A token ring local area network (LAN) testing apparatus is provided. The apparatus is able to conduct a phase jitter measurement without disruption of normal token ring operation. The apparatus transmits a frame that includes phase jitter test data. This frame is transmitted into the token ring, together with a clock signal. After return of the frame with the phase jitter test data, a comparison is made between the recovered clock signal accompanying this frame and a reference clock signal. The result of this comparison provides a sample used in determining phase jitter. In making the measurement, an interpolation circuit is utilized to increase the resolution of the time difference between the recovered clock signal and the reference clock signal. A clamp circuit is used to ensure that each interpolation begins at the same reference voltage, as well as ensuring that the interpolation process is properly discontinued.