To provide a method of measuring a frequency response of a jitter.

In this method, a clock signal from a data input stream 26 is restored to provide a restored clock signal 32. The restored clock signals are counted in a selected period to provide a restored clock count. Reference signals 30 are counted in the selected period to provide a reference signal count. The restored clock count is compared with the reference signal count. A frequency of the restored clock signal is calculated in the selected period.