The Inner Tracking System (ITS) of the ALICE experiment contains six coaxial
cylindrical layers. Layers 3 contains 14 ladders each one hosting 6 SDDs (Silicon
Drift Detectors), while layer 4 contains 22 ladders each one hosting 8 SDDs. The
CARLOS end ladder board is placed on both sides of each ladder with the purpose of
acquiring and compressing data coming from each SDD before sending them towards the
data concentrator card CARLOSrx in counting room through optical fibers. The board
contains the compression chip CARLOS that performs a bi-dimensional compression of
the data coming from the SDD front-end electronics. CARLOS 16-bit output bus is
encoded with 8B/10B Ethernet protocol and sent to a 800 MBit/s single mode optical
fiber using a 1310 nm optical laser.
The CARLOS end ladder board receives the trigger signals and the configuration
parameters through a 40 MBit/s serial signal coming from CARLOSrx through an optical
fiber and converted using a photodiode.
An other photodiode is used for receiving the 40.08 MHz clock coming from the TTC
system. The QPLL ASIC on the end ladder board allows to obtain a clock with a
peak-to-peak jitter lower than 50 ps that is used both for the serializer (ASIC GOL)
of the board and for the front-end electronics.
A special control unit has been developed with the purpose of monitoring parameters
such as voltage, current and temperature related to the whole readout chain. The
control unit is remotely controlled from the DCS (Detector Control System) board
through the I2C bus. The CARLOS end ladder board also provides power for the analog
and digital voltages of the front-end boards under the control of the DCS system.
The board has been tested using the complete setup from the front-end electronics to
the data concentrator card CARLOSrx: the test of the board was successful both for
what concerns data transmission and DCS features. Beside that an FPGA based board has
been developed as an emulator of the front-end boards with the purpose of generating
known patterns after receiving a trigger signal: this allowed us to obtain an
estimation of the optical link Bit Error Rate (BER) lower than 10-16.
The CARLOS end ladder board fulfils the requirements on space (54x49 mm with a
maximum thickness of 16 mm) and on construction materials for its final use in the
ALICE experiment. Up to now 500 boards have been produced and the test of the
production is being carried out.