SCT Links: Lessons Being Learnt Update

SCT Links: Lessons Being Learnt Update • Monitoring & Quality of TX links from RODs  detector • Monitoring & Quality of RX links from detector RODs Mini Opto Workshop March '10

TX Links • 12 way arrays of VCSELs on plug-in PCBs • Driven by BPM-12 ASIC • Mounted in BOCs • 8 way arrays for pixels. • Very high death rate during operation in 2008: 1 or 2 per day. • Strong Circumstantial evidence for ESD • Dead channels showed shifted IV curves compared to working channels. • Simulated delayed failures using ESD gun. • We know that ESD precautions were not perfect during production. Mini Opto Workshop March '10

New Production TXs • Decided to replace all TXs during 2009. • New production with • Very strict ESD control • Extra QA step to look for very low level ESD. Mini Opto Workshop March '10

QA for ESD Mean leakage current versus reverse bias voltage Shows breakdown behaviour ~ 16V as expected. I (pA) Mini Opto Workshop March '10 V

Reverse bias voltage at which at least 1 channel has leakage > 1 nA Difference in reverse bias voltage between 1st channel with I > 1nA and all channels I > 1 nA This spread was demonstrated to be very sensitive to low level ESD. None of the dead channels show suspicious values DeltaV Mini Opto Workshop March '10

TX Deaths 2009/10 • SCT • 4 channels failed • ¼ installed end of May, rest end of July • no failures since mid September • Pixels • 6 channels failed (all early). • Post-Mortems for first 8 failures all showed shifted IV curves as expected for ESD/EoS. • Deaths worrying but still compatible with manufacturer’s expected “delayed” infant mortality ~ 0.1%. Mini Opto Workshop March '10

EoS • Symptoms of EoS are indistinguishable from ESD. • First look at EoS • Measured BPM-12 driver PSRR. • Turned out to be ~ 0 dB  should be considered in design of driver! • Scope measurements in-situ inconclusive. • Making monitoring PCB to measure spikes on VDD and readout values to Db. Mini Opto Workshop March '10

Long Term Monitoring TX • Measure optical power of TX VCSELs using photo-currents in on-detector p-i-n diodes. • Distribution of Ipin • Long term trends Mini Opto Workshop March '10

Distribution of PIN Currents Quality generally good I> 0.1 mA Easy to spot dead channels Mini Opto Workshop March '10

Long term monitoring <Ipin> Strange rise in <Ipin> with time ? Not obviously correlated with temperature ? T Mini Opto Workshop March '10

RX Links • Monitoring using Rxthreshold scans • Results from 2008 and 2009 • Improved monitoring using direct current measurements. Mini Opto Workshop March '10

RX threshold monitoring Send fixed data pattern and scan RXthreshold find min and max values for no errors RXmin RXmax usually overflow RXmax RXmin proportional to power Mini Opto Workshop March '10

RX Monitoring RXmin • <RX_min> stable • Fluctuations in apparent numbers of dead channels are spurious need better method. RXmax Number “Dead” Mini Opto Workshop March '10

Direct BOC Ipin Measurements • Can measure total pin current in 12 channel pin-array in BOC. • With modules waiting for trigger (not sending light) measure pedestal: I_ped • Configure modules so that one link out of 12 is sending light: measure I_signal • Pedestal subtracted current I=I_signal-I_ped Mini Opto Workshop March '10

Sanity Check: compare Ipin with RX_threshold scans Mini Opto Workshop March '10

RX links Ipin Distribution • Very clear spike at 0  spot dead channels. • More accurate monitor of VCSEL power for long term monitoring Mini Opto Workshop March '10

Stability Check Compare data 22/2/10 and 1/3/10 Mini Opto Workshop March '10

RX links Summary • 61 links out of 8176 dead • At this level negligible loss of data because of redundancy system. • No deaths in 2009  deaths were early failures. • 1 death detected 1/3/10. • Very low death rate would not be a problem but … Mini Opto Workshop March '10

SCT Links: Lessons Being Learnt Update