Bunch crossing determination with the IFR

Genova IFR group

Last updated: 4/2/1998

 

The good time resolution (~1 ns) of the RPCs equipping the IFR can be profitably used for the determination of the bunch crossing time (or, equivalently, of the bunch number) related to a given event accepted by the BABAR trigger.

Time information for the IFR will be provided by the IFR TDC Board (ITB), which gets as input 96 FAST-OR signals. Each of them is generated from a Front End Card (FEC) as the logical OR of 16 alternated RPC strips (i.e. every other strip is read by the same FEC).

The TDC chip has a time bin of 500 ps and a dynamic range of 21 bit, corresponding to about 1 ms. All read values are relative to the last occurence of a Sync command to the front end electronics. The measured time value, tmeas, can then be written:

tmeas = tbunch + tflight + tstrip + tcable - tSync,

where:

- tbunch is the actual bunch crossimg time;

- tflight is the time of flight needed for the particle traversing the IFR to reach it from the interaction point;

- tstrip is the propagation time along the RPC strip;

- tcable is the time needed for the signal at the strip end to be taken to the TDC input; this includes: propagation time along the cable to the FEC; electronics delay inside the FEC to read the signal, generate the FAST-OR and send it to the output; propagation time along the cable to the ITB; electronics delay of the ITB input stage;

- tSync is the arrival time of the Sync signal to the TDC (this is shifted by a fixed number of 60 MHz clock periods with respect to the arrival of the command to the FEE crate).

The quantity of interest is (tbunch - tSync): the actual bunch crossing time can then be extracted from here by using the 64-bit time stamp associated to each L1 by the FCTS and written with the event (see the DAQ Programmer's Guide, cap. 13). Thus it is necessary to have a precise determination of the other three contributions: of these, tflight and tstrip are related to each particular event, while tcable should be determined once and for all (or at least vary slowly with time).