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SCT Links Operation. Performance from QA and operation Setting up links during operation for barrel and EC. TX RX Operational problems Fatalities Difficulties How can we track performance? Acknowledgements
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SCT Links Operation • Performance from QA and operation • Setting up links during operation for barrel and EC. • TX • RX • Operational problems • Fatalities • Difficulties • How can we track performance? • Acknowledgements • thanks to Peter Phillips, Bilge Demirkoz, Caroline Magrath and Nigel Hessey for discussions of SCT procedures Pixel Opto November '06
Performance from QA • On-detector VCSEL power • Two VCSELs in one opto-package have single VCSEL current setting Spread of power between two VCSELs important. Pixel Opto November '06
PIN Arrays & DRX-12 Count bit errors in 32k bit stream Blue region is error free. • Slow tail to signal limits dynamic range • Saturation of DRX-12 at very high input current ~ 800 mA Pixel Opto November '06
RX links dynamic range (1) • RX link designed to work at power > 40 mW • Only tested on one channel but could operate with negligible BER at 35 mW (no error in 1 hour) Bit Error Rate Scans Different Optical power BER RX DAC
RX links dynamic range (2) • Look at range of RXmax/RXmin • Can’t set RXmax > 255 Need RXmin < 100 reasonable operating window RXmax ~ 500 Power ~ 1000 mW • Would be better to have a larger range for DRX-12 threshold but can only easily increase this by 20%. Does this help? • However system should work over dynamic range > ~ 10. • Is this true in practice? Pixel Opto November '06
Off-Detector VCSELs • High mean power but large spread. • Change 10 mA 15 mA increases power by ~50% Pixel Opto November '06
Setting Up Links • TX links • Initially set TX DAC = 165 nominal VCSEL current of 10 mA. Newer TXs have higher fibre coupled power new default TX DAX = 140 • Check IPIN. Adjust if <100 (200) or > 700 (600) mA for barrel (EC). • Usually ok after this but if there are problems perform TX DAC scan (see later). Pixel Opto November '06
RX Links • DC coupled links are more difficult to set-up. • Old procedure for barrel • Perform RX threshold scan. • Send configuration data and repeat for 10 triggers (8000 bits). Change RX DAC value and repeat coarse scan of BER vs RX DAC. Optimal setting RX_DAC=0.3*RXmin+0.7*RXmax figure. Works but is slow. • Set timing to global value (barrel) or perform RXdelay scan (Endcaps). • New method: superfast scan (7s) • 2d scan of delay and Rxthreshold in clock/2 mode (ABCD returns 20 MHz clock). Works for most modules but use slower Rxthreshold based on configuration data for problematic channels. Pixel Opto November '06
RX threshold Scan • Good links wide operating window. Not typical! Most VCSEL too bright so don’t see top edge of scan. RX_Threshold Errors at low RX DAC are not due to noise but are due to the slow tail in the p-i-n signals (off-detector array). RXmin scales with power Pixel Opto November '06 Time Bin
MSR Adjustment (1) • Need accurate 50:50 MSR for 20 MHz clock low jitter BC clock recovered by DORIC don’t try to design perfect clock but allow for adjustments with MSR register. • Adjustment will be required on all channels to optimise timing but if it is very wrong can cause problems even in calibration runs. i/p clock with non 50:50 MSR o/p clock has large jitter Pixel Opto November '06
MSR Adjustment (2) • MSR adjustment can work produce low jitter BC clock. Obtain jitter (full width) < 0.5 ns for optimal MSR but jitter could be very big if MSR wrong • Warning: we found a few VCSEL arrays that had very low MSR and could not be tuned to give 50:50 MSR Register
MSR Adjustment (3) • How to set MSR values in full system? • Idea is to tune system so that duty cycle of clock/2 is independent of BPM phase. • MSR scan • (1) Set clock/2 mode • (2) SetMSR value • (3) Measure width of on period with scan of RXdelay. • (4) send odd number of bits (SR) of TTC data to flip phase of BPM signal and repeat (3) • (5) Change MSR register 3 • (6) Plot duty cycle for the two phases as a function of MSR register figure • Plan to develop faster 2D scan in future Pixel Opto November '06
MSR Adjustment (4) • Scan does work but is very time consuming. Only use it now if we have problems with calibration runs
Operational Problems • Fatalities • ESD deaths. ~ 0.4 % of on-detector VCSELs died after burn-in (72 hours @ 10 mA and 50C and full QA (~ 30 minutes operation) rely on redundancy. Maybe lower level damage on more VCSELs will compromise the lifetimes? • Slow turn on VCSELs (figure). QA used DC or PSBR data. Real operation involves long gaps between bursts of data. ~ 0.25% of on-detector VCSELs are not usable. Pixel Opto November '06
Slow Turn on VCSEL • Link 1 has no good value for RX DAC use redundancy • Link 0 illustrates intermediate case: can find good RX DAC but lose dynamic range. Pixel Opto November '06
Operational Problems • Difficulties • RX links: Intermediate cases of slow turn on VCSELs can be used but much more care required to set Rxthreshold slow scan. • TX links: some require larger IPIN than normal, may be linked to slow turn on off-detector VCSELs? Pixel Opto November '06
Problem Solving Recipes • Runs do crash. Often after many minutes in noise occupancy scans because long events suffer more from slow turn on VCSEL problems. • Fixes • Hard reset ( module CLK/2) and repeat • If RXmin>100 reduce VCSEL current • If RXmin<100 increase VCSEL current • Redo RX threshold scan and RX delay scan. • Look at error codes: if they are L1 or BC ID errors error at start of run probably slow turn on VCSEL. • perform TXscan: scans TX DAC value and sends and returns configuration data (cf RX threshold scan). Increase or decrease IPIN if necessary. • (ABCD errors increase VDD to 4.2V) • Have succeeded in simultaneous operation of large number of modules: 672 Barrel 6 in Oxford and 462 barrel modules in SR1 Cosmic run but required a lot of tweaking of parameters. Pixel Opto November '06
Tracking Performance • TX links • Measure IPIN and check if we see expected decrease in responsivity with radiation damage • RX links. • Can track performance roughly with RXmin fig. • Better if we could measure I_PIN in array for each channel Pixel Opto November '06
Tracking I(p-i-n) Difference I(p-i-n) in a run cf reference run. Mean change in I(p-i-n) versus run number 10% spread due to off-detector VCSEL variation but means show stability over ~ 2 months Pixel Opto November '06
RX min correlation with Power Weak correlation Better way: use current measurement in off-detector p-i-n array 12 bits with LSB = 6 mA ~ O(2%) precision.
Outlook • Should take time to debug anomalous channels. • Look at signals with optical probe. • Easier to debug in SR1 than in pit • Can we make DAQ more robust so that single bit errors don’t cause system to crash in calibration runs? Pixel Opto November '06