Provisional requirements for analogue optical link:

• transport of multiplexed output of 8 Helix chips; 256 analogue outputs
• output signals described in HELIX documentation (see also below)
• medium is fibre; driven by LED or Laser
• frequency is 10 MHz (HERA-B: 20 MHz 10-bit, 40 MHz 8-bit)
• linearity and dynamic range requirements to be provided; tentatively 1-3% precision, 8-bits range
• transport is uni-directional
• length is 50 m
• transmitter boards in cryotower; located in crate
• receiver boards in rucksack; located in (e.g. NIM) crate; e.g. 8 -12 inputs/module (matched to ADC)
• calibration facility (charge injection); connection with control/clock for control functions
• tranferfunction: if 1 then ADC cards can be coupled also directly to input boards. Option to change the gain of the optical output boards (e.g. by changing feedback resisters of the output buffer).
• differential type  output from optical receiver. Then the signals can be handled in a similar way as AO and AOD.

Points of concern:

• costs
• development time and manpower required (frequency is high!)

Alternatives:

1. ADC's at Cryotower -> digital transfer
2. copper solution; amplifiers in Cryotower +coax cables

ad 1) Care should be taken to keep the MTBF so large ,that in practice we would not suffer from the lack of accessibility. Moreover, if one module fails the rest of the system should be able to continue running.

as 2) Means more volume and weight (and more noise?), but a simple solution.

  HELIX output buffer, as specified in the HELIX user manual:

Please mail suggestions/corrections/updates to Leo Wiggers (wiggers.nikhef.nl)