actually more involved because there are synchronizer issues and current switching is not instantaneous. Also, the interpolator must calibrate the height of the ramp to a clock period. Vernier Vernier interpolator The vernier method is more involved. The method involves a triggerable oscillator and a coincidence circuit. At the event, the integer clock count is stored and the oscillator is started. The triggered oscillator has a slightly different frequency than the clock oscillator. For sake of argument, say the triggered oscillator has a period that is 1 ns faster than the clock. If the event happened 67 ns after the last clock, then the triggered oscillator transition will slide by −1 ns after each subsequent clock pulse. The triggered oscillator will be at 66 ns after the next clock, at 65 ns after the second clock, and so forth. A coincidence detector looks for when the triggered oscillator and the clock transition at the same time, and that indicates the fraction time that needs to be added. The interpolator design is more involved. The triggerable clock must be calibrated to clock. It must also start quickly and cleanly. Vernier method The Vernier method is a digital version of the time stretching method. Two only slightly detuned oscillators (with frequencies and ) start their signals with the arrival of the start and the stop signal. As soon as the leading edges of the oscillator signals coincide the measurement ends and the number of periods of the oscillators ( and respectively) lead to the original time interval : Since highly reliable oscillators with stable and accurate frequency are still quite a challenge one also realizes the vernier method via two tapped delay lines using two slightly different cell delay times . This setting is called differential delay line or vernier delay line. In the example presented here the first delay line affiliated with the start signal contains cells of D-flip-flops with delay which are initially set to transparent. During the transition of the start signal through one of those cells, the signal is delayed by and the state of the flip-flop is sampled as transparent. The second delay line belonging to the stop signal is composed of a series of non-inverting buffers with delay . Propagating through its channel the stop signal latches the flip-flops of the start signal's delay line. As soon as the stop signal passes the start signal, the