Synchronization Algorithm in Time-to-Digital Converters Networks

Nuclear physics experiments and medical imaging techniques like Positron Emission Tomography (PET) are moving through time- resolved experiments. Furthermore, the increasing complexity of the phenomena under observation makes mandatory to build a set-up by means of the interconnection of an instrumentation cluster. In this sense, classical Time-to-Digital-Converter (TDC) structure is no more satisfactory, since the number of channels of one measuring unit is no more sufficient. In this contribution, the implementation of a network of TDCs based on Field Programmable-Gate Arrays (FPGAs) is presented. Aim of this contribution is to give the possibility to reach a huge amount of channels, simply by connecting TDCs hosted in different devices without loss in term of precision and resolution. Furthermore, to guarantee fast prototyping and flexibility, all the synchronization algorithm and the TDC are implemented in programmable logic. The idea behind this implementation is a distributed architecture composed of multiple high-resolution TDCs (units of picoseconds) and obtain the timestamps as if they were coming from a single device. The challenge is mainly dictated by the differences in clock frequencies of the devices that compose the network, and can be referred to two problems: the offset leading to different values for the timestamps on different devices, and the gain error, which determines different Least Significant Bits (LSBs) for the measurements. The most relevant problem, the second one, it is addressed by adding an extra channel in each TDC, called sync, which takes care of the synchronization between all the devices which compose the network. In this way, the sync is in charge to measure the clock skew between the TDCs respect to a common signal and consequently normalize all the timestamps.