A field-programmable gate array (FPGA) TDC for Fermilab SeaQuest (E906) experiment and its tests with a novel external wave union launcher

We developed a field-programmable gate array (FPGA) TDC module for the tracking devices of Fermilab SeaQuest (E906) experiment, including drift chamber, proportional tube and hodoscope. The requirement of time resolution is better than 4 ns. This 64-channel TDC is made of a 6U VMEbus unit equipped with a Microsemi ProASIC3 Flash based FPGA of low power and high radiation tolerance. The new FPGA firmware (Run2-TDC) is designed to reduce the data volume and data acquisition (DAQ) deadtime. The firmware digitizes multiple input hits of both polarities while allowing users to turn on a multiple-hit elimination logic to remove after-pulses in the wire chambers and proportional tubes. A scaler is fitted into the firmware of each channel to allow for recording the number of hits in its respective channel. The TDC resolution is determined by internal cell of 450 ps, and a measurement precision of 200 ps has been achieved. As a demonstration of constructing the Wave Union TDC using an existing multi-hit TDC without modifying its firmware, external wave union launchers are utilized in our test to improve the TDC´s measurement precision. Measurement precision is nearly halved (108 ps) in data based on a four-edge wave union. An even larger improvement of measurement precision (69 ps) has been reached by combining the approaches of Wave Union TDC and multiple-channel ganging.