I am designing a board-level circuit to measure pulse width, in which I need a high-precision timer. I want to use a high-speed counter to implement the timer. Since the timer requires an accuracy of 500ps, this implies that the clock to the counter has to run at a frequency of 2GHz. Since I am not designing chip-level circuit, I would like to purchase such an external timer/counter which achieves such precision. Does Anyone have an idea what to buy? If such high precision timer is not easily available in the market, is there anyway I can solve the problem through other means?