Tony Weidberg Opto WG April '08 1

SCT Links Long Term Monitoring

• Summary quality installed links

• Techniques for monitoring long term performance– Data links– TTC links

• Improvements for SLHC

Tony Weidberg 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).

Tony Weidberg 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.

Tony Weidberg 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

Tony Weidberg 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

Tony Weidberg 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.

2008 data2007

data

<RX Min>

<RX Max>

Tony Weidberg 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.

Tony Weidberg 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 k series resistor + DVM.

Ipin

0.1 mA

Tony Weidberg Opto WG April '08 9

Ashley's Ipin correlation

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Ipin (LV0)

Ipin

(As

hle

y)

Correlation (DVM + 1k vs LV0

Correlation ok at high Ipin but poor at low values of Ipin

Tony Weidberg Opto WG April '08 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).

Tony Weidberg 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 …

Tony Weidberg 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.