A digitally controlled delay unit is described that provides subnanosecond step sizes. The unit is constructed as two modules, one based on a 100 MHz oscillator, giving a virtually unlimited choice of delay length in 10 ns increments, and the other giving a fine interpolation between these 10 ns steps. Because of the modular construction each unit can be used independently of the other if desired. In particular, the fine delay unit can be used in conjunction with an existing lower-resolution delay unit to extend its range inexpensively. The unit described has been used to produce delays with a step size of 500 ps, although in principle step sizes below 100 ps appear possible ultimately limited by jitter.