A delay measurement apparatus for measuring a delay unit comprising: a clock; clock counter; a digital signal source that is uncorrelated with the clock; a first detector arranged to detect transitions of the digital signal entering the delay unit; a first accumulator arranged to accumulate the current clock counter value based on the output of the first detector; a second detector arranged to detect transitions of the digital signal exiting the delay unit; a second accumulator arranged to accumulate the current clock counter value based on the output of the second detector; a measurement counter arranged to count the number of transitions of the digital signal passing through the delay unit; and a calculation device arranged to calculate an average number of clock cycles that elapse while a transition of the digital signal passes through the delay unit based on the first accumulator, the second accumulator and the measurement counter.