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SCT Links Long Term Monitoring

This article discusses the quality of installed data and TTC links in the SCT, including the status before installation in the pit and improvements made in April '08. Techniques for monitoring long-term performance, such as RX threshold scans and pin current measurements, are also presented. The importance of monitoring BER and the need for better design considerations are highlighted.

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SCT Links Long Term Monitoring

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  1. SCT Links Long Term Monitoring • Summary quality installed links • Techniques for monitoring long term performance • Data links • TTC links • Improvements for SLHC Opto WG April '08

  2. Data Links Quality • Data links status before installation in pit. • Dead : 0.6% • Problematic: 0.3% • Deaths: ESD VCSELs, broken single fibres on detector, damaged tracks on Al/kapton tapes. • Problematic: “slow turn”-on VCSELs (see next slide). Opto WG April '08

  3. SCT Slow-turn on VCSELs • Send fixed data pattern • Scan DAC RX threshold • Check to see if correct bit pattern returned. Opto WG April '08

  4. Data Links Monitoring - 1 • RX Max and RX Min distributions from the RX threshold scans. • Insufficient dynamic range  RXmax usually >255 so can’t be determined. • Best way to monitor optical output of on-detector VCSELs is using RXmin • Long term monitoring of number of links with low values RXmax will indicate if some VCSELs are going to die . RX Max RX Min Opto WG April '08

  5. Data Links Monitoring - 2 Scan RX thresholds for data links, RXmin  optical power received. Plot ratio RXmin ratio (Pit/RAL reception) See decrease of 1 dB as expected from attenuation in long cable Histogram RXmin ratio Pit/RAL Opto WG April '08

  6. Data Links Monitoring - 3 • No dramatic changes but need to start long term monitoring. • Ideally we should have several data sets before beam to understand stability and performance before radiation. <RX Min> 2008 data 2007 data <RX Max> Opto WG April '08

  7. TTC Links Quality • TTC links before installation in pit. • Dead : 0.2% • Problematic: 0.7% • Deaths: broken single fibres on detector, damaged tracks on Al/kapton tapes. • Problematic: low p-i-n current channels • We initially thought this was a very widespread problem but it turned out to be mainly an artifact of oscillations in the current monitoring circuitry in the power supply.  replace up to 10% VCSEL arrays in ROD. Opto WG April '08

  8. TTC Links Monitoring • Measure the currents for the on-detector pin diodes (Ipin) using the LV0 power supplies in the counting room. • Recent measurements of Ipin for SCT barrel in the pit. • Mean <Ipin>=0.24 mA ok • 7.7% links with Ipin<0.1 mA (we require Ipin>0.1 mA to minimise SEU). • Measure Ipin for all suspect low channels with simple card with battery + 1 kW series resistor + DVM. 0.1 mA Ipin Opto WG April '08

  9. Correlation (DVM + 1kW) vs LV0 Correlation ok at high Ipin but poor at low values of Ipin

  10. Status Low Ipin Channels • Measured 1941 channels • From measurements of 150 suspect channels only 14 confirmed low (of which 7 read 0) • To be checked with optical power meter. • 0.8% low channel replace ~ 9% of TXs (12 way arrays). Opto WG April '08

  11. Monitoring BER • Can monitor BER for data links using • Preamble (fixed pattern) • Header (some bits fixed) • Trailer (fixed pattern) • TTC links • BCID and L1ID counters in the ROD. • ROD receives full BCID and L1ID from L1 Trigger. • FE chips (ABCD) on module count BCs and L1s and the 4/8 LS Bits of these numbers are transmitted to the RODs. • Compare LS bits of returned BCID & L1ID with ROD data. • Any discrepancy  bit error(s). Note that we expect finite BER from SEU at high luminosity. • 95% modules show no errors, 5% have errors, mainly BCID and L1Id; under investigation … Opto WG April '08

  12. Conclusions • We have tools available and we will be able to track any long term degradation in performance of data or TTC links. • Data links: perform RX threshold scans. • TTC links: measure pin currents. • BER for data/TTC links from BC and cross checks. • Monitoring could be better if more thought at the design stage: • More accurate measurements of pin currents for low values essential for determining whether TTC link performance is acceptable. • better dynamic range for RX DAC (data links) so that digital scans would always determine meaningful values of RX_Max. • by measuring individual p-i-n currents in RX. Opto WG April '08

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