SCT Links Operation

1
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

2
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.

3
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

4
RX links dynamic range (1)

• RX link designed to work at power gt 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
5
RX links dynamic range (2)

• Look at range of RXmax/RXmin
• Cant set RXmax gt 255 ? Need RXmin lt 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 gt
  10.
• Is this true in practice?

6
Off-Detector VCSELs

• High mean power but large spread.
• Change 10 mA ? 15 mA increases power by 50

7
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 lt100 (200) or gt 700 (600)
  mA for barrel (EC).
• Usually ok after this but if there are problems
  perform TX DAC scan (see later).

8
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_DAC0.3RXmin0.7RXmax ? 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.

9
RX threshold Scan

• Good links ? wide operating window.

Not typical! Most VCSEL too bright so dont 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
Time Bin
10
MSR Adjustment (1)

• Need accurate 5050 MSR for 20 MHz clock ? low
  jitter BC clock recovered by DORIC ? dont 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 5050 MSR
o/p clock has large jitter
11
MSR Adjustment (2)

• MSR adjustment can work ? produce low jitter BC
  clock. Obtain jitter (full width) lt 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 5050

MSR Register
12
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

13
MSR Adjustment (4)

• Scan does work but is very time consuming. Only
  use it now if we have problems with calibration
  runs

14
Operational Problems

• Fatalities
• ESD deaths. 0.4 of on-detector VCSELs died
  after burn-in (72 hours _at_ 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.

15
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.

16
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?

17
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 RXmingt100 reduce VCSEL current
• If RXminlt100 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.

18
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

19
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
20
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.
21
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 dont cause system to crash in calibration
  runs?