sct links: lessons being learnt update

Tony Weidberg Mini Opto Workshop March '10 1

SCT Links: Lessons Being Learnt Update

• Monitoring & Quality of TX links from RODs detector

• Monitoring & Quality of RX links from detector RODs


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.


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.


Mean leakage current versus reverse bias voltage

Shows breakdown behaviour ~ 16V as expected.

I (p

A)

V

QA for ESD


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


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%.


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.


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


Distribution of PIN Currents

Easy to spot dead channels

Quality generally good

I> 0.1 mA


Strange rise in <Ipin> with time ?

Not obviously correlated with temperature ?

Long term monitoring <Ipin>

T


RX Links

• Monitoring using Rxthreshold scans– Results from 2008 and 2009

• Improved monitoring using direct current measurements.


RX threshold monitoringSend fixed data pattern and scan RXthreshold find min and max values for no errors

RXmax usually overflow

RXmin proportional to power

RXmin

RXmax


RX Monitoring

• <RX_min> stable

• Fluctuations in apparent numbers of dead channels are spurious need better method.

RXmin

RXmax

Number “Dead”


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


Sanity Check: compare Ipin with RX_threshold scans

Rx_min link 0 Correlations

y = 0.0054x + 0.0115

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50 60 70 80 90 100

RX_min (DAC counts)

BO

C I

pin

(m

A)


RX links Ipin Distribution

• Very clear spike at 0 spot dead channels.

• More accurate monitor of VCSEL power for long term monitoring

BOC Ipin

0

100

200

300

400

500

600

700

800

900

1000

-0.1

-0.0

40.

020.

080.

14 0.2

0.26

0.32

0.38

0.44 0.

50.

56M

ore

I (mA)

Fre

qu

en

cy


Stability CheckBOC Ipin Correlations

y = 0.9967x

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

22/3/10

01/0

3/10 Correlations

Linear (Correlations)

Compare data 22/2/10 and 1/3/10


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 …

sct links: lessons being learnt update

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