Abstract: An apparatus for measuring jitter in a digital signal that includes an offset unit arranged to form an offset reference clock signal, being offset by a predetermined frequency amount from the digital signal. The apparatus also includes a sampler arranged to sample the digital signal at sampling times determined by the offset reference clock signal such that, in the absence of jitter and the offset by a predetermined frequency, there are a predetermined number of sampling times in each bit of the digital signal. The apparatus further includes at least one detector arranged to detect occasions when the number of sampling items in any bit of the digital signal is different from the predetermined number, and a counter arranged to count the occasions over a predetermined time. Also the apparatus includes an analyzer arranged to derive at least one measure of jitter from the counting of the occasions.