High speed digital TDC for D0 vertex reconstruction

A high-speed digital TDC (time-to-digital converter) has been built as part of the Level 0 trigger for the Dφ experiment at Fermilab. The digital TDC is used to make a fast determination of the primary vertex position by timing the arrival times of beam jets detected in the Level 0 counters. The vertex position is then used by the Level 1 trigger to determine the proper sin θ weighting factors for calculating transverse energies. Commercial GaAs integrated circuits are used in the digital TDC to obtain a time resolution of σ_t=226 ps. The current design is limited to measuring a maximum time difference of ≈100 ns, but it could be easily extended by cascading additional counter stages