NIEL equivalents they calculate, and of the comparison with observed damage. The NIEL ratio of 1 MeV nuetrons to 24 GeV protons is 1:5.3 according to Juerg, while the ratio for ISIS neutrons to 24 GeV protons is 1:3.2. He expects the value appropriate for the neutron spectrum from the Dynmatron to be in between these values. He also pointed out that the damage constants should be measured before any annealing takes place, and that the irradiations done with LEDs under bias do not satisfy this condition. This could explain the differences observed for the damage constants. **end of revision The PINs had responsivities of 0.3-0.55 A/W before proton irradiation and were annealed to values from 0.3 to 0.45 A/W. In the subsequent neutron irradiation the 3 devices showed a marked loss of responsivity, down to values in the range 0.02 to 0.05 A/W. In addition the dark current rose sharply from <20nA to 300-500nA. The PIN behaviour under proton irradiation was as had been expected based on information from GEC. However, the neutron irradiation results are much worse than expected. Several new GEC packages with 3 new-style epitaxial silicon PINs (these have faster rise times at 5V than the devices in the packages irradiated to date) should be available within 1-2 weeks. Tony expressed concern about possible rate effects (the dynamitron is a very hot source) and will arrange an irradiation at ISIS as soon as possible. ACTION > > > > Irradiate new PINs at ISIS au plus vite * * * Juerg Beringer presented further results on irradiation and lifetimes studies of GEC LEDs at Bern. Improved statistics are now available, as are new results from neutron irradiation. The fluences in the neutron irradiation were 0.7*10^14 n/cm^2 and 4.4*10^14 n/cm^2. The devices have been annealing at 50mA and -10C for 8 days. RLOs at present are near 85% and 50% for the two batches, respectively. Juerg showed that the GEC LEDs that have a high initial light output appear to anneal more rapidly than initially weaker devices; the asymptotic behaviour is however, not yet known. Juerg also finds good agreement between proton and neutron damage constants and the NIEL calculations, but in his case he finds n:p=1:3.1 and calculates a ratio of 1:3.2 after folding in the neutron energy spectrum from ISIS. The aging studies are in progress. The aging acceleration factor, partly from continuous DC operation (i.e. time spent at high current) and partly from temperature effects (test being run at 50C), is at least 120 (12 from continuous operation, >=10 from temperature). A few LEDs (3/119 ABB and 1/31 GEC) can be considered dead, but all of these failures occurred before the high temperature phase of the lifetime study. The duration of the lifetime test now corresponds to 20-30 years operation in the SCT. About 80 additional GEC LEDs will be added once they have finished annealing. The aging tests should be finished in mid-April. Reirradiation of ABB LEDs, GEC LEDs, and VCSELs is in progress to study the effect of interim annealing on the RLO versus total fluence. VCSELs: 63 Sandia VCSELs have been operating for the same period, with one post-irradiation failure. The aging acceleration factor in this case is not yet known. About 80 additional devices will be added once they have finished annealing. The LAr group is still investigating Sandia VCSELs as an option and have been studying packaging (with Amphenol). The relevant physicists will be here during the week of March 10-14. Their meeting is Tuesday, March 11 at 9:30. Bob will attend, and other interested parties are encouraged to do so. ACTION > > > > Bob will attempt to arrange for a private discussion with those colleagues working on VCSEL packaging. ACTION > > > > Bob will contact Sandia about VCSELs. * * * Tony Weidberg reported on results using a prototype link. The link uses an existing GEC pigtailed package with the LEDs driven by a CMOS rad-soft version of the LDC, and receives the signals on a fast PIN diode followed by a low noise transimpedance amplifier (from Pierre Jarron) and a separate commercial discriminator. In a production system the amplifier and discriminator would be combined into one chip, and a slower PIN with higher responsivity would be used. Tony showed a plot of signal to noise as a function of the amplifier rise time (which can be tuned), and found a broad maximum with S/N>200 for rise times between 8ns and 25ns. He used this measurement to estimate the safety factor of LED-PIN links after irradiation. The table with the results will appear in the TDR; the final S/N after irradiation is expected to be of the order of 50. It was agreed to show the distribution of S/N one expects after irradiation based on the distribution of RLO for the GEC LEDs measured at Bern. This plot will be included in the TDR. ACTION > > > > Juerg will produce a plot of GEC LEDs irradiated with approximately 2*10^14 p/cm^2 as a function of RLO (and expected S/N). * * * Stefan Marconi presented information on the twisted pair option. Measurements of stp transmission properties have been made using the board designed by Peter Lichard of the TRT. Signal properties over 10m of twisted pair cables show good performance at both 40 and 80 Mbit/s, both for the full 400mV LVDS voltage swing and for transmission at 1/2 LVDS levels. The cables contained 27 pairs, 20 with signals and 7 dummies for measuring cross talk and noise. The cross talk was at the level of 5mV r.m.s., to be compared to signal swings of 1/2 LVDS levels (approx. 250mV). The connection point for the shields had no noticeable effect on the transmission. Common mode signals (from DC to 40MHz) of 1V, injected between the driver and receiver circuits, had no effect on the transmission. Common mode levels of 2V still allowed acceptable transmission. Plots can be seen at "http://wwwcn.cern.ch/m/marconi/www/public/stp/100stp.html" Measurements on longer (20m-80m) lengths of standard gauge (AWG 28) cables indicate that compensation will be needed for lengths beyond 50m (see "http://wwwcn.cern.ch/m/marconi/www/public/stp/fullink.html"). Stefan mentioned plans for full prototype twisted pair links, which will comprise a 10m length of 24 pairs of fine stp, a repeater based on existing LVDS-TTL, TTL-LVDS chips, and 80m of standard twisted pair cable. A compensation circuit for the 80m section will be necessary, and is under study. Three 24-pair links will be made, and will be available for use in the TSP-97 program. His calculations of the stp cable material budget in the inner detector indicate a worst-case total of 0.9% X0 up to the last SCT hit and 2.5% X0 to the last TRT hit (see "http://wwwcn.cern.ch/m/marconi/www/public/stp/budget.html"). In addition the space needed in the cable runs within the inner detector was calculated, and found to be about 2 cm^2 per 36 modules (i.e. 6 24-pair cables). This exceeds the space reserved for optical fibres (0.4 cm^2) in the current engineering layout. * * * John Dowell reported on discussions with GEC-Marconi regarding the mounting of VCSELs in place of LEDs in the existing packages. The alignment would be done to <10um via solder bump bonding. A revised package using less silicon and more ceramic is proposed to reduce cost. The VCSEL packaging could proceed as soon as VCSELs are made available. ACTION > > > > Bob will determine if Sandia can supply VCSELs before cutting to allow post-processing by GEC, and will also inquire about possibilities for VCSEL packaging from Sandia. * * * Tony Weidberg proposed a scheme for a compensated DC-coupled system for the data receiver. The use of an NRZ scheme rules out pure AC coupling, and pure DC coupling makes unrealistic demands on the stability of the amplifier. A scheme using a microprocessor to do a calibration was proposed (the idea comes from Alan Rudge). It will be described in the TDR. Some discussion ensued regarding the timescale on which calibrations would need to be performed. It was pointed out that such a system provides monitoring "for free." * * * Bob Kowalewski proposed a procedure for arriving at a technology choice for the SCT data transmission. The proposal was slightly modified after discussion, and can be found at "http://wwwcn1.cern.ch/~kowalews/SCTlinks/choice.html" Further comments are welcome. ********************************************************************* These minutes can be found from web page http://www.cern.ch/Atlas/GROUPS/INNER_DETECTOR/sctnew/Electronics/links/