The LED-driver is for testing the accuracy of photomultiplier tubes (PMTs) which are housed in the so called Superdrawers of the Tile Calorimeter. The TileCal is one of the calorimeters in the ATLAS detector which measures the energy of certain particles like protons and neutrons. There are over 10000 PMTs in the Superdrawers that have to be kept in check.
What does a PMT do? It takes a light signal (photons) and produces an electrical signal: When a photon with enough energy strikes metal an electron is emitted. This is called the photoelectric effect and PMTs use this at their input stage at the photocathode (see picture). These electrons are not enough to produce a decent electrical signal. Therefore they are accelerated towards an anode and strike a metal plate (dynode) which produces scattering of more electrons. This process is repeated through many stages of dynodes; a cascade that finally results in an electrical pulse.
Most importantly the voltage output is stronger but proportional to the input light. To check if a PMT is working properly, feed it a known amount of photon energy and see if the output voltage amplitude corresponds as expected.
Coming back to the actual TileCal detector; during actual measurements these PMTs receive light pulses that are very short. So it is necessary to check if the PMTs respond correctly for very short pulses. How short?
The LED-driver has to simulate a short pulse of 20 nano-seconds in width. This is 0.00000002 seconds, which is 5 million times shorter than a split-second. The ‘shaper’ is part of the LED-driver circuit responsible for creating this short duration of the pulse. It does so in TTL which is the same type of logic used in your computer, where 5V represent a logic-1 and 0V a logic-0.
This short 5V 20ns pulse is then increased to 20V, still 20ns wide. This signal is replicated for two LED inserts at each Superdrawer. The Old LED driver picture is of the LED-driver that is currently in use. The design is over 10 years old and many of the components are obsolete. Therefore an upgrade is underway.
The new LED-driver design uses propagation delay of successive NAND-gates to create the 20 nano-second wide pulse. A further change to the design is that the LED-driver only requires one input voltage level, whereas the previous required 5V, 24V and -5.2V.
The above picture displays the outdated LED-driver. Notice the extension to the bottom right of green PCB. This was added post production to include an optocoupler between the input signal from the main motherboard and the LED-driver analog circuit. This electrically isolates the two boards therefore a malfunction of either board will not affect the other. This optocoupler has now been incorporated into the updated design.
The schematics of the new LED Driver had to be captured from schematics in PDF format to a digital format known as a capture file and the program used for this is called Orcad Capture. This is the establishment of the logical connections between each of the components. Thereafter the part footprints are made for each of the components. The part footprints are a reference to the physical aspects of the component i.e. the physical contacts, pin numbers etc. Finally the logical connections and footprints are exported to a PCB Editor. In this program (Orcad PCB Editor) the physical layout of the PCB is designed: the dimensions of the board, the placement of all components and the routing (electrical paths) between the components.
The upgrade design has taken on a various forms as the schematic design was scrutinized. Alongside is a picture of testing a possible ‘Shaper’ design on a breadboard (within yellow ellipse). The ‘shaper’ is the part of the circuit that is responsible of the 20ns pulse width.