The Outer Tracker of the LHCb detector is composed of 55000 straw
drift tubes of 5mm diameter.
The requirements for the OT electronics is the precise (1ns) drift
time measurement, permitting a track resolution of 200 um in
the bending plane, at 6% occupancy and 1MHz readout.
The readout electronics is based on three ASIC chips, the ASDBLR
amplifier from ATLAS, the OTIS TDC developed at the ASIC lab in
Heidelberg and the GOL (gigabit optical link) serializer from CERN EP.
All on-detector electronics for one module end is housed in a shielded
The charge signal from the straw anode wires is amplified, shaped and
discriminated by an 8 channel ASDBLR chip with 12ns shaping time.
Four ASDBLRs on 2 PCBs are connected to one OTIS TDC.
The relative time between the discriminated hit signal and the bunch
clock is determined by the OTIS TDC with the help of a delay locked
loop. The 64 inverters in the DLL lead to a time resolution of
25ns/64 = 390ps. Drift times are 6-bit encoded and written to a
240 bit wide L0-buffer. At L0-accept hits within a 75ns window are
readout and a header containing OTIS location, L0-id, Bunch-Id plus
status is prepended.
L0 accepted data from 4 TDCs runs to the GOL chip.
The GOL serializes 32
input bits to a 1.6 Gbit/s output, driving a VCSEL diode. Optical
fibers carry the data 120m to the TELL1 acquisition board.
On the TELL1 daughter boards with 12 inputs each convert the optical
serial data back to paralell electrical data. FPGAs on the TELL1 are
programmed to synchronize, zero suppress and reformat the data.
At 1MHz events are transmitted through multiple Gigabit ports to a more than 1000
node computing farm.
The newly developed outer tracker electronics was used for data
taking in an e- test beam. Reading out 512 channels the full chain
proved excellent performance.