A modular multi-channel time counter using programmable devices

Przedstawiono budowę, zasadę działania i wyniki badań wielokanałowego modułowego licznika czasu. Umożliwia on równoczesny pomiar relacji czasowych pomiędzy impulsami wejściowymi (START), pochodzącymi z maksymalnie sześciu niezależnych źródeł zegarowych, a wspólnym dla wszystkich kanałów impulsem odniesienia (STOP). Moduły pomiarowe licznika wykonano z użyciem układów programowalnych FPGA Spartan-3 (Xilinx). Licznik charakteryzuje się zakresem pomiarowym do 1 s oraz precyzją pomiarów nie gorszą niż 250 ps.We present the design, operation and test results of a modular multichannel time counter built with the use of programmable devices. Its resolution is below 50 ps and the measurement range reaches 1 sec. The design of the counter is shown in Fig. 1. It consists of six independent measurement modules. Each measurement module contains a 2-channel time interval counter (Fig. 2) implemented in a general-purpose reprogrammable device Spartan-3 (Xilinx). To obtain both high precision and wide measurement range, the counting of periods of a reference clock is combined with a two-stage interpolation within a single period of the clock signal [6]. The interpolation involves a four-phase clock in the first interpolation stage [8] and a time delay coding line in the second interpolation stage. The reference clock module contains an integrated digital synthesizer [7], that provides the reference clock signal of 250 MHz for measurement modules, and is driven by an external clock source of 5 MHz or 10 MHz. The standard measurement uncertainty of the time counter was tested (Figs. 3 and 4) carefully and it did not exceed 250 ps in the full measurement range. As the acid test of the time counter, the differences between signals of 1 PPS from the tested clock sources and the reference 1 PPS signal were also verified (Figs. 5 and 6). The modular design makes the multi-channel time counter easy to modify to meet requirements of various applications